1. Chapter 5 Peer-to-Peer Protocols and Data Link Layer
PART I:
General Overview
Peer-to-Peer Protocols
Data Link Layer Functions

2. Peer-to-Peer Protocols
Peer-to-Peer protocols: many protocols involve the interaction between two peers
May be implemented at different layers. For example:
Data Link Layer (PPP, HDLC, etc)
Transport Layer (e.g., TCP)
Many networking services can be implemented at different layers: e.g., Error Detection & Retransmission, Sequencing, Timing, Flow Control, Segmentation & Blocking, Multiplexing, Security (Encryption, Authentication) etc
Focus mainly on Data Link Layer (DLC) services (in this part)

3. Service Models
The service model specifies the information transfer service layer-n provides to layer-(n+1)
Important distinction is whether the service is:
Connection-oriented
Connection-less
Also, whether functions are implemented as:
End-to-End
Hop-by-Hop

4. Connection-Oriented Transfer Connection-Oriented Service
Connection Establishment
Connection must be established between layer-(n+1) peers
Layer-n protocol must: Set initial parameters, e.g. sequence numbers; and Allocate resources, e.g. buffers
Message transfer phase
Exchange of SDUs
Disconnect phase
Example: PPP (Layer 2), TCP (Layer 4)
n + 1 peer process
send
receive
Layer n connection-oriented service
SDU

5. Connectionless Transfer Service
No Connection setup, simply send SDU
Each message is 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

6. End-to-End vs. Hop-by-Hop
A service feature can be provided by implementing a protocol
end-to-end across the network
across every hop in the network
Example:
Perform error control at every hop in the network or only between the source and destination?
Perform flow control between every hop in the network or only between source & destination?
There are tradeoffs between the two approaches

7. Example: Error control in Data Link Layer
Physical A B
Packets
Frames 3 2 1
Medium
Data Link operates over wire-like, directly-connected systems
Frames can be corrupted or lost, but arrive in order
Data link performs error-checking & retransmission
Ensures error-free packet transfer between two systems
(a)
(b) 1 2
Physical layer entity
Data link layer entity 3
Network layer entity

8. Example: Error Control in Transport Layer
Transport layer protocol (e.g. TCP) sends segments across network and performs end-to-end error checking & retransmission
Underlying network is assumed to be unreliable
Physical
layer
Data link
End system A
Network
Transport
Messages
Segments B

9. End-to-End Approach Preferred1 2 5
Data
ACK/NAK
Hop-by-Hop 3 4
Hop-by-hop cannot ensure E2E correctness,
Works well when PHY layer is very noisy (e.g., wireless)
But allows faster recovery
Simple inside the network
End-to-End
Very efficient when PHY layer is reliable (e.g., fiber)
More flexible if complexity at the edge 1 2 3 4 5
Data
Data
Data
Data

10. Data Link Layer Data Links Services Framing Error control Flow control
Physical A B
Packets
Frames
Data Links Services
Framing
Error control
Flow control
Multiplexing
Security: Authentication & Encryption
etc
Examples
Point-to-Point Protocol (PPP)
Ethernet (Wired) LAN
IEEE (Wireless) LAN
Directly connected, wire-like
Losses & errors, but no out-of-sequence frames
Applications: Direct Links; Computer LANs

11. Framing Mapping stream of physical layer bits into frames
Mapping frames into bit stream
Frame boundaries can be determined using:
Character Counts
Control Characters
Flags
Framing
received
frames
transmitted

12. Data bits from higher layer
Framing & Bit Stuffing
Flag
Address
Control
Information
FCS
Data bits from higher layer
Frame boundary defined by flag bits (e.g., )
Example: 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

13. Example: Bit stuffing & de-stuffing
Data to be sent
After stuffing and framing
(a)
* *
Data received
After destuffing and deframing
(b)

