THE DATA LINK LAYER

Out of order with the book

Dr. John P. Abraham

University of Texas, Panam

The data link layer

Provides data transport across a physical link

Handles physical addressing, network topology, line discipline,Frame synchronization, error notification, orderly delivery of frames and flow control.

Services provided by data link layer Unacknowledged connectionless

service Acknowledged connectionless service Acknowledged connection-oriented

service

Unacknowledged connectionless service

No connection established. Send frames. No acknowledgment by

receiving machine. If lost, no attempt to recover by this layer.

Recovery is left up to the higher layers Good for reliable lines such as fiber, use

for speech, video, etc. where late is worse than lost frames.

Acknowledged connectionless service

No connection established Frames are sent and each is individually

acknowledged Frames are resent if acknowledgement

received within timeout period. Useful for unreliable channels such as

wireless.

Acknowledged connection-oriented service

Reliable service Source and destination establish

connection All frames are received in the right order After transmission the connection is

releasedaming

Framing

In order to provide service to the network layer, the data link layer must use the service provided to it by the Physical layer. The physical layer only accept a raw bit stream. For a long bit stream, if error occurred during transmission, the entire stream has to be sent again. Therefore, it is broken to smaller frames.

Each frame will have to be distinguished somehow. Three methods are used

Character count Starting and ending characters, with

character stuffing Starting and ending flags, with bit stuffing

Character count

The first byte indicates how many characters are in the frame including the first byte.

Trouble with this scheme is that if the first byte of any frame gets corrupted, the destination will get out of sequence, having no way to find where the beginning of the next frame is.

This method is rarely used any more.

Start and end with control chars with character stuffing

Resynchronize at the beginning of each frame Start with ASCII chars DLE & STX

– Data Link Escape, Start of Text

End with the same sequence If these ASCII char is part of the text then double it

– like Jim’’s book - in pascal Problem - must use 8 bit ASCII which is not

suitable for all communications

Start and end with flags with stuffing

Each frame begins with bit pattern 01111110 If senders data has five consecutive ones then

stuff it with a zero which is deleted at the receiving end

This bit pattern called flag byte is added at the end of each frame as well

With this method boundary between two frames are easily recognized

Error control

How to make sure that all data arrived at the destination correctly and in the proper order.

The receiver sends special control characters indicating positive or negative acknowledgement about incoming frames.

What to do if the acknowledgment gets lost? -the computer will hang.– Introduce timers for each frame.– The time has to be set long enough to send/receive in both

directions – The time will vary depending upon distance, conditions, etc.– We will examine this in more detail later.

Receives packets from network layer The packet is treated as pure data, even though

it may contain data and control information. It is the data link’s job to transmit it to the

receiver’s data link layer, which in turn gives it to the network layer.

Data link encapsulates it in a frame by adding data link header and trailer to it.

Appropriate checksums are computed and included in the frame.

Elementary data link protocols

computes the checksum if checksum does not match send error

message If checksum is ok notify sending data link layer Check all header trailer information If all OK strip off the header and trailer and

pass the packet to the data link layer

Receiving data link layer

Wait for an event to happen Fetch packet from the network layer for

transmission on the channel Deliver information from inbound frame to

network layer Go get an inbound frame from the physical layer Pass the frame to physical layer for transmission Start clocks and stop clocks as necessary etc, etc

Pseudocode for data link layer routine

Sliding Window Protocols

For full duplex two transmission lines are required One to send data and the other to receive data and/or

ACKs Sending ACKs in a packet of its own is waste of resources May use piggybacking The acknowledgement is attached to an outgoing data

frame(ack field in the frame header) Data link layer must wait until it has some thing to send

before it can ack. This may cause time out and the frame may be re-sent.

The essence of sliding window protocols is that at any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending window. Similarly, the receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept. It is NOT necessary for the sender and receiver to have same number limits.

Sequence numbers of frames sent but not acknowledged are kept in the senders window.

Th receivers window corresponds to the frames it may accept.

One bit sliding window protocol

uses stop and wait scheme, since the sender transmits a frame and waits for its acknowledgement before sending the next one.

The acknowledgement field of the frame contains the number of the last frame received without error. If this number agrees with the sequence number of the frame the sender is trying to send, the sender knows that it is done and can proceed with the next frame.

