Ssap

Control

LLC data field

1 byte 1 byte 1-2 bytes 42-1497 bytes [4-LLC header-»[4-Payload-

1 byte 1 byte 1-2 bytes 42-1497 bytes [4-LLC header-»[4-Payload-

Fig. 11.21. LLC, IEEE 802.2 frame format.

Destination Service Access Point (DSAP) and Source Service Access Point (SSAP) defines the location of the end points in communication link for either connection oriented or connectionless service. The MAC information passes information to DSAP. That determines which protocol the incoming information belongs to such as IP, Network or DeCNet. The SSAP determines which protocol is sent to the destination protocol such as IP or DeCNet.

The control field identifies the frame type. The control field determines the type of information in the information field, such as supervisory frame, the unnumbered frame and the information frame. If the LLC frame is used to encapsulate, a higher level protocol within the pay load field, 802.2 provides for a means, known as the sub network assess protocol (SNAP), to identify the protocol. SNAP formats are recognized by DSAP and SSAP. In response to the address, LLC protocol will interpret the payload as a SNAP format. Actual source and destination address are included in the protocol format of the data area in the payload.

MAC layer. The function of the MAC is to access the network. It defines, how different stations can access the transmission medium. The MAC uses CSMA/CD protocol. In an Ethernet network, each station uses CSMA/CD protocol to access the network in order to transmit information. The MAC and CSMA/CD are explained under the CSMA/CD protocol heading.

CSMA/CD Protocol. The MAC mechanism is based on the Carrier Sense Multiple Access with Collision Detection (CSMA/CD). The CSMA/CD works as follows:

1. If a station wants to transmit, the station senses the channel.

2. If the channel is busy, it continues the sensing of the channel. When the channel becomes idle, the station starts transmit data.

3. After sending data, the station senses for the collision, as there is a possibility that two station may send data at the same time.

4. If the collision detected, the station which detected first sends a jamming code (32 bits) on the bus, in order to indicate the other station that there is a collision on the bus.

5. If no collision, transmission completed.

6. The two stations, which enveloped in collision, wait according to back-off algorithm. Back off algorithm is a method used to generate waiting time for stations that were involved in collision.

When a collision occurs, the stations involved senses it and stops the transmission and resend the frames. This is called collision detection.

Fig. 11.22 shows the IEEE 802.3 MAC format. Each fields of the frame are explained below :

LLC frame format

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