Network Devices - Switch

Switches :

A network switch or switching hub is a computer networking device that connects network segments ( i.e different LANs ).

Like network bridge that the Switch processes and routes data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the network layer (layer 3 and above) are often referred to as Layer 3 switches or multilayer switches.

When packets are received at the destination, it is checked for the errors and if errors are found, data will be retransmitted. When the packets are arrives, the header is checked to determine which packet is destined for which segment and then it is forwarded to that segment. Switches can be used in heavily loaded network to combine the data flow and improve performance.

Role of switches in networks
Switches may operate at one or more OSI layers, including physical, data link, network, or transport layer. A device that operates simultaneously at more than one of these layers is known as a multilayer switch.

Layer-specific functionality
While switches may learn about topologies at many layers, and forward at one or more layers, they do tend to have common features. Other than for high-performance applications, modern commercial switches use primarily Ethernet interfaces, which can have different input and output bandwidths of 10, 100, 1000 or 10,000 megabits per second. Switch ports almost always default to Full duplex operation, unless there is a requirement for interoperability with devices that are strictly Half duplex. Half duplex means that the device can only send or receive at any given time, whereas Full duplex can send and receive at the same time.

At any layer, a modern switch may implement power over Ethernet (PoE), which avoids the need for attached devices, such as an IP telephone or wireless access point, to have a separate power supply. Since switches can have redundant power circuits connected to uninterrupted power supplies, the connected device can continue operating even when regular office power fails.

Layer-1 hubs versus higher-layer switches
A network hub, or repeater, is a fairly unsophisticated network device. Hubs do not manage any of the traffic that comes through them. Any packet entering a port is broadcast out or "repeated" on every other port, except for the port of entry. Since every packet is repeated on every other port, packet collisions result, which slows down the network.

There are specialized applications where a hub can be useful, such as copying traffic to multiple network sensors. High end switches have a feature which does the same thing called port mirroring. There is no longer any significant price difference between a hub and a low-end switch.

Layer 2

A network bridge, operating at the Media Access Control (MAC) sublayer of the data link layer, may interconnect a small number of devices in a home or the office. This is a trivial case of bridging, in which the bridge learns the MAC address of each connected device. Single bridges also can provide extremely high performance in specialized applications such as storage area networks.

Classic bridges may also interconnect using a spanning tree protocol that disables links so that the resulting local area network is a tree without loops. In contrast to routers, spanning tree bridges must have topologies with only one active path between two points. The older IEEE 802.1D spanning tree protocol could be quite slow, with forwarding stopping for 30 seconds while the spanning tree would re-converge. A Rapid Spanning Tree Protocol was introduced as IEEE 802.1w, but the newest edition of IEEE 802.1D-2004, adopts the 802.1w extensions as the base standard. The IETF is specifying the TRILL protocol, which is the application of link-state routing technology to the layer-2 bridging problem. Devices which implement TRILL, called Bridges, combine the best features of both routers and bridges.

While "layer 2 switch" remains more of a marketing term than a technical term,[citation needed] the products that were introduced as "switches" tended to use micro-segmentation and Full duplex to prevent collisions among devices connected to Ethernets. By using an internal forwarding plane much faster than any interface, they give the impression of simultaneous paths among multiple devices.

Once a bridge learns the topology through a spanning tree protocol, it forwards data link layer frames using a layer 2 forwarding method. There are four forwarding methods a bridge can use, of which the second through fourth method were performance-increasing methods when used on "switch" products with the same input and output port bandwidths:

1. Store and forward: The switch buffers and, typically, performs a checksum on each frame before forwarding it.
2. Cut through: The switch reads only up to the frame's hardware address before starting to forward it. There is no error checking with this method.
3. Fragment free: A method that attempts to retain the benefits of both "store and forward" and "cut through". Fragment free checks the first 64 bytes of the frame, where addressing information is stored. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames that are in error because of a collision will not be forwarded. This way the frame will always reach its intended destination. Error checking of the actual data in the packet is left for the end device in Layer 3 or Layer 4 (OSI), typically a router.
4. Adaptive switching: A method of automatically switching between the other three modes.

Cut-through switches have to fall back to store and forward if the outgoing port is busy at the time the packet arrives. While there are specialized applications, such as storage area networks, where the input and output interfaces are the same bandwidth, this is rarely the case in general LAN applications. In LANs, a switch used for end user access typically concentrates lower bandwidth (e.g., 10/100 Mbit/s) into a higher bandwidth (at least 1 Gbit/s). Alternatively, a switch that provides access to server ports usually connects to them at a much higher bandwidth than is used by end user devices.

Layer 3
Within the confines of the Ethernet physical layer, a layer 3 switch can perform some or all of the functions normally performed by a router. A true router is able to forward traffic from one type of network connection (e.g., T1, DSL) to another (e.g., Ethernet, WiFi).

The most common layer-3 capability is awareness of IP multi-cast. With this awareness, a layer-3 switch can increase efficiency by delivering the traffic of a multi-cast group only to ports where the attached device has signaled that it wants to listen to that group. If a switch is not aware of multi-casting and broadcasting, frames are also forwarded on all ports of each broadcast domain, but in the case of IP multi-cast this causes inefficient use of bandwidth. To work around this problem some switches implement IGMP snooping.

Layer 4
While the exact meaning of the term Layer-4 switch is vendor-dependent, it almost always starts with a capability for network address translation, but then adds some type of load distribution based on TCP sessions.

The device may include a statecraft firewall, a VPN concentrator, or be an IPSec security gateway.

Layer 7
Layer 7 switches may distribute loads based on URL or by some installation-specific technique to recognize application-level transactions. A Layer-7 switch may include a web cache and participate in a content delivery network.

Types of Switches :
Many types of switches exist, including ATM Switches, LAN Switches and WAN Switches.

ATM Switch ( Asynchronous Transfer Mode) :
ATM switch is a high performance, cell oriented technology that utilizes fixed-length information unit known as Cell for high speed transmission, and support voice, video and data applications. ATM allows customers to replace their separate voice and data networks with single network to handle, data and other multimedia contents such as video.

