DATA LINK LAYER

Prepared by: anil shresthaTribhuvan university

Data link layera)Services Provided to the Network Layer

b)Framing

c)Error Control

d)Flow Control

DATA LINK LAYERDLL purpose? The goal of the data link layer is to provide reliable, efficient communication between adjacent machines connected by a single communication channel. Specifically:1. Group the physical layer bit stream into units called frames. Note that

frames are nothing more than ``packets'' or ``messages''. By convention, we'll use the term ``frames'' when discussing DLL packets.

2. Sender checksums the frame and sends checksum together with data. The checksum allows the receiver to determine when a frame has been damaged in transit.

3. Receiver recomputes the checksum and compares it with the received value. If they differ, an error has occurred and the frame is discarded.

4. Perhaps return a positive or negative acknowledgment to the sender. A positive acknowledgment indicate the frame was received without errors, while a negative acknowledgment indicates the opposite.

5. Flow control. Prevent a fast sender from overwhelming a slower receiver. For example, a supercomputer can easily generate data faster than a PC can consume it.

6. In general, provide service to the network layer. The network layer wants to be able to send packets to its neighbors without worrying about the details of getting it there in one piece.

DATA LINK LAYER

Functions of the Data Link Layer:

a)Provide service interface to the network layerb)Dealing with transmission errorsc)Regulating data flow

1.Slow receivers not swamped by fast senders

Functions of the Data Link Layer (2)

Relationship between packets and frames.

Services Provided to Network Layer

(a) Virtual communication.(b) Actual communication.

Services Provided to Network Layer

Placement of the data link protocol.

Framing Framing by character count.

A character stream. (a) Without errors. (b) With one error.Problem: Even if the error is detected, the receiver cannot figure out

where the next frame starts ... its cannot resynchronize.

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

Frames that need to be send in a bit stream:

FlagFlag

The sender sends the following bit stream:

FlagFlag Esc

The receiver will ignore this flag.

Frames that need to be send in a bit stream:

FlagEsc

The sender sends the following bit stream:

Esc Esc Flag FlagEsc

The receiver will ignore this Esc, and accept the flag.

The receiver will ignore this flag.

Framing (3)The goal is to have 01111110 as a unique

bit pattern.

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.

Error Detection and Correction

a)Error-Correcting Codesb)Error-Detecting Codes

Error-Correcting Codes Include enough redundancy to detect and correct errors. To understand errors, consider the following: Messages (frames) consist of m data (message) bits

and r redundancy bits, yielding an n = (m+r)-bit codeword.

Hamming Distance. Given any two codewords, we can determine how many of the bits differ. Simply exclusive or (XOR) the two words, and count the number of 1 bits in the result.

Significance? If two codewords are d bits apart, d errors are required to convert one to the other.

A code's Hamming Distance is defined as the minimum Hamming Distance between any two of its legal codewords (from all possible codewords).

In general, all   possible data words are legal. However, by choosing check bits carefully, the resulting codewords will have a large Hamming Distance. The larger the Hamming distance, the better able the code can detect errors.

Error-Correcting Codes

Use of a Hamming code to correct burst errors.

Error-Detecting Codes Error-correcting codes are widely used on wireless links

that are noisy. However, they generate too large transmission overhead

for reliable links such as copper wire or fiber. Therefore, here error-detection codes are used.

When error is detected, the data is retransmitted. The goal for error correcting codes it to add redundancy to

the data so that the errors are not only detected but can be at the same time corrected (without retransmission).

For error-detecting codes the goal is to only detect the errors with the minimal transmission overhead. They are based on polynomial code also known as CRC (Cyclic Redundancy Check)

A k-bit frame is regarded as polynomial with coefficients 0 and 1 with terms from xk-1 to x0

For example: 110001 -> x5 + x4 + x0

Polynomial arithmetic is done modulo 2 using the rules of algebraic field theory.Both addition and subtraction are identical to exclusive OR. For exampe:

10011011 11110000+11001010 -10100110-------------- ------------- 01010001 01010110

The sender and receiver must agree on a generator polynomial G(x).G(x) must have the first and last bit equal to 1.For a given frame, we consider its polynomial M(x) (longer than G(x)).The checksum is the reminder from the division M(x)*xr / G(x), where r is the degree of G(x).Polynomial T(x) obtained as M(x)*xr - checksum represents the check-summed frame that is divisible by G(x).

An example division is shown on the next page, where the frame is 1101011011 (corresponds to M(x))and the generator polynomial G(x) = x4 + x + x0 -> 10011.M(x)*xr -> 11010110110000 (we added 4 zeros at the end)

Calculation of the polynomial code checksum.