14. Error Detection & Retransmission
Purpose: to ensure a sequence of information packets is delivered in order and without errors or duplications despite physical layer problems
Technique known as: Automatic Repeat Request (ARQ)
Stop-and-Wait ARQ
Go-Back N ARQ
Selective Repeat ARQ
Basic elements of ARQ:
Error-detecting codes
ACKs (acknowledgment messages)
Timeout mechanisms

15. Segmentation & Blocking
To accommodate arbitrary message size, a layer may have to deal with messages that are too long or too short for its protocol
Segmentation & Reassembly: a layer breaks long messages into smaller blocks and reassembles these at the destination
Grouping & Ungrouping: a layer combines small messages into bigger blocks prior to transfer
1 long message
2 or more blocks
2 or more short messages
1 block

16. Pacing and Flow Control
Messages can be lost if receiving system does not have sufficient buffering to store arriving messages
If destination layer-(n+1) does not retrieve its information fast enough, destination layer-n buffers may overflow
Flow Control provide backpressure mechanisms that control transfer according to availability of buffers at the destination
Examples: TCP and HDLC

17. Timing
Applications involving voice and video generate units of information that are related temporally
Destination application must reconstruct temporal relation in voice/video units
Network transfer introduces delay & jitter
Timing Recovery protocols use timestamps & sequence numbering to control the delay & jitter in delivered information
Examples: RTP & associated protocols in Voice over IP

18. Multiplexing
Multiplexing enables multiple layer-(n+1) users to share a layer-n service
A multiplexing tag is required to identify specific users at the destination
Examples: UDP, IP

19. Privacy, Integrity, & Authentication
Privacy: ensuring that information transferred cannot be read by others
Integrity: ensuring that information is not altered during transfer
Authentication: verifying that sender and/or receiver are who they claim to be
Security protocols provide these services and are discussed in Chapter 11

20. Chapter 5 Peer-to-Peer Protocols and Data Link Layer
ARQ Protocols and Reliable Data Transfer

21. Automatic Repeat Request (ARQ)
Purpose: to ensure a sequence of information packets is delivered in order and without errors or duplications despite transmission errors & losses
Combines error detection and retransmission to ensure data is delivered accurately
We will look at:
Stop-and-Wait ARQ
Go-Back N ARQ
Selective Repeat ARQ
Basic elements of ARQ:
Error-detecting codes
ACKs (acknowledgment messages)
Timeout mechanisms

22. Timer set after each frame transmission
Stop-and-Wait ARQ
Transmit a frame, wait for ACK
Error-free
packet
Packet
Information frame
Transmitter
(Process A)
Receiver
(Process B)
Timer set after each frame transmission
ACK Control frame
CRC
Information
packet
Header
Information frame
Control frame: ACKs
CRC
Header

23. Information Frame:
Header: for control info
CRC Check: covers header & Info bits
CRC design: ensures error detection w high probability
Control Frame:
Header: provides control info
ACKs: acknowledge receipt of a frame or group of frames
NAKs: a frame has been received in error

24. Need for Sequence Numbers
(a) Frame 1 lost A B
Frame 1
ACK
Time
Time-out 2
(b) ACK lost (Duplication of frames) A B
Frame 1
ACK
Time
Time-out 2
For station A, cases (a) & (b) are the same; A acts in same way
But in case (b) the receiving station B accepts Frame 1 twice
Question: How does B know the second frame is also Frame 1?
Answer: Add frame sequence number in header
Errors in the reverse channel
Loss of ACK results in duplicate packet
Use Slast : sequence number of most recent transmitted frame

25. Sequence Numbers
(c) Premature Time-out (Gaps in delivered packet sequence) A B
Frame
ACK 1
Time
Time-out 2
The transmitting station A misinterprets duplicate ACKs
Incorrectly assumes second ACK acknowledges Frame 1
Question: How does the transmitter know second ACK is for Frame 0?
Answer: Add frame sequence number in ACK header
Use Rnext : sequence number of next frame expected by the receiver
Implicitly acknowledges reception of all prior frames
Problem with sequence numbers?