ATM Switch

LAN Switches :

A separate connections for each one in a company's internal network are known as LAN network. Essentially, a LAN creates a series of instant network that contains only two devices communicating with each other at that particular moment. Lan switches are designed to switch data frames at high speed.

LAN Switch

WAN Switches :

A WAN Switch is a multi-port inter-networking device used in carrier networks.Typically these devices switch traffic such as Frame Relay, X.25 and operate at the data link layer of the OSI Reference Model.

WAN Switch

Network Devices - Bridges

Network Bridges :

Bridges are used to interconnect LAN segment in cheap and easily configured manner. It can also device the networks to isolate traffic or problems. For example, if the amount of traffic between two computers are more, slowing down the entire operation, a bridge can isolate quality of those computers.

Bridging, is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the MAC address is stored so as to preclude the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area networks.

A network bridge connects multiple network segments at the (data link layer) Layer 2 of the OSI model. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D standard. A bridge and switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.

Bridges are similar to repeaters or network hubs, devices that connect network segments at the (physical layer) Layer 1 of the OSI model. However, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. Bridges are more complex than hubs or repeaters. Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network.

Bridges can be used to :

• Expand the distance of the segment
• Reduce network traffic resulting from an excessive number of attached computers. A bridge can handle overload by dividing it into two separate networks, reducing the amount of traffic on each segment and improve its performance.
• Connect unlike network segment such as Ethernet and token ring, bridge can be used to forward data between them.

A bridge which operates at the Media Access Control sub layer performs the following functions

• Listens to all traffic
• Checks the source and destination MAC address in the packet
• Builds up a MAC table when information is available
• Once it knows where the nodes are located, it forwards traffic; if the destination is not listed in the routing table, the bridge forwards to all segments and if the destination address is found the it forwards to that segment.

Bridges built routing table based on the address of the computer that had transmitted the data on the network. specifically, it uses source address of the device that initiates the transmission on create routing table.

Advantages of Network Bridges :

• Self-configuring
• Simple bridges are inexpensive
• Isolate collision domain
• Reduce the size of collision domain by micro-segmentation in non-switched networks
• Transparent to protocols above the MAC layer
• Allows the introduction of management/performance information and access control
• LANs interconnected are separate, and physical constraints such as number of stations, repeaters and segment length don't apply
• Helps minimize bandwidth usage

Disadvantages of Network Bridges :

• Does not limit the scope of broadcasts [broadcast domain cannot be controlled]
• Does not scale to extremely large networks
• Buffering and processing introduces delays
• Bridges are more expensive than repeaters or hubs
• A complex network topology can pose a problem for transparent bridges. For example, multiple paths between transparent bridges and LANs can result in bridge loops. The spanning tree protocol helps to reduce problems with complex topologies.

Types of Bridges :

• Local Bridge : are used where the network is being locally segmented. Ex., in same building or same floor.
• Remote Bridge : are used in pairs and where the network is remotely segmented. The segments are physically apart, or in different buildings. Remote bridges can have different communication media in between.

OSI Model - Physical Layer

Layer 1 - Physical Layer

Physical layer determines the physical and electrical characteristics of network like type of cables, length, and connectors. At the physical layer, data is transmitted onto the medium cables) in the form of bits (ones and zeros). There are number of types of cables that can used for data transmission but the most common are coaxial cable, fiber cable and twisted pair cables. Physical layer determines the voltage level, timings of voltage changes, physical data rates, maximum transmission distance and physical data rates.

Some of the functions of physical layer involve

• Transmits raw bit stream over physical cable
• Establishments and termination of connections to communication medium such as cables.
• Sharing of Communication resources among multiple users
• Conversion of digital data and signal to transmit over network.

Repeater and hubs operate at physical layer. Repeater connects two segments of network cable. It is a device, which receives the signal, regenerates them to proper signal and sends it to all ports. Repeaters require small amount of time to regenerate the signal. A hub is really a multiple port repeater. Like repeater, hubs does not check the traffic, instead it sends all the data to all the ports. Every device connected to hub or hubs has to listen if the device transmit. It receives the data from one device and sends it to another.

OSI Model - Data Link Layer

Layer 2 - Data Link Layer
Data Link layer (DLL) provides the physical transmission of data and handles error notification, network topology and flow control. Here, source and destination address is added to the data. This data frames are then checked for errors with the help of Frames Check Sequence ( FCS ), which are placed at the end of the frame. If errors are found, data link layer rejects the frame, if not the data will be passed to the Network layer.

Data Link Layer is also concerned with unique identification of each device and provides physical means of data transfer.

Some of the responsibility of data link layer includes

• Sharing the media among the devices.
• Data flow control between the receiver and the transmitter.
• Packaging the data into frames
• Detecting ans correcting errors from the physical layer on the receiving end.

The important functions of Data Link Layer is

• Data framing : It is concerned about encapsulation of higher level messages into frames which are sent across the network at the physical layer.
• Addressing : It is concerned with labeling the data with a particular destination locations. Every device on a network has unique hardware address or MAC address which is used by the data link protocol to ensure that data intended for a particular device gets it properly.

Error detection and recovery
Data Link Layer check the errors which may occur in a data. For example : cycle Redundancy Check (CRC) field is used to allow the receiving device to detect if data received properly.

The Institute of Electronics Engineers has divided data link layer into two sublayer

1. Media Access Layer
2. Logical Link Layer

Media Access Layer ( MAC) :
The lower sublayer of data link layer, Media Access Layer performs functions such as controlling access and encoding data into a valid format. MAC sublayer is concerned with the physical address of device which is usually given by the hardware manufacturer. Every device in a network should have unique MAC address which ensures proper transmission of data. MAC layer is concerned with adding source and destination address. Bridges and Switches which operate at the data link layer filters and forwards the data based on the MAC address.

