ch3datalink-1

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The Data Link Layer

Chapter 2



2

Data Link Layer

a) The main tasks of the data link layer are:

• Transfer data from the network layer of one machine to the

network layer of another machine

• Convert the raw bit stream of the physical layer into groups of

bits (“frames”)

Network

Layer

Data Link

Layer

Physical

Layer

Network

Layer

Data Link

Layer

Physical

Layer



3

MAC and LLC

a) The MAC protocol are implemented in the MAC sublayer which is

the lower sublayer of the data link layer

b) The higher portion of the data link layer is often called Logical Link

Control (LLC)

Logical Link

Control

Medium Access

Control D a t a L i n k

L a y e r

to Physical Layer

to Network Layer



Adapter Communication

a) It is a board /card that typically contains RAM ,DSP Chips, a host

bus interface and link interface

b) Commonly known as network interface Card

c) For most part, this layer protocols are implemented in an adapter.



Data Link Layer Design Issues

a) Services Provided to the Network Layer b) Framing and its synchronization

c) Error Control

d) Flow Control

e) Addressing

f) Link management-control and data on same

link,their should be distinction.



Link layer protocol Services

a) Provide service interface to the network layer

b) Framing, Link Access

c) Reliable Delivery

d) Flow control

e) Error Detectionf) Error Correction

g) Half Duplex or full Duplex



Functions of the Data Link Layer (2)

Relationship between packets and frames.



Services Provided to Network Layer

(a) Virtual communication.

(b) Actual communication.



Framing

A character stream. (a) Without errors. (b) With one error.

Problem: Even if the error is detected, the receiver cannot figure out wherethe next frame starts ... its cannot resynchronize.

Framing by character count.



Framing (2)

(a) A frame delimited by flag bytes.

(b) Four examples of byte sequences before and after stuffing.

Problem: Too tied to the 8-bit per character format ... UNICODE uses 16-bits/char



Framing (3)

Bit stuffing(a) The original data.

(b) The data as they appear on the line.

(c) The data as they are stored in receiver‟s memory after destuffing.

The goal is to have 01111110 as a unique bit pattern.



Elementary Data Link Protocols

a) An Unrestricted Simplex Protocol

b) A Simplex Stop-and-Wait Protocol

c) A Simplex Protocol for a Noisy Channel

Each protocol is increasing in complexity

and drops unrealistic assumptions.



Flow Control: Initial Assumptionsa) Simplex Channel

b) Infinite buffer capacity with the receiverc) Error free transmission

d) Network layer at the senders end is always ready with data

e) No need for flow control



Unrestricted

Simplex

Protocol



Some assumptions dropped

a) Infinite capacity in the buffer of the receiver.

b) Need for “flow control”

c) Stop-n-Wait protocol

– Sender sends a frame – And waits for a signal in the form of a dummy frame

– No seq no. is required since the line is still error free



Simplex

Stop-and-

Wait

Protocol



Problem of Duplicate frames

a) Solution : – Keep a sequence number for each frame to distinguish between the new frame and a duplicate frame.

b) How large should be the sequence number? Or

c) What should be minimum number of bits requiredfor the sequence number? – The only ambiguity at the receiver is between two

successive frames.. Say m and m+1 and never betweenm-1 and m +1 .. Its only after the ack for m-1 reaches

back intact mth frame is sent.. And once mth framereaches intact at the receiver‟s end .. Story of m-1thframe is over .. However depending upon whether ackof mth frame reaches back intact or not either mth orm+1 th frame is sent .. hence only 1 bit is sufficient



A Simplex Protocol for a Noisy Channel

A positive acknowledgement with retransmission protocol.

Continued



Simplex protocol for noisy channel

* Data frame can be lost

* Ack can be lost

Lost data frames:

This means that sender will never get an ack.Sender can implement timer for each frame sent. If timer expires retransmit.

But what if the data frame arrived but the ack was lost?

Using above method, sender will retransmit frame that receiver has already received.

Result: Duplicate frame at sender .

So receiver needs some way of distinguishing duplicates.

Answer: Use sequence numbers.

What should be the field size of these sequence numbers ?

1 bit.



Unrestriced simplex protocol

Assumption 1. Receiver can process data infinitely fast. Assumption 2. Channel never looses data

Sendersends as fastas possible

Receiver canalwaysprocess fastenough



Simplex stop-and-wait protocol Assumption 2. Channel never looses data

Sendersendsframe

Senderwaits forack

Rcvr sendsack afterprocessing

Sendersendsframe



Sliding Window Protocols

a) A One-Bit Sliding Window Protocol

b)A Protocol Using Go Back N

c) A Protocol Using Selective Repeat

„Read them from book‟

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