26. 1-Bit Sequence Number is Sufficient
Transmitter A
Receiver B
Slast
Rnext
0 1
Timer
Global State:
(Slast, Rnext)
Error-free frame 0
arrives at receiver
(0,0)
(0,1)
ACK for
frame 0
arrives at
transmitter
ACK for
frame 1
arrives at
transmitter
Error-free frame 1
arrives at receiver
(1,0)
(1,1)

27. Stop-and-Wait ARQ Transmitter Ready state Wait state Receiver
Await request from higher layer for packet transfer
When request arrives, transmit frame with updated Slast and CRC
Go to Wait State
Wait state
Wait for ACK or timer to expire; block requests from higher layer
If timeout expires
retransmit frame and reset timer
If ACK received:
If sequence number is incorrect or if errors detected: ignore ACK
If sequence number is correct (Rnext = Slast +1): accept frame, go to Ready state
Receiver
Always in Ready State
Wait for arrival of new frame
When frame arrives, check for errors
If no errors detected and sequence number is correct (Slast=Rnext), then
accept frame,
update Rnext,
send ACK frame with Rnext,
deliver packet to higher layer
If no errors detected and wrong sequence number
discard frame
send ACK frame with Rnext
If errors detected

28. Stop-and-Wait Efficiency
Stop-and-wait works well over channels with small propagation delay
Inefficient when prop delay is much larger than time to transmit frame
Example:
Frame = 1000 bits over 1.5 Mbps channel
Time from beginning of Tx to receipt of ACK = 40 ms
No of bits that can be tx is 40 ms X 1.5Mbps = 60,000 bits
Efficiency = 1000/60,000 = 1.6%
Large delay-BW prod = prod of bit rate and delay before an action takes place
Delay-BW prod = LOST OPPORTUNITY in terms of Tx bits

29. Stop-and-Wait EfficiencyB
First frame bit enters channel
Last frame bit enters channel
Channel idle while 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
10000-bit 1 Mbps takes 10 ms to transmit
If wait for ACK = 1 ms, then efficiency = 10/11= 91%
If wait for ACK = 20 ms, then efficiency =10/30 = 33%

30. Stop-and-Wait Model tf time tprop tack tproc
frame
tf time A B
tprop
tack
tproc
t0 = total time to transmit 1 frame
bits/info frame
bits/ACK frame
channel transmission rate

31. S&W Efficiency on Error-free channel
bits for header & CRC
Effective transmission rate:
Transmission efficiency:
Effect of
frame overhead
Effect of
Delay-Bandwidth Product
Effect of
ACK frame

32. Example: Impact of Delay-Bandwidth Product
nf=1250 bytes = bits, na=no=25 bytes = 200 bits
2xDelayxBW Efficiency
1 ms
200 km
10 ms
2000 km
100 ms
20000 km
1 sec km
1 Mbps
103
88%
104
49%
105 9%
106 1%
1 Gbps
107
0.1%
108
0.01%
109
0.001%
Stop-and-Wait does not work well for very high speeds or long propagation delays

33. S&W Performance with Transmission Errors
1 successful transmission
i – 1 unsuccessful transmissions
Efficiency:

34. Example: Impact of Bit Error Rate
nf=1250 bytes = bits, na=no=25 bytes = 200 bits
Find efficiency for random bit errors with p=0, 10-6, 10-5, 10-4
1 – Pf Efficiency
p=0
p=10-6
p=10-5
p=10-4
1 Mbps
& 1 ms 1
88%
0.99
86.6%
0.905
79.2%
0.368
32.2%
Bit errors impact performance as nfp approach 1

35. Go-Back-N ARQ Improve Stop-and-Wait efficiency by not waiting!
Keep channel busy by continuing to send frames
Allow a window of Ws non-acknowledged frames
Use m-bit sequence numbering
If ACK for oldest frame arrives before window is exhausted, we can continue transmitting
If window is exhausted, Go Back and retransmit all outstanding frames (starting from the oldest non-acknowledged one)