The important functions of MAC are

• Physical addressing
• Error notification and ordered delivery of frames
• Examples : CSMA/CD (used in Ethernet and IEEE 802.3), Token bus (IEEE 802.4), Token Ring(802.5) and Token passing (used in FDDI)

MAC Sublayer Frames

Preamble	Destination Address	Source Address	Type	Data	Frame Check Sequence
8 Byte	6 Byte	6 Byte	2 Byte	Variable	4 Byte

Definitions of MAC sub-layer frames are

1. Preamble : A sequence of 8 bytes having alternating 1 and 0 values that are used for synchronized.
2. Destination Address and Source Address : These addresses are referred as MAC layer address and are unique to each device.MAC address is 6 byte (48 bit) address. The destination MAC address field identifies which station should receive the frame. A Destination MAC address may specify either an "individual address" or a "multi-cast address". The source MAC Address identifies the station that has intended the frame. It always specifies individual address.
3. Type/Length : The length field identifies the length of MAC frame in bytes.
4. Data : This field contains the data transferred from the source station to destination. The data field contains LLC information and information from the user.
5. Frame Check Sequence (FRC) : This field contains a 4 byte (32 bit) Cyclic Redundancy Check (CRC) value which is created by the sending device and is recalculated by the receiving device to check for damaged frames. ( CRC contains block of data called as frames which are used to detect errors.)

Logical Link Control (LLC) Layer:
LLC Layer, which is concerned with managing traffic over the physical medium. LLC protects the type of higher level layer from concern with the specific LAN implementation. It is concerned with managing traffic over physical medium. It establishes and controls a logical link between devices on a network. The LLC Layer controls the flow of date and checks errors.

LLC Sublayer frames :
LLC frame type is divided into SAP (Service Access Point ) and SNAP ( Subnetwork Access Protocol ). The frame type depends upon the type of application which runs on a computer. SAP is the logical point at which services are provided. Typically, the protocols in the network layer (like IP) bind at specific SAP in the LLC layer for accessing the services provided by it.

Devices used in DLL : Switches and Bridges operate at this layer

OSI Model - Network Layer

Layer 3 - Network Layer
Network layer, that defines how Internet works or interconnected network functions. It adds headers containing logical address of the device. Network layer is concerned with sending data from one computer to another computer even if it is on a remote network.

In WAN (Wide Are Network ), two end devices are separated by number of networks. Between the two end devices the data has to be passed through a series of intermediate nodes. These nodes are called as Routers.

Routers are special devices, which are capable of making complex routing decisions. Routers evaluate all available paths to a destination and determine which to use. After selecting the best path, router can continue with switching packets.

Network layer provides functional and procedural means of transferring data from source to destination over one or more networks. Routing and forwarding data are the important functions of network layer. Apart from this, it is concerned with physical and logical addressing of device, error detection, flow control, segmentation and sequencing of data. Internet Protocol ( IP ) handles physical and logical addressing of workstations.
Two important aspects required to send data from one device to another are

1. Logical address related to source and destination.
2. Path through which the data has to be sent to reach the destination.

Network Layer functions

• Logical Addressing (Host Addressing) : Every host in the network needs to have a unique address which determines where it is. This address will normally be assigned from a hierarchical system, so you can be "Fred Murphy" to people in your house, "Fred Murphy, Main Street 1" to Dubliners, or "Fred Murphy, Main Street 1, Dublin" to people in Ireland, or "Fred Murphy, Main Street 1, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addresses.
• Routing (Message Forwarding) : Moving the data packets through a series in a network is the main function of network layer. This work is done but the device and software which function at network layer like router. The function include handling incoming packets from several sources, and determining their final destination. Network layer encapsulates the message received from the higher layers by placing network layer headers into the datagram (Datagram Encapsulation). Since many networks are partitioned into subnetwork and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.
• Fragmentation : The network layer has to send message to the data link layer for transmission . But the data link layer has limits on the length that can be sent from the network layer. If the packets that has to be sent from the network layer is very long, the network layer has to break into pieces and send each piece to the data link layer.

The information added to the routing table includes

• Network Address : It is protocol specific . It maintains a unique table for each protocol.
• Interface : It is an interface used by router to send information to reach particular network.
• Metric : Metric indicates the cost of the route. And it is determined by the number of routers that must be crossed to reach the destination (hops). This metric is used to decide between multiple matching routers.

Protocols used in this layers

• IPv4/IPv6, Internet Protocol
• DVMRP, Distance Vector Multi-cast Routing Protocol
• ICMP, Internet Control Message Protocol
• IGMP, Internet Group Multi-cast Protocol
• PIM-SM, Protocol Independent Multi-cast Sparse Mode
• PIM-DM, Protocol Independent Multi-cast Dense Mode
• IPsec, Internet Protocol Security
• IPX, Internetwork Packet Exchange
• RIP, Routing Information Protocol
• DDP, Datagram Delivery Protocol

OSI Model - Transport Layer

Layer 4 - Transport Layer :

The Transport Layer provides transparent transfer of data between end stations on the network. This layer is responsible for ordering and reassembling of packets. The transport connection provided by transport layer is an error free virtual circuit channel that delivers data packets in the same order in which it is sent. This layer checks the maximum size of the data packets and breaks up the request into smaller packets as required. Transport layer also checks the validity of data packets, sequencing and handling duplicate packets.

It is providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/de-segmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also provides the acknowledgment of the successful data transmission and sends the next data if no errors occurred.

Transport layer protocol is typically categorized as either connection-less or connection oriented protocols. The two types of protocols used in this layers are Transmission Control Protocol (TCP) and User Data-gram Protocol (UDP). TCP is connection oriented protocol and UDP is connection-less protocol.

There are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes are shown in the following table:

Connection-mode transport protocols

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.

Connection Oriented Communication versus Connection-less Communication

Connection describes communication between two end points in which a message can be sent from one end point to another without any prior arrangement. The device at one end point can transmit data to other, without first ensuring that the recipient is ready to receive the data. Internet Protocol and User Data-gram Protocol are connectionless protocol.

Connection-oriented communication describes the means of transmitting data in which devices at the end Points use protocols to establish connection before sending any data. Transmission control protocol is connection-oriented protocol. The four steps in connection-oriented communication are as follows

1. It establishes connection between the sending and receiving station
2. The two station notify their operating system that the connection is about to start (Synchronization).
3. When connection between the two devices are complete, the transfer of information begins.
4. Two devices communicate with each other and checks that the data is delivered correctly.  

