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
layer
Physical A B
Packets
Frames
Examples
PPP
HDLC
Ethernet LAN
IEEE (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
Framing
received
frames
transmitted

6. Control Characters Data to be sent A DLE B ETX STX E
Transmission of printable characters using ASCII
Octets with HEX value < 0x20 are nonprintable
Use control characters: STX (start of text) = 0x02; ETX (end of text) = 0x03.
Data to be sent A
DLE B
ETX
STX E
After stuffing and framing
DLE B
ETX
STX A E
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
Address
Control
Information
FCS
HDLC frame
any number of bits
Frame delineated by flag character
HDLC uses bit stuffing to prevent occurrence of flag 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
Data to be sent
After stuffing and framing
(a)
* *
Data received
After destuffing and deframing
(b)

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.
Channel
Encoder
b1…bk
Decoder
c1…cn

12. Single Parity Check All codewords have even # of 1s n=k+1
Information bits: b1, b2, b3, …, bk
Check Bit: bk+1= b1+ b2+ b3+ …+ bk modulo 2
Codeword: (b1, b2, b3, …, bk,, bk+!)
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. P[undetectable error] = 0.0014
Probability of Error
P[error detection failure]
= P[undetectable error pattern]
= P[all error patterns with even number of 1s]
= p2(1 – p)n p4(1 – p)n-4 + … n 2 n 4
Example: Evaluate above for n = 6, p = 0.01
P[undetectable error] =

15. Two-Dimensional Parity Check
column check bit
row check bits

16. Error-detecting capability
Arrows indicate failed check bits
Two errors
One error
Three errors
Four errors (undetectable)
1, 2, or 3 errors can always be detected; Not all patterns >4 errors can be detected

17. Hamming Codes Class of linear block codes
Capable of correcting all single-error patterns
For each m > 2, there is a (2m–1, n-m) Hamming code m
n = 2m–1
k = n–m
Code rate k/n 3 7 4
4/7 15 11
11/15 5 31 26
26/31

18. m = 3 Hamming Code Information bits are b1, b2, b3, b4
Parity checks (binary addition/multiplication)
b5 = b b3 + b4
b6 = b1 + b b4
b7 = b2 + b3 + b4
Linearity
24 = 16 codewords

19. Hamming (7,4) code Information Codeword Weight b1 b2 b3 b4
b1 b2 b3 b4 b5 b6 b7
w(b) 4 3 7

20. Parity Check Equations
Rearrange parity check equations:
0 = b b3 + b4 + b5
0 = b1 + b b b6
0 = b2 + b3 + b b7
In matrix form: b1 b2
0 = b3
0 = b4 = H bt = 0
0 = b5 b6 b7
All codewords must satisfy these equations
Note: each nonzero 3-tuple appears once as a column in check matrix H

21. Hamming Code: Error Detection1
s = H e = =
Single error detected
s = H e = = =
Double error detected
s = H e = = = 0
Triple error not detected

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 b1 b2 o
Set of n-tuples within distance 1 of b1
Set of n-tuples within distance 1 of b2
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 > 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
P[undetectable error]
= P[ error is a codeword]
≈ (# of codewords with dmin) x pdmin
Animated example

24. Hamming Codes: Error-correctionb e R +
(Receiver)
(Transmitter)
Error pattern
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
s = H R = He
s = 0
No errors in
transmission
Undetectable
errors
Correctable
Uncorrectable
(1–p)7
7p3
1–3p 3p 7p
7p(1–3p)
21p2

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 + b 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 + b 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
Reg 0 +
Encoder for g(x) = x3 + x + 1
Reg 1
Reg 2
0,0,0,i0,i1,i2,i3
g0 = 1
g1 = 1
g3 = 1

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. Layer n connection-oriented service
Connection setup
Message transfer
Connection release
Example: TCP, PPP
n + 1 peer process
send
receive
Layer n connection-oriented service
SDU

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
SDU
Layer n connectionless service

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. Timer set after each frame transmission
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
(a) Frame 1 lost A B
Frame 1
ACK
Time
Time-out 2
(b) ACK lost A B
Frame 1
ACK
Time
Time-out 2
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: Slast=SN of most recent transmitted frame.

