1 Data Link Layer
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Data Link Layer
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Data Link Layer Functionality
Services provided by the link layerFrame creationError detection and/or correctionFlow controlReliable delivery
Creating the illusion of a reliable link
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Framing
The data unit at the data link layer is called a “frame”A frame is a group of bits, typically in sequenceIssues:
Frame creationFrame delineation
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Frame Delineation
How to tell when a new frame starts:Frame tags with character stuffingFrame tags with bit stuffing
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Frame tagging with character stuffing
Use starting and ending characters (tags) to mark boundaries of frameProblem: What if tag characters occur in the data or control portions of the frame?
Frame tags (special characters)
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Character stuffing
Insert extra escape characters when a tag appears in data field
STXDLE
ETXDLE
DLEDLE
Start Tag
End Tag
Character Stuffed DLE code
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Character Stuffing Example
DLEI am a jerk trying to DLE crash your network! ETX
DLEI am a jerk trying to DLE crash your network! ETXSTXDLE DLE DLE ETXDLE
DLEI am a jerk trying to DLE crash your network! ETX
Tagging and Character Stuffing
Untagging and Character Unstuffing
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Character StuffingPPP
PPP = Point-to-Point Protocol, used widely in the InternetPPP uses frame tags with character or byte stuffing
01111110 Control Fields Payload (up to 1500 bytes) Checksum 01111110
PPP Frame Format
Character Frame Tags
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Frame tagging with bit stuffing
Bit strings may be used instead of character sequences to delineate framesMore efficient
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Bit stuffing
Each frame begins with a start and end bit sequence, e.g., 01111110When sender’s data link layer sees five 1’s in a row, it stuffs a zero bitThe receiver “unstuffs” a zero after five consecutive 1’s.
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Bit unstuffing
When a pattern of five 1s appears, examine sixth bitIf the sixth bit is 0, remove it (stuffed bit)If the sixth bit is 1, check seventh bit
If 0, accept as flag (BOF or EOF)If 1, error
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Bit Stuffing Example
Tagging and Bit Stuffing
Untagging and Bit Unstuffing
00110010111111100010100
0111111000110010111110110001010001111110
00110010111111100010100
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Bit StuffingHDLC
HDLC = High-level Data Link Control. Used in X.25 networksHDLC uses frame tags with bit stuffing
01111110 Control Fields Payload (arbitrarily long) Checksum 01111110
HDLC Frame Format
Character Frame Tags
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Error Control
No physical link is perfectBits will be corruptedWe can either:
detect errors and request retransmissionor correct errors without retransmission
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Error Detection
Parity bitsPolynomial codes or checksums
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Parity bits
Append a single parity bit to a sequence of bitsIf using “odd” parity, the parity bit is chosen to make the total number of 1’s in the bit sequence odd.If “even” parity, the parity bit makes the total number of 1’s in the bit sequence even
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Parity bits: Example
00010101 even
01111 even
11111111 odd
10011 odd
TransmittedSequence Parity
ParityBit
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Parity Bits
Only detects when there are an odd number of bit errorsDoes not detect an even number of bit errorsIt also has high overhead; it requires 1 extra bit for every several bits transmitted
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Checksum
Consider data as 16 bit words.Add all the data values (1s complement of this sum is the checksum)Receiver adds all the words and the checksum. If no errors, then sum is all 1s
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Polynomial Codes
Can detect errors on large chunks of dataHas low overheadMore robust than parity bitRequires the use of a “code polynomial”
Example: x2 + 1
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Cyclic Redundancy Check
CRC: Example of a polynomial codeProcedure:1. Let r be the degree of the code polynomial. Append r zero
bits to the end of the transmitted bit string. Call the entire bit string S(x)
2. Divide S(x) by the code polynomial using modulo 2 division.3. Subtract the remainder from S(x) using modulo 2 subtraction.
The result is the checksummed message
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Generating a CRCExample
Message: 1011 1 x x3 + 0 x x2 + 1 x x + 1 = x3 + x + 1
Code Polynomial: x2 + 1 (101)
Step 1: Compute S(x)
r = 2
S(x) = 101100
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Generating a CRCExample (cont’d)
Step 2: Modulo 2 divide
101100101101
001000
01000010010101
1001
Remainder
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Generating a CRCExample (cont’d)
Step 3: Modulo 2 subtract the remainder from S(x)
101100- 01101101
Checksummed Message
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Decoding a CRC
Procedure1. Let n be the length of the checksummed message
in bits2. Divide the checksummed message by the code
polynomial using modulo 2 division. If the remainer is zero, there is no error detected.
