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CS 457 - Computer Networks
Data Link Layer
Introduction Synchronization Error Detection Flow Control Error Control (via Retransmission)
© Jorg Liebeherr, 1998
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CS 457 - Computer Networks
Introduction Main Task of the data link layer: n Provide error-free transmission over a link
© Jorg Liebeherr, 1998
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
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Introduction n
The PDU at the Data Link Layer (DL-PDU) is typically called a Frame. A Frame has a header, a data field, and a trailer
n
Example
01111110 8bits
Address 8bits
Control 8bits
Data >=0
Header
Checksum 16bits
01111110 8bits
Trailer
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Framing n n
Problem: Identify the beginning and the end of a frame in a bit stream Solution (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") 01111110
n
Data
01111110
Problem: – The sequence '01111110' must not appear in the data of the frame
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Bit-Oriented Framing and Bit Stuffing n
'Bit stuffing': If the sender detects five consecutive '1' it adds a '0' bit into the bit stream. The receiver removes the '0' from each occurrence of the sequence '111110' Original bit sequence:
After stuffing bits at sender: After stuffing bits are removed by receiver:
n
0110111111111111111100 011011111 0111110111110100 Stuffed bits
0110111111111111111100
Note: The flags itself are not bit-stuffed.
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Error Control Two basic approaches to handle bit errors: –
Error-correcting codes -
–
Error-detecting codes plus retransmission -
© Jorg Liebeherr, 1998
Used if retransmission of the data is not possible Data are encoded with sufficient redundancy to correct bit errors Examples: Hamming Codes, Reed Solomon Codes, etc.
Used if retransmission of corrupted data is feasible Receiver detects error and requests retransmission of a frame. 6
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Error Detection Techniques n
Error Detection Techniques: –
Parity Checks
–
Cyclic Redundancy Check
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Parity Checks General Method: n Append a parity bit to the end of each character in a frame such that the total number of '1' in a character is: -
n
even (even parity) or odd (odd parity)
Example: – – –
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With ASCII code, a parity bit can be attached to an 7-bit character ASCII "G" = 1 1 1 0 0 0 1 with even parity = with odd parity = 8
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Cyclic-Redundancy Codes (CRC) General Method: n The transmitter generates an n-bit check sequence number from a given k-bit frame such that the resulting (k+n)-bit frame is divisible by some number n n
The receiver divides the incoming frame by the same number If the result of the division does not leave a remainder, the receiver assumes that there was no error
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Cyclic-Redundancy Codes (CRC) n
CRC is used by all advanced data link protocols, for the following reasons: – Powerful error detection capability – CRC can be efficiently implemented in hardware
n
We discuss the CRC method in five easy steps: 1. Prerequisites 2. CRC Encoding Method 3. Example 4. Error Detection with CRC 5. Capabilities of CRC
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Step 1: Prerequisites n
n
Modulo-2 Arithmetic: +
1
0
*
1
0
1
0
0
1
1
0
0
1
0
0
0
0
Examples of polynomials based on Modulo-2 operations: 1. (x+1)(x+1) = 2. (x2+1)(x3+x+1) = 3. If F(x) contains (x+1) as a factor, then F(1) = 0
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Step 2: CRC Encoding Method n
Let us view each block of data as a polynomial with binary coefficients: 1 0 1 1 0 1 is viewed as x 5 + x 3 + x 2 + 1
Define: n M(x) Data block is a polynomial (= Message, Frame) n P(x) "Generator Polynomial" which is known to both sender and receiver (degree of P(x) is n)
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Step 2: CRC Encoding Method (I) (II) (III)
Append n zeros to M(x), i.e., M(x) xn Divide M(x) xn by P(x) and obtain: M(x) xn = Q(x) P(x) + R(x) Set T(x) = M(x) xn + R(x). T(x) is the encoded message
Note:
n
T(x) is divisible by P(x). Therefore, if the received message does not contain an error then it can be divided by P(x).