37. Sequence Numbers What if ACK only if Slast=Rnext?
(c) Premature Time-out A B
Frame
ACK 1
Time
Time-out 2
The transmitting station misinterprets duplicate ACKs
Question: How is the receiver to know second ACK is for frame 0?
Need SN in ACK: Rnext=SN of next frame expected by the receiver.
Implicitly acknowledges receipt of all prior frames.
What if ACK only if Slast=Rnext?

38. How many bits for SN? 1-Bit SN Suffices Rnext Slast Timer
Transmitter A
Receiver B
Slast
Rnext
0 1
Timer

39. Finite State Machine Slast Transmitter A Receiver B Rnext Frame 0
(0,0)
(0,1)
(1,0)
(1,1)
Global State:
(Slast, Rnext)
Error-free frame 0
arrives
ACK 0
ACK 1
Error-free frame 1
Frame 0
lost/error
Frame 1

40. S/W Efficiency t Last frame bit enters channel ACK arrives
First frame bit enters channel
Last frame bit enters channel
Transmitter waits for ACK
Last frame bit arrives at receiver
Receiver processes frame and
prepares ACK
ACK arrives
First frame bit arrives at receiver t

41. S/W Transmission Time tprop tack tprocB
tprop
tack
tproc
t0 = total time to transmit 1 frame if no error
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
nf=10,000 bits, na=no=200 bits
2xDelayxBW Efficiency
1 ms
200 km
10 ms
2000 km
100 ms
20,000 km
1 sec
200,000 km
1 Mbps
103
88%
104
49%
105 9%
106 1%
1 Gbps
107
0.1%
108
0.01%
109
0.001%
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/ (1–Pf ).
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 Go-Back-4: Time A Bfr
Time 1 2 3 4 5 6
ACK1
out of sequence frames
Go-Back-4:
4 frames are outstanding; so go back 4 7 8 9
ACK2
ACK3
ACK4
ACK5
ACK6
ACK7
ACK8
ACK9
Rnext
Frame transmission are pipelined to keep the channel busy
Frame with errors and subsequent out-of-sequence frames are ignored

48. Choose Window Size > RTTA B
Time fr
Time-out 1
ACK1
Stop-and-Wait ARQ
Receiver is looking for Rnext=0 A B fr
Time 1 2 3
Receiver is looking for Rnext=0
Out-of-sequence frames
If window exhausted, go back N 4 5 6
Go-Back-N ARQ
ACK1
ACK2
ACK3
ACK4
ACK5
ACK6

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
Timer
Slast
Slast+1
Srecent
Slast+Ws-1
Transmitter
...
Buffers
Send Window (size Ws)
Frames
transmitted
and ACKed
most recent transmission
oldest un-ACKed frame
max SN allowed
Receiver
Receive Window (size 1)
Rnext
Frames
received
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. A B fr
Time 1 2 3
ACK1
ACK
ACK2
ACK3
Transmitter goes back 4
Receiver has Rnext= 0, but it does not know whether this is the old frame 0 or a new frame 0
Rnext
Example: Go-Back-3 is good. A B fr
Time 1 2
ACK1
ACK2
ACK3
Receiver has Rnext= 3 , so it rejects the old frame 0
Rnext

52. ACK Piggybacking in Bidirectional GBN
In bi-directional communication, ACKs are often piggybacked on data frames to reduce overhead.
Transmitter
Receiver
SArecent RA next
SBrecent RB next

53. Choice of Timeout & Window SizeTf
Tproc
Tprop
Tout
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 > 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 A B fr Time 1 2 3 4 5 6 ACK1 8 9 7 10 11 12 ACK2
Time 1 2 3 4 5 6
ACK1 8 9 7 10 11 12
ACK2
NAK2
ACK7
ACK8
ACK9
ACK10
ACK11
ACK12

