Prepared by:

anil shrestha

Tribhuvan university

a)Services Provided to the Network Layer

b)Framing

c)Error Control

d)Flow Control

DLL 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.

Functions of the Data Link Layer:

a)Provide service interface to the network layer

b)Dealing with transmission errors

c)Regulating data flow

1.Slow receivers not swamped by fast senders

Relationship between packets and frames.

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

Placement of the data link protocol.

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.

(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.

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.

a)Error-Correcting Codes

b)Error-Detecting 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.

Use of a Hamming code to correct burst errors.

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 - checksumrepresents 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

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).

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

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.

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.

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

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.

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

a)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.

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

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

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

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)

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

Simple

Inefficient

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

Receiver detects error in frame i

Receiver sends rejection-i

Transmitter gets rejection-i

Transmitter retransmits frame i and all subsequent

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

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

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

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