Chapter 4 Data Link Layer

1
Chapter 4 Data Link Layer

• Framing
• Error control
• Flow control
• Multiplexing
• Link Maintenance
• Security

2
Services

• Transfers frames across direct connections
• Directly connected (can be wireless), wire-like
• Losses errors, but no out-of-sequence frames
• More detailed services
• Framing (bits ? frames)
• Error control (protection from impairment)
• Flow control
• Multiplexing
• Link Maintenance
• Security Authentication Encryption

3
Data Link Protocols

• Examples
• PPP
• HDLC
• Ethernet LAN
• IEEE 802.11 (WiFi) LAN

4
Framing (Chapter 5.4 in Leon-Garcia)
5
Framing

• Bit stream - frames
• Frame boundaries can be determined using
• Character Counts
• Control Characters
• Framing Bits
• Framing by illegal code

6
Control Characters

• Transmission of printable characters using ASCII
• Octets with HEX value lt 0x20 are nonprintable
• Use control characters STX (start of text)
  0x02 ETX (end of text) 0x03.

• What about transmission of data (including
  non-printable characters)?
• Introduce DLE (data link escape) 0x10
• DLE STX (DLE ETX) used to indicate beginning
  (end) of frame
• Insert extra DLE in front of occurrence of DLE
  STX (DLE ETX) in frame
• All DLEs occur in pairs except at frame
  boundaries.

7
Bit Stuffing

• Frame delineated by flag character
• HDLC uses bit stuffing to prevent occurrence of
  flag 01111110 inside the frame
• Transmitter inserts extra 0 after each
  consecutive five 1s inside the frame
• Receiver checks for five consecutive 1s
• if next bit 0, it is removed
• if next two bits are 10, then flag is detected
• If next two bits are 11, then frame has errors

8
Example Bit stuffing
9
Example Framing in Ethernet

• Ethernet complies to standard IEEE 802.3
• An illegal manchester coding is used for framing.
• A character count is also included in the header.
• All frames have an integral number of bytes. If
  not, the frame is considered to be received in
  error.

10
Error Control Coding(Chapter 3.9 in Leon-Garcia)
11
Error Control

• Two approaches
• Forward error correction (FEC)
• Error detection retransmission (ARQ)
• Add redundancy (admit only codewords with a
  certain pattern)
• Blindspot when channel transforms a codeword
  into another codeword
• (n,k) block code
• There are capacity-achieving codes
• Usually with somewhat high complexity
• When bandwidth is abundant it suffices to use
  simpler codes.

12
Single Parity Check

• nk1

• All codewords have even of 1s
• All error patterns that change an odd number of
  bits are detectable
• Others undetectable
• Redundancy overhead 1/(k 1)

13
Example

• Information (7 bits) (0, 1, 0, 1, 1, 0, 0)

14
Probability of Error
Perror detection failure Pundetectable
error pattern Pall error patterns with even
number of 1s p2(1 p)n-2
p4(1 p)n-4

• Example Evaluate above for n 6, p 0.01

Pundetectable error 0.0014
15
Two-Dimensional Parity Check
16
Error-detecting capability
1, 2, or 3 errors can always be detected Not
all patterns gt4 errors can be detected
17
Hamming Codes

• Class of linear block codes
• Capable of correcting all single-error patterns
• For each m gt 2, there is a (2m1, n-m) Hamming
  code

18
m 3 Hamming Code

• Information bits are b1, b2, b3, b4
• Parity checks (binary addition/multiplication)

b5 b1 b3 b4 b6 b1 b2
b4 b7 b2 b3 b4

• Linearity
• 24 16 codewords

19
Hamming (7,4) code
20
Parity Check Equations

• Rearrange parity check equations

0 b1 b3 b4 b5 0 b1 b2
b4 b6 0 b2 b3 b4
b7

• In matrix form

• All codewords must satisfy these equations
• Note each nonzero 3-tuple appears once as a
  column in check matrix H