Functions of Transport Layer Protocol 1.) Flow Control : Flow control is a process of controlling the rate at which a device sends data. Depending on the particular protocol, the sending and receiving device participate in the process of flow control. The three methods of implementing flow control are

• Buffering
• Congestion avoidance
• Windowing

Buffering : Under normal circumstance the input buffer of the device can hold enough data packets. But sometimes data comes in at faster rate than the receiving device can process it. Under such condition, the buffer overflow and data will be lost.Flow control helps to prevent buffer overflow by signaling the sending device to stop the transmission allowing the receiving device to empty buffer before continuing again.

Congestion avoidance : Congestion can occur during data transfer for two reasons. First, a high speed computer might be capable of generating traffic faster than a network can transfer it. Second, if many computers simultaneously need to send datagrams to a single destination, that destination can experience congestion, although no single source caused the problem.

When datagrams arrive too quickly or gateway to process, they are temporarily stored in memory. It the traffic continues, the host or gateway eventually exhausts its memory and must discard additional datagrams that arrive.

Instead of allowing data to be lost, the transport function can issue a "not ready" indicator to the sender. Acting like a sop sign, this indicator signals the sender to sender to stop data. When the receiver can handle additional data, the receiver sends a "ready" transport indicator. When this indicator i received, the sender can resume the segment transmission.

In the figure below, the computer receiving the data notices that its buffers are filling.This causes separate PDU or field in the header to be send towards the sender signaling the sender to pause the transmission of data.

Congestion avoidance

Windowing : A Window size is the maximum data the sender can transmit without getting any acknowledgment. If the no acknowledgment is received and if the window is full, then the sender has to wait for acknowledgment. In the figure the line indicate the time difference the sending the PDU and its receipt.

Windowing

2.) Acknowledgment : Reliable data transfer assures that the data won't be duplicated or lost. This is achieved by a process known as positive acknowledgment with retransmission. In this process the receive machine has to communicate with the sender by sending an acknowledgment, The sending device waits for the acknowledgment before sending the next segment. When it sends a segment, the sending machine starts a timer ad retransmit the segment if the timer expires before acknowledgment is received from the receiving machine.

In the above figure the sending machine sends segment 1,2 and 3. The receiving device sends acknowledgment and request for the next segment (segment 4). When it receives the acknowledgment, the sending device sends segment 4,5 and 6. If the segment 5 doesn't reach the destination the receiving device acknowledges that event with a request for the segment to be resent. The sending device then resend the lost segment and waits for the acknowledgment which is required to send the next segment.

OSI Model - Session Layer

Layer 3 - Session Layer

The session layer establishes a one-to-one communication session between sending and receiving computers. Data conversion can be established, controlled and terminated in this layer. It enables application running at two workstations to coordinates their communication into a single session. The request and responses between the application is synchronized with the help for protocol that are added in this layer. Only after a completion of conversation, the data or request will be sent to the presentation layer.

The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check pointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

Some of the protocols that are implemented in the session layer are

• Network File Transfer (NFS) : It is enveloped by Sun Microsystems and used with TCP/IP and unix workstations to allow transport access to remote resources.
• Structured Query Language (SQL) : It is developed by IBM to provide users with a simpler way to define their information requirement on the both local and remote system.
• Remote Procedure Call (RPC) : Its procedures are created on client and performed on servers
• X Window : It is used by intelligent terminals for communicating with remote Unix computer allowing them to operate even they have locally attached monitor.
• AppleTalk Session Protocol (ASP) : Another client/server mechanism which establishes and maintains session between AppleTalk client and server machine.
• Digital Network Architecture Session Control Protocol (DNA SCP) : A DECnet session layer protocol.

OSI Model - Presentation Layer

Layer 2 - Presentation Layer

Presentation layer ensures that the data transmitted by the application layer of one system will be readable by the application layer on the another system. It gives the standard format for transmitting information across various systems, so that the data can be understood regardless of the type the system involved. The presentation layer is concerned with only the format and representation of the data but also with the data structure used by the programs.

The Presentation layer is concerned with the conversion of protocol such as encryption, decryption and graphic expansion of the application. It is also concerned with data compression, which can be used to reduce the size of the data packets. And cryptography, which is essential for privacy and authentication.

The Presentation Layer establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.

This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.

Presentation layer provides

• Data Conversion : The sending device can converts text or data into general format (ASCII) for transmission over a network. The receiving computer converts this format (ASCII) to a format required by the destination applications.
• Data Encryption : Data is converted into encoded form, which can't be read by unauthorized persons
• Protocol Translation : Data is converted from one form to another and it is transmitted between dissimilar operating systems.

Some of the standard Graphical image formats are :- TIFF (Tagged Image File Format ) & JPEG ( Joint Photographic Expert Group )

Some of standard visual image representations are :- MIDI ( Musical Instrument Digital Interface ) used for digitized music. MPEG ( Moving Picture Experts Group )

OSI Model - Application Layer

OSI model consists of two layers : upper layer and lower layer. The upper layer are oriented more towards services to the application and lower layer are more oriented towards the data flow from end to end through the application. The upper layer describes the network function with respect to the application and operating system. The four lower layers of OSI reference model are Physical, Data Link, Network and Transport Layer. It describes how the data is transferred across physical devices. The lower layer is implemented with hardware and software.

OSI Model

Layer 1 - Application Layer

The Application Layer supports user and application tasks and overall system management, including resource sharing, file sharing, remote file servers, database and network management.


The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include:

• On OSI Model:
  • FTAM File Transfer and Access Management Protocol
  • X.400 Mail
  • Common management information protocol (CMIP)
• On TCP/IP Model:
  • Hypertext Transfer Protocol (HTTP),
  • File Transfer Protocol (FTP),
  • Simple Mail Transfer Protocol (SMTP)
  • Simple Network Management Protocol (SNMP) 

Important functions of Application Layer :

1. Identifying intended communication partners
2. Determining the resource availability of intended communication
3. Synchronizing communication
4. Authentication of intended communication partners

ISO-OSI Model Introduction

Introduction :

One of the best ways to understand networking is to study by which computers and other networking device transfer data. OSI model describes how data is transferred from an application on one computer to another. OSI model comprises seven different layers. Each layer handles specific function in transferring data over a network.