Exercise: Encode the frame 1 1 0 1 0 1 1 0 1 1
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Step 3: Encoding M (x) = x9 + x8 + x6 + x 4 + x3 + x + 1 -
(I) (II) (III)
Generator polynomial is P(x) = x4 + x + 1 Degree of P(x) is 4
M(x) x4 = x13 + x 12 + x10 + x8 + x7 + x 5 + x4 M(x) x4 = Q(x) P(x) + R(x) =(x9 + x8 + x3 + x) P(x) + x3 + x2 + x T(x) = M(x) x4 + R(x) = x13 + x12 + x10 + x8 + x7 + x5 + X4 + x3 + x2 + x
Transmitted Bit Sequence: 1 1 0 1 0 1 1 0 1 1 1 1 1 0 © Jorg Liebeherr, 1998
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Step 4: Error Detection with CRC n
Errors can be expressed as Error Polynomials
n
For example, Sent Message: 1011101 Received Message: 1 1 1 1 0 0 1 ______________________________ Error: 0100100
n
In the example, the Error Polynomial E(x) is given by:
E(x) = x 5 + x 2 © Jorg Liebeherr, 1998
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Step 5: Error Detection Capability of CRC Note: n Errors will not be detected by CRC if
E(x) / P (x) = B (x) + 0, i.e., E(x) contains P(x) as a factor n
Observe the following trade-off: – Large values of n introduce a lot of overhead, but improve the error detection capability.
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Step 5: Error Detection Capability of CRC n
n
All single-bit errors are detected, if P(x) has a factor with at least 2 terms: E(x) = x i If P(x) = (x+1)P’(x), then x i cannot contain (x+1) All double-bit errors are detected if P(x) has a factor with at least 3 terms E(x) = x i+x j
n
= x j (x k+1)
(for k = i-j, j1
Sliding Window Flow Control – Allows transmission of multiple frames – Assigns each frame a k-bit sequence number – Range of sequence number is [0..2k-1], i.e., frames are counted modulo 2k
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n
Sending Window: -
At any instant, the sender is permitted to send frames with sequence numbers in a certain range
-
The range of sequence numbers is called the sending window Frames already transmitted
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Operation of Sliding Window
Window of frames that may be transmitted
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Frame sequence number
© Jorg Liebeherr, 1998
Last frame transmitted Window shrinks as frames are sent
Window expands as acknowledgements are received 32
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n
Receiving Window: -
The receiver maintains a receiving window corresponding to the sequence numbers of frames that are accepted
Frames already received
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Operation of Sliding Window
Window of frames that are accepted by receiver
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Last frame acknowledged Window shrinks as frames are received
Window expands as acknowledgements are sent
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Operation of Sliding Window n
Operations at the sender: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
send a single packet (with sequence number "0"):
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
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Operation of Sliding Window n
Operations at the sender: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
Receive a single acknowledgement (ACK 1)
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
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Operation of Sliding Window n
Operations at the receiver: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
Receive a single packet (with sequence number "0"):
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
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Operation of Sliding Window n
Operations at the receiver: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
Transmit a single acknowledgement ("ACK 1"):
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
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n
n
How is “flow control” achieved? –
Receiver can control the size of the sending window
–
By limiting the size of the sending window data flow from sender to receiver can be limited
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Operation of Sliding Window
Interpretation of ACK N message: –
© Jorg Liebeherr, 1998
Receiver acknowledges all packets until (but not including) sequence number N
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Example Transmitter
Receiver
0 1 2 3 4 5 6 7 0 1 2 3 F0 F1 F2
0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 ACK4 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
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Example Continued Transmitter
Receiver
0 1 2 3 4 5 6 7 0 1 2 3 F3 0 1 2 3 4 5 6 7 0 1 2 3 F4 F5 F6 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
© Jorg Liebeherr, 1998
ACK3 0 1 2 3 4 5 6 7 0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3
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n
Define: –
We use the same parameters for as in Stop-andWait
–
To simplify notation we set: F/C = 1 I =a
–
(Normalization)
W = Maximum window size (iddentical for sender and receiver)
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Analysis of Sliding Windows 1st frame received a a+1
0
Wth frame received a+W
... 0
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CS 457 - Computer Networks
Analysis of Sliding Windows
1
2
W
Receiver
Sender 2a+1 1st ACK received
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n