21
Hamming Code Error Detection
22
Minimum distance

• Undetectable error pattern must have 3 or more
  bits
• At least 3 bits must be changed to convert one
  codeword into another codeword

Set of n-tuples within distance 1 of b2
Set of n-tuples within distance 1 of b1
Distance 3

• Spheres of distance 1 around each codeword do not
  overlap
• If a single error occurs, the resulting n-tuple
  will be in a unique sphere around the original
  codeword

23
General Hamming Codes

• For m gt 2, the Hamming code is obtained through
  the check matrix H
• Each nonzero m-tuple appears once as a column
• The resulting code corrects all single errors
• Pundetectable error
• P error is a codeword
• ( of codewords with dmin) x pdmin
• Animated example http//www.systems.caltech.edu/EE
  /Faculty/rjm/SAMPLE_20040708.html

24
Hamming Codes Error-correction

• Compute syndrome
• s HR H (b e) Hb He He
• If s 0, then the receiver accepts R as the
  transmitted codeword, find the corresponding
  k-bit message
• If s is nonzero, then an error is detected
• Hamming decoder assumes a single error has
  occurred
• Each single-bit error pattern has a unique
  syndrome
• The receiver matches the syndrome to a single-bit
  error pattern and corrects the appropriate bit

25
Hamming Codes Performance

• Assume bit errors occur independent of each other
  and with probability p

26
Other Error Control Codes

• Good practical codes for error detection
• Internet Check Sums
• CRC Polynomial Codes
• They can detect the vast majority of errors.
• Good codes for error correction
• Turbo codes
• Low-density parity-check (LDPC) codes

27
Internet Checksum

• Several Internet protocols (e.g. IP, TCP, UDP)
  use check bits to detect errors in the header
• A checksum is calculated for header contents and
  included in a special field.
• Treating each 16-bit word in data as an integer,
  find
• x b0 b1 b2 ... bL-1 modulo 216-1
• The checksum is then given by
• bL - x modulo 216-1
• Thus, the headers satisfy the following pattern
• 0 b0 b1 b2 ... bL-1 bL modulo
  216-1

28
Polynomial Codes

• Convenient mathematical formulation of coding
• Polynomials as codewords
• Implemented using shift-register circuits
• Called cyclic redundancy check (CRC) codes
• Excellent for detecting burst errors

29
Automatic Repeat Request (ARQ) (Chapter 5 in
Leon-Garcia)
30
Peer-to-Peer Protocols

• Each layer provides a service to the layer above.
• It does so by executing a peer-to-peer protocol.
• The protocol uses the the services of the layer
  below.

n 1
n 1
n
n
n 1
n 1
31
Service Models

• The service model specifies the manner in which
  information is transferred.
• Connection-oriented
• Connectionless

32
Connection-Oriented

• Connection setup
• Message transfer
• Connection release
• Example TCP, PPP

33
Connectionless Transfer Service

• No setup
• Each message sent independently
• Must provide all address information per message
• Simple quick
• Example UDP, IP

n 1 peer process send
n 1 peer process receive
Layer n connectionless service
SDU
34
Automatic Repeat Request (ARQ)

• Purpose To pass to the receiver every frame
  correctly, only once, in order.
• Bad things can happen Error, arbitrary delay,
  out-of-order arrival, or loss. Aim at very high
  reliability.
• Assume if frames arrive, they arrive in-order for
  now. We save the out-of-order problem for later.
• Basic elements
• Error-detecting code
• ACKs (positive acknowledgments)
• NAKs (negative acknowledgments)
• Timeout mechanism

35
Stop-and-Wait ARQ
Transmit a frame, wait for ACK
Error-free packet
Packet
Information frame
Transmitter
Receiver
Timer set after each frame transmission
Control frame
36
Need for Sequence Numbers

• In cases (a) (b) the transmitting station A
  acts the same way
• But in case (b) the receiving station B accepts
  frame 1 twice
• Question How is the receiver to know the second
  frame is also frame 1?
• Need a sequence number SlastSN of most recent
  transmitted frame.