OSI Origin and Evolution

When network was first developed, computers could typically communicate with other computers from the same manufactures. For example: companies run either complete DECnet solution or an IBM solution but they couldn't use both. In order to overcome this problem, Open Systems Interconnection (OSI) reference model was developed by International Organization for Standardization (ISO).

OSI Model or OSI Reference model describes  the flow of data on a computer network layer has a composed of seven layers that describes the functions of data communication protocol. Easy layer of OSI model describes a  particular network function.

One layer in OSI model uses the services provided by the layer immediately below it. For example : the network layer has to deliver data from end to end. To do this, it uses the services provides by the data link layer to forward data to the next successive device along the end-to-end path.

The protocols in OSI model look like blocks stacked on upon another. Because of appearance, OSI model is also called as stack or protocol stack. In this, data is transmitted from one layer to the next, down the stack.
By dividing network function into logical smaller pieces, network related problems could be easily solved. OSI model reduces the impact of technological change on the entire network and allows extensibility. New application and network hardware can be easily added or replaced without requiring redesigning.

Seven layers in OSI Model

7.Application

6.Presentation

5.Session

4.Transport

3.Network

2.Data Link

1.Physical

Note : The easiest way to remember the seven layers is sentence: Please Do Not Throw Sausage Pizzas Away.


Advantages of OSI Model

• It divides network communication into smaller pieces, making it easier for people to understand and learn. People can easily understand protocol specification.
• It standardizes network components to allow multiple vendor development and support.
• Different types of network hardware and software can communicate with each other.
• Changes in one layer will not affect other layer.

Interaction between same layers on different computers

Each successive layer of the OSI model interacts with the layer one above it, or below it and its peer layer in order networked computer system. For example : Data Link Layer of Host A communicates with Network Layer, Physical Layer of the same systems and Data Link layer of Host B.

Layer D ( or Layer 4) has to interact with Layer D on another computer to successfully implement its function. For example: the transport layer can send data only when it receives the acknowledgment that the data is received, otherwise the sender will not know when it has to perform error recovery. Same way, the sending device encodes the distinction network layer address. It the router does not help by performing their network layer tasks, the data packets will not be delivered the current location.

In this figure, the application layer on HOST A communicates with application layer on HOST B. Same way, the transport, Session and Presentation Layer on HOST A communicate with respective layers in Host B. The lower three layers of the OSI model is concerned with the delivery of data and Router 1 is used for the purpose. Router 1 also communicates with Host B's Physical, Data Link and Network Layer.

Data Encapsulation

Each layer in OSI model is responsible for interacting with the peer layer in other networked computer systems. For example : when two computer communicate, one layer such as Data Link layer has to communicate with exact same layer ( in this case it is Data Link layer ) on the other computer. This is possible with the help of the own protocol. Each layer's protocol exchange information in the form of packets known as Protocol Data Unit ( PDU ).

Even though this type of communication takes place logically between peers, each layer depends on the layer below it for the actual delivery of data. Each layer transmits its PDU to the layer below. In this manner, the data for the lower layer is the PDU of the layer immediately above it.

This process of adding one PDU and encoding it with another is called as data encapsulation. This process is similar to the mail delivery system. Once the composition of the layer is ready, it is then placed in an envelope that gets the address to the destination. Then the mail is placed into the mail carrier's bag. The bag is then placed into the delivery truck. At the destination the bag is opened and the letter will be given to the appropriate mail carrier. The letter reaches the recipient who open it and reads the content. The layers is the OSI Model function in the same way.

Step 1 : The application message is converted into data for communication on the network.
Step 2 : The data obtained from the above step is added with a segment header by the transport system.
Step 3 : The segment is given the network header that contains source and destination logical addresses.
Step 4 : The packet or data-gram that is obtained from the above step is given frame header, specifies to the physical medium used.
Step 5 : The frame is then converted into series of bits and impressed as  electrical signal generated on the physical medium

The above five steps can be summarized as follows.

OSI Origin and Evolution

Geo IP Locator

Geo IP Locator means, giving Geographical location to the IP/domain where it is existed.

Powered by GeoIPView

International Organisation for Networking

Here are some of the standards organizations that you are likely to encounter when reading about networking and the Internet:

• International Organization for Standardization (ISO) : Probably the biggest standards organization in the world, the ISO is really a federation of standards organizations from dozens of nations. In the networking world, the ISO is best known for its OSI Reference Model.
• American National Standards Institute (ANSI) : ANSI is the main organization responsible for coordinating and publishing computer and information technology standards in the United States. While they are commonly thought of as developing and maintaining standards, they do neither. Instead, they oversee and accredit the organizations that actually create the standards, qualifying them as Standards Developing Organizations or SDOs. ANSI also publishes the standards documents created by the SDOs, and serves as the United States' representative to the ISO.
• Information Technology Industry Council (ITIC) : ITIC is a group of several dozen companies in the information technology (computer) industry. ITIC is the SDO approved by ANSI to develop and process standards related to many computer-related topics. It was formerly known as the Computer and Business Equipment Manufacturers Association (CBEMA).
• National Committee for Information Technology (NCITS) : A committee established by the ITIC to develop and maintain standards related to the information technology world. NCITS was formerly known by the name Accredited Standards Committee X3, Information Technology, or more commonly, just X3. It maintains several sub-committees that develop and maintain standards for various technical subjects.
• Institute of Electrical and Electronics Engineers (IEEE) : The IEEE (pronounced “eye-triple-ee”) is a well-known professional organization for those in the electrical or electronics fields, including computers and networking. IEEE's main claim to fame in the networking industry is the IEEE 802 Project, which encompasses many popular networking technologies including Ethernet.
• Electronic Industries Alliance (EIA): The EIA is an international industry association that is best known for publishing electrical wiring and transmission standards.
• Telecommunications Industry Association (TIA) : The TIA is the communications sector of the EIA, and is responsible for developing communications standards. Since communications, wiring and transmission are all related, and since the TIA and EIA organizations are also related, standards produced by the EIA or TIA are often labeled with the combined prefixes “EIA/TIA” or “TIA/EIA”.
• International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) : ITU-T is another large international body that develops standards for the telecommunications industry. The ITU-T was formerly named the International Telephone and Telegraph Consultative Committee or CCITT (the abbreviation was of the French version of the organization's name, Comité consultatif international téléphonique et télégraphique.)
• European Telecommunications Standards Institute (ETSI) : An organization with members from dozens of countries both within and outside Europe that is dedicated to developing telecommunications standards for the European market (and elsewhere). ETSI is known for, among other things, regulating the use of radio bandwidth in Europe and developing standards such as HiperLAN.
• Electronics Industry Association (EIA) : A  Group that specifies electrical transmission standard. EIA's well known standard is EIA/TIA-232.
• Internet Activities Board (IAB) : Soen of request for comments documents by the IAB as Internet standards including Transmission Control Protocol (TCP) and Simple Network Management Protocol (SNMP).

