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