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