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Decoding a CRCExample
101101 Checksummed message (n = 6)
1011 Original message (if there areno errors)
101101101101
001000
01000010110100
1001
Remainder = 0(No error detected)
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Decoding a CRCAnother Example
When a bit error occurs, there is a large probability that it will produce a polynomial that is not an even multiple of the code polynomial, and thus errors can usually be detected.
101001101101
000000
00000000100001
1000
Remainder = 1(Error detected)
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Choosing a CRC polynomial
The longer the polynomial, the smaller the probability of undetected errorCommon standard polynomials:(1) CRC-12: x12 + x11 + x3 + x2 + x1 + 1(2) CRC-16: x16 + x15 + x2 + 1(3) CRC-CCITT: x16 + x12 + x5 + 1
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Flow Control
What happens if the sender tries to transmit faster than the receiver can accept?Data will be lost unless flow control is implemented
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Controlling the Flow of Data
Slow JoeFast Frank
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Some Flow Control Algorithms
Flow control for the ideal networkStop and Wait for noiseless channelsStop and Wait for noisy channelsSliding window protocolsSliding window with error control
– Go Back N– Selective Repeat
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Flow control in the ideal network
Assumptions:Error free transmission link,Infinite buffer at the receiver
No acknowledgement of frames necessarySince the data link is error-free and the receiver can buffer as many frames as it likes, no frame will ever
be lost
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Flow control in the ideal network (cont’d)
Slow JoeFast Frank
Infinite bucket
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Stop and Wait with Noiseless Channels
Assumptions:Error free transmission link,Finite buffer at the receiver
Problem of Buffer overflow at the receiverBuffer overflow may happen at the receiver when the sending router sends frames at a rate faster than the receiving router
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Stop and Wait with Noiseless Channels (cont’d)
Slow JoeFast Frank
Finite bucket (once full, ball is lost)
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Stop and Wait with Noiseless Channels (cont’d)
Solution: Stop-and-WaitThe receiver sends an acknowledgement frame telling the sender to transmit the next data frame. The sender waits for the ACK, and if the ACK comes, it transmits the next data frame.
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Stop and Wait with Noiseless Channels (cont’d)
Data
ACK
Data
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Stop and Wait (cont’d)
Note that we assume an error-free transmission link and therefore ACK frames will not be lostIn this flow control protocol, there are two types of frames: data frames and ACK frames. The ACK frames don’t contain any particular information, since only the arrival of the ACK frame at the sender is important.
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Stop and Wait for Noisy Channels
Assumptions:Transmission link may cause errors in frames,
Finite buffer at the receiver
ACK frames may now be lost
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Problems introduced by a noisy line
Problem 1: Loss of a data or ACK frameSince the transmission link is not error-free, a data or ACK frame may be lost, causing the sender to wait indefinitely for an ACK
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Loss of an ACK frame
ACK
Data
error
?
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Problems introduced by a noisy line
Can we solve problem 1 by introducing a timeout period for the sender? Yes, but...
Problem 2: Duplicated framesIf the ACK frame for a certain data frame is lost, the sender will retransmit the same frame after a time-out period, and the receiver will then have two copies of the same frame
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Duplicated data frames
Data
ACK
Data
TimedOut
Duplicate data
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Stop and Wait for Noisy Channels (cont’d)
Solution:The sender uses a timer to retransmit data frames when an ACK has not arrivedThe sender includes a sequence number in each frame to distinguish one frame from another. This way, the receiver knows when it has received duplicate frames.
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Stop and Wait for Noisy Channels (cont’d)
Data
ACK
Data1
1
11
TimedOut
receiverknows to discard thisdata
1
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Is Stop and Wait the best we can do?