A protocol using Go-Back-N

Takes into account of round trip transmission time. Enough frames should be send to stuff the band with to

avoid channel idling Consider a 50-kbps satellite channel with 500 m-sec

round-trip propagation delay– Let us send 1000 bit frames– At t=0 starts sending the first frame, at t=20 msec the frames has been

send– Receiver fully receives only at t=270msc, and ack will be received at

t=520 msec.– Sender would be blocked 96% of time- only 4% of available band

width was used.

Go-Back-N continued.

Instead of sending 4 frames, the sender should have sent 26 frames. This will take t=520 msec.

at t=520 the acknowledgement for the first frame will be received. The senders maximum window size is 26.

This technique is called pipelining. If a frame in the middle of a stream gets damaged, what should the

receiver do? Should all frames starting at that point be re-sent? Go-Back-N simply discards all frames including the damaged one,

not sending acks. The time out at the sender will resend all subsequent frames.

Can waste a lot of bandwidth in an unreliable channel.

Selective Repeat

Receiver stores all good frames after the damaged frame.

Notifies the sender of the bad frame, and sender re-sends that one.

This technique requires the data link layer to have large amount of memory for the cache.

Examples of Data Link Protocols

HDLC - HIGH LEVEL DATA LINK CONTROL– DERIVED FROM IBM’S SNA - LAYER CALLED SDLC

(SYNCHRONOUS DATA LINK PROTOCOL).

– ANSI MODIFIED IT TWICE FINALLY IT IS NOW HDLC

– CCITT MODIFIED HDLC AS A PART OF X.25 CALLED LAPB (LINK ACCESS PROCEDURE B)

ALL ARE BIT ORIENTED PROTOCOLS– FRAME EXAMPLE– 01111110/ADDRESS/CONTROL/DATA/CHECKSUM/01111110

– CONTROL FIELD IS FOR FRAME SEQUENCE NOS

DATA LINK LAYER IN THE INTERNET

HOME USER DIALS UP THE INTERNET PROVIDER THE PC AT HOME FUNCTIONS AS A CHARACTER

ORIENTED TERMINAL LOGGED INTO THE INTERNET SERVICE PROVIDER’S TIME SHARING SYSTEM. -This is called a SHELL ACCOUNT. CAN’T USE WWW.

THE HOME PC CAN CALL THE INTERNET PROVIDER’S ROUTER AND THEN ACT LIKE A FULL-BLOWN INTERNET HOST. CAN USE WWW. TO DO THIS SOME POINT-TO-POINT DATA LINK PROTOCOL IS REQUIRED.

FRAMING, ERROR CONTROL AND OTHER DATA LINK LAYER FUNCTIONS

TWO POINT-TO-POINT PROTOCOLS– SLIP - SERIAL LINE IP

– PPP - POINT-TO-POINT PROTOCOL

POINT-TO-POINT DATA LINK PROTOCOL

DEVISED IN 1984 TO CONNECT SUNS TO INTERNET USING MODEMS

SLIP DOES NOT DO ERROR CORRECTION ONLY SUPPORTS IP -NOT IPX EACH SIDE MUST KNOW EACH OTHER’S IP

ADDRESS IN ADVANCE SLIP DOES NOT PROVIDE USER AUTHENTICATION SLIP IS NOT AN INTERNET APPROVED STANDARD

THEREFORE MANY VERSIONS EXIST

SLIP (SERIAL LINE IP)

APPROVED INTERENT STANDARD HANDLES ERROR CORRECTION SUPPORTS MULTIPLE PROTOCOLS IP ADDRESS CAN BE NEGOTIATED AT CONNECT

TIME PERMITS AUTHENTICATION A LINK PROTOCOL CALLED LCP (LINK CONTROL

PROTOCOL) CONNECTS, NEGOTIATES, AND TERMINATES CONNECTIONS

PPP - POINT TO POINT PROTOCOL

PC CALLS PROVIDER’S ROUTER VIA MODEM, ROUTER ESTABLISHES A PHYSICAL CONNECTION, THE PC THEN SENDS THE ROUTER A SERIES OF LCP (LINK CONTROL PROTOCOLS) TO NEGOTIATE PPP PERAMETERS TO BE USED.

THEN A SERIES OF NCP(NETWORK CONTROL PROTOCOLS) ARE SENT TO CONFIGURE THE NETWORK LAYER. A NETWORK ADDRESS IS ISSUED - THE PROVIDER HAS A LIMITED NUMBER.– IF TOTAL CUSTOMER CONNECTION EXCEEDS THE

AVAILABLE NO OF IP ADDRESSES CAN’T CONTINUE.

EXAMPLE OF PC CONNECTION