Internetworking Concept

Network Components :

The main idea of network is to help all organization increase its productivity by connecting all the computer or networks. Connection between networks has helped organization in many ways in spite of variation in time, location and type of computer or devices.

By being a part of network, a company can access information, processes and ways of doing business changes. It is very important for every organization to manage a corporate inter-network that helps to optimize its resources.

Network is characterized based on group employees in the following ways

• Main Office : It is the location where all computers are connected to a LAN. It is a data center where all information related to company is centralized. Main office might contain thousands of employees who depend on networks to do their work.
•  The other areas of office where require access to the resources at main office and each other are

• Branch Office : These are smaller groups of employees and they are connected through LAN. Even through some of the data's are stored in the branch office, most of the time they access the data from the main office. How frequently they access the data from the main office determines the type of WAN connection. It can be permanent of dial up connection.
• Telecommuters : Employees who access the resources from the outside are telecommuters.
• Mobile users : Some of the employees work from different locations and they require connection to the network. When they work at main or branch office, they connect to LAN. When they work outside the office, they use dial-up service to connect to the network.

In order to understand what types of equipment and services to install in your network, it is very important to understand the user and the business needs, then you can subdevide the network into smaller components,

Corporate Networking Strategy :

Three Layer Hierarchial Network Model :

To properly build an inter-network, a three layered hierarchical model is used to organize traffic flow. Hierarchical model also simplifies the task required for two computers to communicate and divides it into three layers. Each layer is focused on a specific function, as mentioned below:

• Access layer provides work-group/user access to the network
• Distribution provides policy based connectivity
• Core or backbone layer provides optimal transport between sites.

Figure show various aspects of hierarchical model.

Access Layer :

This layer includes hubs and switches. Access layers is also called as Desktop Layer because it focus on connecting client nodes, such as workstations on the network. This layer ensures that packets are delivered to end user computers. Access Layers has ability to expand or contract collision domains using repeater, hub or switch. Switches in access layer are not high-powered device such as those found at the core layer. Rather it is advanced version of a hub.
Access layer controls work-group access to the locally available resources. Traffic that flows to and from the local resources is restricted between resources, switches and users. In most of the network, access to the services such as database, centralized storage is denied. In this case the request  or traffic will be sent to next layer distribution layer.
At the access layer, user can

• Enable MAC address filtering : it is possible to program a switch to allow only certain systems to access the connected LANs.
• Create separate collision domain: A switch can create separate collision domain for each connected node to improve performance.
• Share bandwidth : Allows the same network connection to handle all data. Allows moving data from one network to another to perform load balancing.

Note : A collision domain describes a portion of a network where any communication sent by a node is received by any other node on the network. In broadcast domain any part or any node of a network can broadcast to any node of the network

Distribution Layer:
Distribution layer is also referred as Work-group layer. Usually routing between access layer and core layer is handled by distribution layer. It also provides policy based connectivity; it can read the packet and prioritize the delivery based on the set by user.
Functions of distribution layer include

• Packet filtering : Distribution layer processes the packets and transmit data  packets based on the source and destination information to create network borders.
• Access layer aggregation point : distribution layer  serves the aggregation point for access layer switches.
• Distribution layer serves as the boundary for broadcast and multi-cast domains.
• performs queuing of packet and provides security service.

It determines the fastest way in which the request can be forwarded to the server. When the path is selected the request will be sent to core layer.

Core Layer :
Core layer is considered as backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by the devices in the layer. Rather, this layer is concerned with speed and ensures reliable delivery of packet.
Usually, the traffic is being transmitted to and from services common to all users. These services are known as global or enterprise services. Example of these services includes email, Internet access and video conferencing. When the user access to enterprise service, then distribution chooses the best path and forwards the request to core layer. Core layer provides the rapid transmit of request to the enterprise service.

Network Symbols

Symbols used to denote Network Diagrms.

Cisco Products:

Cisco Products ( Optical )

Cisco : TelePresence

Cisco : Security

Cisco : IBM

Cisco : WAN

Cisco : LAN

Cisco : Media 

Cisco : Buildings

Cisco : People

Cisco : Multimedia / Voice / Phone 

 Cisco : Video Surveillance

Cisco : Miscellaneous

Network Devices - Ethernet hub

A LAN (Local Area Network) comprises of different communication devices such as

• Hubs
• Bridges
• Switches
• Routers

Ethernet Hubs:
Ethernet Hubs are centralized device in a star topology. A hub takes incoming signal and repeats it out to all ports on a network. A Multi-port twisted pair hub allows several point-to-point segments to be joined in to one network. One end point of the point-to-point link is attached to the hub and other is attached to the computer.

An Ethernet hub (or) active hub (or) network hub (or) repeater hub or hub is a device for connecting multiple twisted pair or fiber optic Ethernet devices together and making them act as a single network segment. Hubs work at the layer 1 ( physical layer ) of the OSI model. The device is a form of multi-port repeater. The Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or Attachment Unit Interface (AUI) connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications.

Hubs don't manage any of the traffic that comes through them, and any packet entering any port is regenerated and broadcast out on all other ports. Since every packet is being sent out through all other ports, packet collisions result — which greatly impedes the smooth flow of traffic.

For 10 Mbit/s networks built using repeater hubs, the 5-4-3 rule must be followed: up to 5 segments (4 hubs) are allowed between any two end stations. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even that is only allowed if the hubs are of the a delay variety

Hubs are classified as ( Layer 1) physical layer devices in the OSI model. At the physical layer, hubs support little in the way of sophisticated networking. Hubs don't read any of the data passing through them and are not aware of their source or destination. A hub, simply receives incoming Ethernet frames, regenerates the electrical signal, and broadcasts these packets out to all other devices on the network.