Upon receiving the check-summed frame, the receiver divides it by G(x):

[T(x) + E(x)] / G(x)

Since T(x) / G(x) is always zero, the result is always E(x) / G(x).

The errors containing G(x) as a factor will slip by, all other errors will be caught.

Single bit errors will be detected: We have E(x)=xi for a single bit error, E(x) / G(x) will not be zero, since G(x) must have the first and last bit equal to 1.

All errors consisting of an odd number of inverted bits will be detectedif G(x) is divisible by (x + 1).E(x) consists of odd number of terms, e.g., x5 + x2 + x0 and therefore, cannot be divisible by (x+1). Since E(x) has an odd number of terms E(1)=1.If E(x) = (x + 1) Q(x), then E(1) = (1 + 1) Q(1) = 0, a contradiction.

The polynomial G(x) used in IEEE 802 standard is

x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1

parity concept

Parity concept

In parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity).

Example

Suppose the sender wants to send the word world. In ASCII the five characters are coded as

1110111 1101111 1110010 1101100 1100100

The following shows the actual bits sent

11101110 11011110 11100100 11011000 11001001

CRC generator and checker

Binary division in a CRC

Binary division in CRC checker

Checksum

Checksum

The sender follows these steps:

• The unit is divided into k sections, each of n bits.

• All sections are added using one’s complement to get the sum.

• The sum is complemented and becomes the checksum.

• The checksum is sent with the data.

Checksum The receiver follows these steps:

• The unit is divided into k sections, each of n bits.

• All sections are added using one’s complement to get the sum.

• The sum is complemented.

• If the result is zero, the data are accepted: otherwise, rejected.

Example

Suppose the following block of 16 bits is to be sent using a checksum of 8 bits.

10101001 00111001 The numbers are added using one’s complement 10101001 00111001

------------Sum 11100010

Checksum 00011101 The pattern sent is 10101001 00111001

00011101

Example Now suppose the receiver receives the pattern

sent in Example 7 and there is no error. 10101001 00111001 00011101 When the receiver adds the three sections, it will

get all 1s, which, after complementing, is all 0s and shows that there is no error.

10101001 00111001 00011101 Sum 11111111 Complement 00000000 means that the

pattern is OK.

Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

Transmission time Time taken to emit all bits into medium

Propagation time Time for a bit to traverse the link

Model of Frame Transmission

Data Link Protocolsa)A Simplex Stop-and-Wait Protocol.

b)Sliding window protocol.

- A One Bit sliding window protocol.

- A protocol using Go Back N.

- A protocol using selective Repeat.

Stop and Wait

Source transmits frame Destination receives frame and replies

with acknowledgement Source waits for ACK before sending next

frame Destination can stop flow by not send

ACK Works well for a few large frames

Fragmentation

Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame

received) On error, retransmission of smaller frames is

needed Prevents one station occupying medium for

long periods Stop and wait becomes inadequate

Stop and Wait Link Utilization

Sliding Windows Flow Control

Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames

without ACK Each frame is numbered ACK includes number of next frame

expected Sequence number bounded by size of

field (k) Frames are numbered modulo 2k

Sliding Window Diagram

Example Sliding Window

Sliding Window Enhancements

Receiver can acknowledge frames without permitting further transmission (Receive Not Ready)

Must send a normal acknowledge to resume

If duplex, use piggybacking If no data to send, use acknowledgement

frame If data but no acknowledgement to send, send

last acknowledgement number again, or have ACK valid flag (TCP)

Stop and Wait

Source transmits single frame Wait for ACK If received frame damaged, discard it

Transmitter has timeout If no ACK within timeout, retransmit

If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Stop and Wait -Diagram

Stop and Wait - Pros and Cons

Simple Inefficient

Go Back N (1)

Based on sliding window If no error, ACK as usual with next frame

expected Use window to control number of

outstanding frames If error, reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Go Back N - Damaged Frame

Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all

subsequent

Go Back N - Lost Frame (1)

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and

retransmits

Go Back N - Lost Frame (2)

Frame i lost and no additional frame sent Receiver gets nothing and returns neither

acknowledgement nor rejection Transmitter times out and sends

acknowledgement frame with P bit set to 1

Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )

Transmitter then retransmits frame i

Go Back N - Damaged Acknowledgement

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out, it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Go Back N - Diagram

Selective Reject

Also called selective retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the

receiver and buffered Minimizes retransmission Receiver must maintain large enough

buffer More complex login in transmitter

Selective Reject -Diagram

THE END

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