Stop and Wait is an effective form of flow control, but…
It’s not very efficient. 1. Only one data frame can be in transit on the link at
a time2. When waiting for an acknowledgement, the sender
cannot transmit any frames
Better solution? Sliding Window
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Sliding Window ProtocolsDefinitions
Sequence Number: Each frame is assigned a sequence number that is incremented as each frame is transmitted
Sender Window: Keeps track of sequence numbers for frames that have been sent but not yet acknowledged
Receiver Window: Keeps track of sequence numbers for frames the receiver is allowed to accept
Maximum Sender Window size: The maximum number of frames the sender may transmit without receiving any acknowledgements
Maximum Receiver Window size: The maximum number of frames the receiver may receive before returning an acknowledgement to the sender
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Simple Sliding Window with Window Size of 1
A sliding window with a maximum window size of 1 frame
Window for a 3-bit sequence number
0
1
2
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5
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7
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Sliding Window example0
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Sender window
Receiver window
(a) (b) (c) (d)
(a) Initial state, no frames transmitted, receiver expects frame 0(b) Sender transmits frame 0, receiver buffers frame 0(c) Receiver ACKS frame 0 (d) Sender receives ACK, removes frame 0
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Simple Sliding Window withWindow size 1 (cont’d)
This protocol behaves identically to stop and wait for a noisy channel
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Sliding Window ProtocolsGeneral Remarks
The sending and receiving windows do not necessarily have the same maximum sizeAny frame whose sequence number falls outside the receiver window is discarded at the receiverThe sender window’s size grows and shrinks as frames are transmitted and acknowledgedUnlike the sender window, the receiver window always remains at its maximum size
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Sliding Window ProtocolsPiggybacking Acknowledgements
Since we have full duplex transmission, we can “piggyback” an ACK onto the header of an outgoing
data frame to make better use of the channel
When a data frame arrives at a router, instead of immediately sending a separate ACK frame, the
router waits until it is passed the next data frame to send. The acknowledgement is attached to the
outgoing data frame.
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Sliding Window with Maximum Sender Window Size WS
With a maximum window size of 1, the sender waits for an ACK before sending another frame
With a maximum window size of WS, the sender can transmit up to WS frames before “being blocked”
This allows the sender to transmit several frames before waiting for an acknowledgement
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Sender-Side Window with WS=2
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(a) (b) (c) (d)
(e) (f) (g) (h)
(a) Initial window state(b) Send frame 0(c) Send frame 1(d) ACK for frame 0 arrives
(e) Send frame 2(f ) ACK for frame 1 arrives(g) ACK for frame 2 arrives, send frame 3(h) ACK for frame 3 arrives
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Pipelining
Sliding window with WS > 1 is also called “pipelined” communication
A B99
data stream51 50
4910
ACK stream
By allowing several frames onto the link before receiving an acknowledgement, pipelining keeps the link from
being idle
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Sliding Window with Maximum Receiver Window Size WR
With a maximum window size of 1, the receiver must receive and process every frame in sequence
With a maximum window size of WR, the receiver can receive and process up to WR frames before
acknowledging them
This is useful when frames are lost: the receiver can still accept and buffer frames after the missing frame
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Receiver-Side Window with WR=2
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(a) (b) (c) (d)
(e) (f) (g) (h)
(a) Initial window state(b) Nothing happens(c) Frame 0 arrives, ACK frame 0 (d) Nothing happens
(e) Frame 1 arrives, ACK frame 1(f) Frame 2 arrives, ACK frame 2(g) Nothing happens(h) Frame 3 arrives, ACK frame 3
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What about Errors?
What if a data or acknowledgement frame is lost when using a sliding window
protocol?
Two Solutions:Go Back N
Selective Repeat
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Sliding Window with Go Back N
When the receiver notices a missing or erroneous frame, it simply discards all frames with greater sequence numbers and sends no ACKThe sender will eventually time out and retransmit all the frames in its sending window
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Go Back N
Discarded byreceiver
Frame witherror
Timeout intervalAC
K 0
Time
Sender
Receiver
Maximumwindow size = 8
Maximumwindow size = 8
0
0 1
1
ACK
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E
3
D
4
D
2 3
2
ACK
2
4 5 6
3 4 5
ACK
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ACK
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ACK
5
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ACK
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Go Back N (cont’d)
Go Back N can recover from erroneous or missing frames
But…
It is wasteful. If there are errors, the sender will spend time retransmitting frames the receiver has already seen
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Sliding Window with Selective Repeat
The sender retransmits only the frame with errorsThe receiver stores all the correct frames that arrive following the bad one. (Note that the receiver requires a frame buffer for each sequence number in its receiver window.)When the receiver notices a skipped sequence number, it keeps acknowledging the last good sequence numberWhen the sender times out waiting for an acknowledgement, it just retransmits the one unacknowledged frame, not all its successors.
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Selective Repeat
Buffered byreceiver
Frame witherror
Timeout intervalAC
K 0
Time
Sender
Receiver
Maximumwindow size = 8
Maximumwindow size = 8
0
0 1
1
ACK
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E
3 4 2 5
2
ACK
4
6
5
ACK
5
6
ACK
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3 4AC
K 1
ACK
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