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this is presentation about data link layer
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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