Types of Hubs :
There are many types of hubs with different features / specification, which provide the type of functionality required to build a network. There are three main types of hubs : Active Hub, Passive Hub and Intelligent Hub.

• Active Hubs : It can regenerate and retransmit the signal the same way as repeater does. In fact, active hubs usually have twelve ports for network computers to connect; they are sometime called as multi-port hubs. Active hubs require electrical power to run. Usually active hubs are more expensive than passive hubs as they provide additional feature.
• Passive Hubs : It acts as a connection point and do not amplify or regenerate the signal; the signal passes through the hub. a Passive hub simply receives on the input port and broadcasts its on the output port without even repairing it. Passive hubs do not require electrical power to run.
• Intelligent Hubs : An intelligent hub has all features of a passive and active hub, also help to improving the performance of network. As active hub helps in finding out where the problem exits., an intelligent hub diagnoses it and tries to correct it without affecting the performance of the network. Intelligent hubs provide features which helps to determine the exact cause and location of the problem and helps in identifying the solution for the problem. An intelligent hub helps in controlling and minimizing data traffic in the network, which results in improved performance of the network.

LAN Hardwares and Softwares

To realize a LAN process, several functions are to be performed. They require hardware and software specially built for such purpose. Some of the basic hardware and software components of LAN include

• Network Interface Controller ( NIC )
• Servers
• LAN Software / Operating System ( NOS )

Network Interface Controller ( NIC ):

 

A network interface card (or) network adapter (or) network interface controller (or) LAN adapter is a computer hardware component that interfaces to a computer network. 

Purpose :

The NIC Card allows computers to communicate over a computer network. It is both an OSI layer 1 (physical layer) and layer 2 (data link layer) device, as it provides physical access to a networking medium and provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wireless.

Every Ethernet network card has a unique 48-bit serial number called a MAC address, which is stored in ROM carried on the card. Every computer on an Ethernet network must have a card with a unique MAC address. Normally, it is safe to assume that no two network cards will share the same address, because card vendors purchase blocks of addresses from the Institute of Electrical and Electronics Engineers (IEEE) and assign a unique address to each card at the time of manufacture.

Whereas network cards used to be expansion cards that plug into a computer bus, the low cost and ubiquity of the Ethernet standard means that most newer computers have a network interface built into the motherboard. These either have Ethernet capabilities integrated into the motherboard chip-set or implemented via a low cost dedicated Ethernet chip, connected through the PCI (or the newer PCI express) bus. A separate network card is not required unless multiple interfaces are needed or some other type of network is used. Newer motherboards may even have dual network interfaces built-in

Implementation :

The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as Ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.
There are four techniques used to transfer data, the NIC may use one or more of these techniques.

• Polling is where the microprocessor examines the status of the peripheral under program control.
• Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system's address bus.
• Interrupt-driven I/O is where the peripheral alerts the microprocessor that it's ready to transfer data.
• DMA is where an intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.

A network card typically has a RJ45, BNC, or AUI socket where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. Network cards are typically available in 10/100/1000 Mbit/s varieties. This means they can support a notional maximum transfer rate of 10, 100 or 1000 Megabits per second.
Sometimes the words 'controller' and 'card' are used interchangeably when talking about networking because the most common NIC is the network interface card. Although 'card' is more commonly used, it is less encompassing. The 'controller' may take the form of a network card that is installed inside a computer, or it may refer to an embedded component as part of a computer motherboard, a router, expansion card, printer interface or a USB device.

Servers :

One of the major advantage of LAN is sharing resources such as storage device, printer etc. This is enabled by providing servers on the LAN. Server is a dedicated computer which controls one or more resources. Two major categories of servers used in LAN are :

• File Server
• Print Server

Networking file server is used to share storage spaces for files. In addition to providing storage apace for files, file server is used to take periodic backup and also provide access to another server within and between LANs.
Same way print server is used to handle printing operations of all workstations connected in the network.

LAN Software / Operating System :

Network Operating System (NOS) is a software application required to control the use of LAN standards. Every NOS provides two sets of software, one is server and client software. Server software runs the network server and client server runs network client. 

Server software enables file server, print server, database server to operate.
Ex.: Windows NT Server, Novell Net-ware

Client software must interact with application layers and computer's own operating system. Some application allow users to share data files and access shared data.
Ex.: Windows NT Workstation, Windows 2000 Professional, Windows XP etc.

LAN Access Method

If two computers put the data onto the cable at the time, the data packets from one computer may collide with another computer. If data is sent over a network from one user to another or accesses from server, then there should be some way for the data to access the cable without disturbing another.

Access methods define a set of rules such as how a computer puts data onto the network cable and takes data from the cable. Access methods should be consistent in the way they handle the data. If different computers use different methods, the network may fail because some methods dominate the cable. Access methods of the cable, but two of these are commonly used, they are

• Carrier-Sense Multiple Access with collision Detection (CSMA/CD)
• Token Passing

Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)
In this access method, each computer on the network checks the cable for network traffic. A computer senses that the cable (carrier-sense) is free. There are usually many computer or nodes in a network which attempts to transmit data (multiple access). This creates a collision, as contents of the data frame will collides and results in corruption of the frame. This collision will be detected by the transmitting nodes. The two nodes involved in the transmission then wait for short random time interval before attempting to retransmit a frame once again.

CSMA/CD is a modification of pure carrier sense multiple access (CSMA) which is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus reducing the probability of a second collision on retry.

CSMA/CD is a layer 2 ( Data Link ) access method, not a protocol of the OSI model. If a station wants to send some information, it uses the following algorithm:

Main procedure

1. Frame ready for transmission.
2. Is medium idle? If not, wait until it becomes ready
3. Start transmitting.
4. Did a collision occur? If so, go to collision detected procedure.
5. Reset retransmission counters and end frame transmission.

Collision detected procedure

1. Continue transmission until minimum packet time is reached to ensure that all receivers detect the collision.
2. Increment retransmission counter.
3. Was the maximum number of transmission attempts reached? If so, abort transmission.
4. Calculate and wait random back off period based on number of collisions.
5. Re-enter main procedure at stage 1.