If the window size is sufficiently large the sender can continuously transmit packets:
n
W ≥ 2a+1: Sender can transmit continuously
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Analysis of Sliding Windows
normalized efficiency = 1 n
W < 2a+1:Sender can transmit W frames every 2a+1 time units normalized efficiency =
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W 1 + 2a 43
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ARQ Error Control n
Two types of errors: – Lost frames – Damaged Frames
n
Most Error Control techniques are based on (1) Error Detection Scheme (e.g., Parity checks, CRC), and (2) Retransmission Scheme
n
Error control schemes that involve error detection and retransmission of lost or corrupted frames are referred to as Automatic Repeat Request (ARQ) error control
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n
All retransmission schemes use all or a subset of the following procedures: – – – –
Receiver sends an acknowledgment (ACK) if a frame is correctly received Receiver sends a negative acknowledgment (NAK) if a frame is not correctly received The sender retransmits a packet if an ACK is not received within a timeout interval All retransmission schemes (using ACK, NAK or both) rely on the use of timers
© Jorg Liebeherr, 1998
University of Virginia
CS 457 - Computer Networks
ARQ Error Control
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n
Note: Once retransmission is used, a sequence number is required for every data packet to prevent duplication of packets
n
Both ACKs and NAKs can be sent as special frames, or be attached to data frames going in the opposite direction (Piggybacking)
01111110
Address
Control
Data
Checksum
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CS 457 - Computer Networks
ARQ Error Control
01111110
0 Send Seq.-No. P/F Recv Seq.-No. For piggybacking © Jorg Liebeherr, 1998
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ARQ Schemes n
n
The most common ARQ retransmission schemes: –
Stop-and-Wait ARQ
–
Go-Back-N ARQ
–
Selective Repeat ARQ
The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme
© Jorg Liebeherr, 1998
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n
Stop-and-Wait ARQ is an addition to the Stop-andWait flow control protocol:
•
Frames have 1-bit sequence numbers (SN = 0 or 1) Receiver sends an ACK (1-SN) if frame SN is correctly received Sender waits for an ACK (1-SN) before transmitting the next frame with sequence number 1-SN If sender does not receive anything before a timeout value expires, it retransmits frame SN
• • •
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University of Virginia
CS 457 - Computer Networks
Stop-and-Wait ARQ
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ACK 1 0 Frame
0 Frame
ACK 1
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Stop-and-Wait ARQ
Timeout
A CS 457 - Computer Networks
1 Frame
ACK 0
ACK 0 ACK 1 ACK 0
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1 Frame
1 Frame
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0 Frame
1 Frame
Lost ACK n
Lost Frame n
Timeout
A
CS 457 - Computer Networks
Stop-and-Wait ARQ
B
B
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n
Go-Back-N uses the sliding window flow control protocol. If no errors occur the operations are identical to Sliding Window
n
Operations: –
A station may send multiple frames as allowed by the window size
–
Receiver sends a NAK i if frame i is in error. After that, the receiver discards all incoming frames until the frame in error was correctly retransmitted
–
If sender receives a NAK i it will retransmit frame i and all packets i+1, i+2,... which have been sent, but not been acknowledged
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Frames 4,5,6 are retransmitted
Lost Frame
A
ACK 6
NAK 4
ACK 3
6 Frame
5 Frame 4 Frame 6 Frame
5 Frame 4 Frame 3 Frame
2 Frame 1 Frame 0 Frame
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Go-Back-N ARQ n
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Go-Back-N ARQ
B Frames 5 and 6 are discarded © Jorg Liebeherr, 1998
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n
Frames 0-4 are retransmitted
Lost ACK Timeout
A
ACK 5
ACK 3
ACK 3
5 Frame 4 Frame 3 Frame
2 Frame 1 Frame 0 Frame
4 Frame 3 Frame
2 Frame 1 Frame 0 Frame
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Go-Back-N ARQ
B Frames 3 and 4 are discarded © Jorg Liebeherr, 1998
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Details Go-Back-N ARQ A n
Frame
B
Scenario 1: A transmits frame i, and B detects error in frame i, but has received frames i-1, i-2,... correctly è B sends NAKi
n
Scenario 2: Frame i is lost or B does not recognize frame i Assume that A sends frame i+1 and B receives it è B sends NAKi, or A will timeout and retransmit frame i and all subsequent frames
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Scenario 3: B receives frame i and sends ACK(i+1) which is lost è B may send an ACK(i+k) later which also acknowledges all frames 2I + Tt
U
n
n
W 1+ 2a
W Tt > 2I + Tt
U = 1− P
= 1
W Tt < 2I + Tt
U=
With Errors
n
W Tt < 2I + Tt
U=
W ⋅(1 − P ) 1 + 2a
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Analysis of ARQ Protocols - Results P=10-3
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n
XModem is a simple data link protocol which used to be popular for communications over modems
n
Xmodem uses Stop-and-Wait ARQ
SOH NUM 1byte
CNUM 1
1
Data
