Link Layer Protocol Services to the Network Layer - Assignment Example

(Keith 2008) The services offered by the link layer to the network layer are usually hidden from the network layer which only sees the services as a reliable communication channel which can send and receive data packets as frames. Framing, addressing, error detection, error correction, flow control, link management and acknowledgement of frames are some of the services provided by the link protocols. The link layer groups bits of the physical layer into frames enabling transmission of data in a form that can be understood by the network layer.

Since various network stations have different speed of operation, the link layer protocol provides flow control service that ensures no station in the network is swamped with data from fast devices within the network. In addition the link layer provides a link management services through collision handling and avoidance. (Nancy 1988) One major services offered by the link layer protocol is error detection and correction. It has error check included in the frame header which provides a more sophisticated error detection and correction since it can detect single bit and a wide range of common multiple -bit errors.

Error detection in link layer uses checksum which is the same as those used in IP in the network layer as well as TCP in the transport layer. . The frames are organized in seven fields: PREAMBLE which is 7 bits long used to warm the receiver that data is coming, SOF indicates that the next bit of data will be the destination address, DA the destination address which identifies the receiver of the data, SA identifies the source address, Length which indicate the length of the payload data, DATA contains the data transmitted its length varies with the size of data and FCS which is used for error checking.

In 10BASE-T the PRE field is used for receiver synchronization while 100BASE-T does not require the PRE field for transmission since it has an electrical encoding that is different from 10BASE-T. The signal of 10BASE-T frame is zero when it is not transmitting while 100BASE-T transmits and idle signal between frames. Collision detection Figure 5.14 According to the figure node D detects collision first before the other node B detects since its transmission start time is greater than the transmission start time of node D.

Node D detects the collision before node B since it started transmission later after D had already started. Furthermore node D has a shorter round trip time as compared to node B. Node A does not realize that detection has occurred between node B and D since it is not involved in transmission. Node A only waits for the signal from B in case of a collision since after the collision Node B will try to retransmit the signal after random times until it reaches its destination. Jamming signal is a signal sent by the data station, informing the other stations not to transmit.

In this case the jamming signal is sent by node D since it is the node that detects a collision