37
Sequence Numbers
(c) Premature Time-out

• The transmitting station misinterprets duplicate
  ACKs
• Question How is the receiver to know second ACK
  is for frame 0?

• Need SN in ACK RnextSN of next frame expected
  by the receiver.
• Implicitly acknowledges receipt of all prior
  frames.
• What if ACK only if SlastRnext?

38
How many bits for SN?
1-Bit SN Suffices
39
Finite State Machine
40
S/W Efficiency
41
S/W Transmission Time
frame tf time
bits/info frame
bits/ACK frame
channel transmission rate
42
Efficiency on Error-free channel
Overhead bits (header, CRC)
Effective transmission rate
Transmission efficiency
Effect of frame overhead
Effect of Delay-Bandwidth Product
Effect of ACK frame
43
Delay-Bandwidth Product

• nf10,000 bits, nano200 bits

S/W inefficient for very high speeds or long
delays
44
Average Transmission Number

• Proposition Let Pf be the frame error
  probability. Then the average number of
  transmissions per successful frame is 1/ (1Pf ).
• Proof The number of transmissions to first
  correct arrival has geometric distribution.
• E.g., if 1-in-10 gets through, then in average 10
  tries to success.

45
Efficiency in Channel with Errors

• Assuming time-out is equal to t0 (it should be
  larger)

Effect of frame loss

• If bit-error-rate is p, then

46
Go-Back-N

• A sliding-window protocol.
• Keep channel busy by continuing to send frames
• Allow a window of up to Ws outstanding frames
• If ACK for oldest frame arrives before window is
  exhausted, we can continue transmitting
• If window is exhausted, pull back and retransmit
  all outstanding frames

47
Go-Back-N ARQ

• Frame transmission are pipelined to keep the
  channel busy
• Frame with errors and subsequent out-of-sequence
  frames are ignored

48
Choose Window Size gt RTT
49
Go-Back-N with Timeout

• Problem with Go-Back-N as presented
• If frame is lost and source does not have frame
  to send, then window will not be exhausted and
  recovery will not commence
• Use a timeout with each frame
• When timeout expires, resend all outstanding
  frames

50
Go-Back-N Transmitter Receiver
Receiver will only accept error-free frame with
SN Rnext. When the frame arrives Rnext is
incremented by 1, so the receive window slides
forward by 1.
51
Maximum Window Size Ws 2m-1
Example M 22 4, Go-Back 4 is not good.
Example Go-Back-3 is good.
52
ACK Piggybacking in Bidirectional GBN

• In bi-directional communication, ACKs are often
  piggybacked on data frames to reduce overhead.

53
Choice of Timeout Window Size

• Timeout value should allow for
• 2 Tprop Tproc
• A frame begins transmission right before the
  first frame arrives Tf
• Next frame carries the ACK, Tf (piggy-back)
• Thus, timeout gt 2 Tprop 2 Tf Tproc
• Ws should be large enough to keep channel busy
  for Tout

54
Efficiency of Go-Back-N

• if channel is error-free and Ws
  is large enough to keep channel busy
• Assume Pf frame loss probability, time to
  deliver a frame is
• tf if first attempt succeeds
• Tf Wstf /(1-Pf) otherwise go back Ws and try
  again

Delay-bandwidth product determines Ws
55
Improvement over Go-Back-N?

• Go-Back-N repeats multiple frames when a few
  errors or losses occur
• How about retransmitting only an individual
  frame?
• Selective Repeat ARQ
• Timeout pinpoints individual frame
• Receiver maintains a window of acceptable SN
• Error-free, but out-of-sequence frames with SN
  within the receive window are buffered
• Arrival of Rnext causes window to slide forward
  by 1 or more
• NAK causes retransmission of oldest un-acked frame

56
Selective Repeat ARQ
57
Selective Repeat ARQ
58
Send Receive Windows
Transmitter
Receiver
59
What size Ws and Wr allowed?