57. Selective Repeat ARQ Transmitter Receiver Buffers Slast Slast+ Ws-1
...
Send Window
Srecent
Frames
transmitted
and ACKed
Timer
Slast+ 1
Slast+ Ws - 1
Frames
received
Receiver
Receive Window
Rnext
Rnext + Wr-1
Rnext+ 1
Rnext+ 2
Rnext+ Wr- 1
...
Buffers
max Seq # accepted

58. Send & Receive Windows 1 2 i i + Ws – 1 2m-1 Slast send window i + 1
Transmitter
Receiver 1 2 i
i + Ws – 1
2m-1
Slast
send
window
i + 1
Moves k forward when ACK
arrives with Rnext = Slast + k
k = 1, …, Ws-1 1 2 i
j + Wr – 1
2m-1
Rnext
receive
window j
Moves forward by 1 or more when frame arrives with Seq. # = Rnext

59. What size Ws and Wr allowed?
Example: M=22=4, Ws=3, Wr=3 A B
fr0
Time
fr1
fr2
ACK1
ACK2
ACK3
Frame 0 resent
{0,1,2}
{1,2}
{2}
{.}
Send Window
{1,2,3}
Receive Window
{2,3,0}
{3,0,1}
Old frame 0 accepted as a
new frame because it falls
in the receive window

60. Ws + Wr = 2m is maximum allowed
Example: M=22=4, Ws=2, Wr=2 A B
fr0
Time
fr1
ACK1
ACK2
Frame 0 resent
{0,1}
{1}
{.}
Send Window
{1,2}
Receive Window
{2,3}
Old frame 0 rejected because it falls outside the receive window

61. Why Ws + Wr = 2m works
The number of bits, m, is enough to label all outstanding frames.
Usually, Ws = Wr = 2m-1
2m-1 1
2m-1 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
nf=10,000 bits, na=no=200 bits
p = 0, 10-6, 10-5, 10-4 and R = 1 Mbps & 100 ms
1 Mbps x 100 ms = bits = 10 frames → Use Ws = 11
Efficiency
10-6
10-5
10-4
S&W
8.9%
8.8%
8.0%
3.3%
GBN
98%
88.2%
45.4%
4.9% SR
97%
89%
36%
GBN >> S&W 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 << nf, and L = 2(tprop+tproc)R/nf =(Ws-1).
Selective-Repeat:
Go-Back-N:
For Pf≈0, SR & GBN same
For Pf→1, GBN & SW same
Stop-and-Wait:

65. Delay-Bandwidth product = 10, 100
ARQ Efficiencies p
Delay-Bandwidth product = 10, 100

66. Standard Data Link Layer Protocols: PPP & HDLC (Chapter 5
Standard Data Link Layer Protocols: PPP & HDLC (Chapter in Leon-Garcia)

67. DLL “Packet” “Frame” Network Network layer layer Data link Data link
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 CRC 16 or CRC 32 All stations are to accept the frame
Flag
Address
Control
Information
FCS
Protocol
CRC 16 or CRC 32
1 or 2
variable
2 or 4
All stations are to
accept the frame
HDLC Unnumbered frame
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
Primary
Commands
Responses
Secondary
Asynchronous Balanced Mode
Used in full-duplex point-to-point links
Primary
Secondary
Commands
Responses
Mode is selected during connection establishment

72. HDLC Frame Format Flag Address Control Information FCS
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 N(S) N(R) P/F 1 2-4 5 6-8 Information Frame
N(S)
N(R)
P/F 1
2-4 5
6-8
Information Frame
Supervisory Frame S
Unnumbered Frame M
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
Primary A
Secondaries B, C
B, RR, 0, P
B, I, 0, 0
B, I, 1, 0
B, I, 2, 0,F X
B, SREJ, 1
C, RR, 0, P
C, RR, 0, F
B, SREJ, 1,P
B, I, 3, 0
B, I, 4, 0, F
B, I, 0, 5
Time
A polls B
RR=receive ready
N(S)
N(R)
N(R)
B sends 3 info
frames
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) I3 I4 I5 I6
RNR5
RR6