Methods for collision detection are media dependent. But on an electrical bus such as 10BASE-5 or 10BASE-2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.

Applications 

• CSMA/CD was used in bus topology Ethernet variants ( is early a versions of twisted-pair Ethernet). In Modern Ethernet networks built with switches and/or full-duplex connections no longer utilise CSMA/CD. The IEEE Std 802.3, which defines all Ethernet variants, for historical reasons still bares the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications".
• Variations of the concept are used in radio frequency systems that rely on frequency sharing, including Automatic Packet Reporting System.

Token Passing

In this method, collision are eliminated by passing a special network frame called Token. When a computer wants to send data, it has to wait for a free token. When it gets the token, it can transmit the data along with the token. The receiving device gets data with token, it sends the token to the sender as an acknowledgment . When sender receives the acknowledgment, the token will be released. Remember when token is in use by one computer the other computer cannot send data, because only one computer at a time can use the token. There is no collision. Token passing is used by Token Ring Topology and it is more expensive than CSMA/CD.

Token ring technology is a local area network protocol which resides at the data link layer of the OSI model. It is using a special three-byte frame called a Token that travels around the ring. Token-possession grants the possessor permission to transmit on the medium. Token ring frames travels completely around the loop.

Token Frame
When no station is transmitting a data frame, a special token frame circles the loop which is repeated from station to station until arriving at a station that needs to transmit data. If a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token. When a transmission error occurs and no token frame, or more than one, is present, a special station referred to as the Active Monitor detects the problem and removes and/or reinserts tokens as necessary. (In 4 Mbit/s Token Ring, only one token may circulate; on 16 Mbit/s Token Ring, there may be multiple tokens).The special token frame consists of three bytes as described below.

Token Ring Frame Format

A data token ring frame is an expanded version of the token frame. It is used by stations to transmit media access control (MAC) management frames or data frames from upper layer protocols and application

Data/Command Frame

SD	AC	FC	DA	SA	PDU from LLC (IEEE 802.2)	CRC	ED	FS
8 bits	8 bits	8 bits	48 bits	48 bits	up to 18200x8 bits	32 bits	8 bits	8 bits

Token Frame

Start Delimiter	Access Control	End Delimiter
8 bits	8 bits	8 bits

Abort Frame

SD	ED
8 bits	8 bits

SD ( Starting Delimiter) : it consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. 

J	K	0	J	K	0	0	0
1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bit

AD ( Access Control ) : this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.

+	Bits 0–2				3				4								5-7
0	Priority				Token				Monitor								Reservation

FC ( Frame Control ) : a one byte field that contains bits describing the data portion of the frame contents.Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.

+	Bits 0–2				Bits 3-7
0	Frame type				Control Bits

Frame type - 01 indicates LLC frame IEEE 802.2 (data) and ignore control bits 00 indicates MAC frame and control bits indicate the type of MAC control frame

DA ( Destination Address ) : a six byte field used to specify the destination(s) physical address.
SA ( Source Address ) : It contains physical address of sending station . SA is six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) of the sending station adapter.
Data : A variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information.Maximum length of 4500 bytes
CRC : (Frame Check Sequence) a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.
ED ( Ending Delimiter ) : The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit.

J	K	1	J	K	1	I	E
1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bit

FA ( Frame Status ) : a one byte field used as a primitive acknowledgment scheme on whether the frame was recognized and copied by its intended receiver. 

A	C	0	0	A	C	0	0
1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bit	1 bi

A = 1 , Address recognized C = 1 , Frame copied

Abort Frame : Used to abort transmission by the sending station.

Token Ring Insertion Process :

Token ring stations must go through a 5-phase ring insertion process before being allowed to participate in the ring network. If any of these phases fail, the token ring station will not insert into the ring and the token ring driver may report an error.

• Phase 0 (Lobe Check) — A station first performs a lobe media check which is wrapped at the MSAU( Multi Station Access Unit ) and is able to send 2000 test frames down its transmit pair which will loop back to its receive pair. The station checks to ensure it can receive these frames without error.
• Phase 1 (Physical Insertion) — A station then sends a 5 volt signal to the MSAU( Multi Station Access Unit ) to open the relay.
• Phase 2 (Address Verification) — A station then transmits MAC frames with its own MAC ( Media Access Unit ) address in the destination address field of a token ring frame. When the frame returns and if the address copied , the station must participate in the periodic (every 7 seconds) ring poll process. This is where stations identify themselves on the network as part of the MAC management functions.
• Phase 3 (Participation in ring poll) — A station learns the address of its Nearest Active Upstream Neighbor (NAUN) and makes its address known to its nearest downstream neighbor, leading to the creation of the ring map. Station waits until it receives an AMP or SMP frame with the ARI and FCI bits set to 0. When it does, the station flips both bits (ARI and FCI) to 1, if enough resources are available, and queues an SMP frame for transmission. If no such frames are received within 18 seconds, then the station reports a failure to open and re-inserts from the ring. If the station successfully participates in a ring poll, it proceeds into the final phase of insertion, request initialization.
• Phase 4 (Request Initialization) — Finally a station sends out a special request to a parameter server to obtain configuration information. This frame is sent to a special functional address, typically a token ring bridge, which may hold timer and ring number information with which to tell the new station about.

LAN Transmission Methods :
LAN transmission method is classified into uni-cast, multi-cast and broadcast. In each type of transmission, a single data packet is transmitted to one or more nodes.

• Uni-cast : a single data packet is sent from one node to another specific node on a network. First, the source node assigns destination node address for the packet. This packet is then sent to the network and finally the packet is sent to the destination.
• Multicast : a single data is copied and sent to specific subset of the nodes on the network but not all the networks. First, a source node addresses the each packet using a multicast address. This packet is then sent to the network, which makes a copy of the packet and sends the copy to the network to the every node which is part of multicast address.
• Broadcast : a single data packet is send to all nodes on the network. In this type, the source node addresses the packet by using the broadcast address. The packet is sent into the network which makes copies and sends the copy to all nodes on the network.

Anycast	Broadcast		Multicast
Unicast		Geocast