• Example M224, Ws3, Wr3

60
Ws Wr 2m is maximum allowed

• Example M224, Ws2, Wr2

61
Why Ws Wr 2m works

• The number of bits, m, is enough to label all
  outstanding frames.
• Usually, Ws Wr 2m-1

0
0
1
2m-1
1
2m-1
Ws Wr-1
Slast
2
2
receive window
Rnext
Ws
send window
Ws-1
62
Efficiency of Selective Repeat

• of transmissions required to deliver a frame
  is
• tf / (1-Pf)

63
Example Impact Bit Error Rate on Selective
Repeat
nf10,000 bits, nano200 bits p 0, 10-6, 10-5,
10-4 and R 1 Mbps 100 ms 1 Mbps x 100 ms
100000 bits 10 frames ? Use Ws 11

• GBN gtgt SW for large delay-bandwidth product
• GBN becomes inefficient as error rate increases
• SR is the best. Efficiency drops as error rate
  increases

64
Comparison of ARQ Efficiencies
Assume na, no ltlt nf, and L 2(tproptproc)R/nf
(Ws-1).
Selective-Repeat
Go-Back-N
For Pf0, SR GBN same
For Pf?1, GBN SW same
Stop-and-Wait
65
ARQ Efficiencies
66
Standard Data Link Layer ProtocolsPPP HDLC
(Chapter 5.5-6 in Leon-Garcia)
67
DLL
Packet
Network layer
Network layer
Frame
Data link layer
Data link layer
Physical layer
Physical layer
68
PPP Point-to-Point Protocol

• A data link layer protocol.
• Encapsulating IP packets over point-to-point
  links.
• Router-router
• Dial-up to router (PC to Internet service
  provider (ISP))
• Functions
• Provides Framing and Error Detection
• Link Control Protocols
• Set up, configure, testing, maintain, terminate
• Authentication Password Authentication Protocol,
  etc.
• Network Control Protocols
• Configure network layer protocols
• E.g., IP, IPX (Novell), Appletalk

69
PPP Frame Format

• Can support multiple network protocols
  simultaneously
• Specifies what kind of packet is contained in
  the payload

70
High-Level Data Link Control (HDLC)

• Bit-oriented data link control
• Derived from IBM Synchronous Data Link Control
  (SDLC)

71
HDLC Data Transfer Modes

• Normal Response Mode
• Used in polling multidrop lines

• Asynchronous Balanced Mode
• Used in full-duplex point-to-point links

• Mode is selected during connection establishment

72
HDLC Frame Format

• Control field gives HDLC its functionality
• Codes in fields have specific meanings and uses
• Flag delineate frame boundaries
• Address identify secondary station (1 or more
  octets)
• Control purpose functions of frame (1 or 2
  octets)
• Information user data length not standardized
• Frame Check Sequence 16- or 32-bit CRC

73
Control Field Format

• S Supervisory Function Bits
• N(R) Receive Sequence Number
• N(S) Send Sequence Number
• M Unnumbered Function Bits
• P/F Poll/final bit used in interaction between
  primary and secondary

Note The information frames and supervisory
frames allow HDLC to implement Stop-and-Wait,
Go-Back-N, and Selective Repeat ARQ. Note The
unnumbered frames implement control functions.
74
Example HDLC Using Polling
Address of secondary
N(S)
N(R)
A polls B RRreceive ready
B sends 3 info frames
N(R)
A rejects fr1
A polls C
C nothing to send
A polls B, requests selective retrans. fr1
B resends fr1 Then fr 3 4
A send info fr0 to B, ACKs up to 4
75
HDLC Flow Control

• Flow control prevents transmitter from
  overrunning receiver buffers.
• Receiver can control flow by delaying
  acknowledgement messages.
• Receiver can also use supervisory frames to
  explicitly control transmitter
• Receive Not Ready (RNR) Receive Ready (RR)