36. Go-Back-N ARQ Go-Back-4: Time A Bfr
Time 1 2 3 4 5 6
ACK1
Out-of-order frames are not accepted
Go-Back-4:
4 old frames not ACKed; so go back 4 7 8 9
ACK2
ACK3
ACK4
ACK5
ACK6
ACK7
ACK8
ACK9
Rnext
Frame transmissions are pipelined to keep the channel busy
(Pipeline: processing of new task before completion of previous task)
Window size larger than delay-BW product to keep channel full
Frame with errors and subsequent out-of-sequence frames are ignored
Transmitter is forced to go back when window of 4 is exhausted

37. Similarities between S&W and Go-back-N
Time fr
Time-out expires 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
Four frames are outstanding; so go back 4 4 5 6
Go-Back-N ARQ
ACK1
ACK2
ACK3
ACK4
ACK5
ACK6

38. Similarities between S&W and go-back-N in error recovery
S&W: error results in loss of Tx time equal to time-out period
Go-back-N: error results in loss of Tx time equal to the Tx of Ws frames
Error Recovery:
S&W: Receiver is looking for frame with correct Rnext(0 or 1)
Go-back-N: Receiver is looking for frame with correct Rnext (0,1,…, N-1)

39. Similarities between S&W and go-back-N in error recovery
S&W: Transmitter keeps sending the frame every time-out period until acknowledged
Go-back-N: Identify oldest outstanding unacknowledged frame and sent it together with the subsequent Ws-1 frames

40. Need window size long enough to cover round trip timeA B
Time fr
Time-out expires 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
Four frames are outstanding; so go back 4 4 5 6
Go-Back-N ARQ
ACK1
ACK2
ACK3
ACK4
ACK5
ACK6

41. 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 be initiated
Use a timeout with each frame
When timeout expires, resend all outstanding frames

42. Go-Back-N Transmitter & Receiver
Receive Window
Rnext
Frames
received
Timer
Slast
Slast+1
Srecent
Slast+Ws-1
...
Buffers
Send Window
Frames
transmitted
and ACKed
most recent transmission
oldest un-ACKed frame
max Seq # allowed
Receiver will only accept
a frame that is error-free and
that has sequence number Rnext
When such a frame arrives, Rnext is incremented by one, and then the receive window slides forward by one
Receiver sends an ACK with R_next; Tx assumes all prior frames received
Send window: S_last to S_last +W_s -1
Window not allowed to slide beyond S_last +W_s -1
Need New Ack

43. Sliding Window Operation
Transmitter
Slast
Slast+Ws-1
...
Srecent
Frames
transmitted
and ACKed
Send Window
m-bit Sequence Numbering 1 2 i
i + Ws – 1
2m – 1
Slast
send
window
i + 1
Transmitter waits for error-free ACK frame with sequence number Slast
When such ACK frame arrives, Slast is incremented by one, and the send window slides forward by one

44. Maximum Allowable Window Size is Ws = 2m-1
M = 22 = 4, Go-Back - 4:
Transmitter goes back 4 fr fr 1 fr 2 fr 3 fr fr 1 fr 2 fr 3
Time A B
ACK1
ACK2
ACK3
ACK
Receiver has Rnext= 0, but it does not know whether its ACK for frame 0 was received, so it does not know whether this is the old frame 0 or a new frame 4 (4=0)
Rnext A B fr
Time 1 2
ACK1
M = 22 = 4, Go-Back-3:
ACK2
ACK3
Transmitter goes back 3
Receiver has Rnext= 3 , so it rejects the old frame 0
Rnext

45. Bidirectional Links
How can improve communication between A and B if information is flowing in both directions
Both A and B are implementing the Go-Back-N ARQ
So, both will have info to send + ACK (control frames)
Can be piggyback (attach) the control frames on the info frames?

46. Bidirectional Links What is the adv. of this piggybacking?
What if a frame arrives (carrying ACK) is in error?
What if a frame arrives (carrying ACK) is out of sequence?
What happens if the receiver has no info frames to send at this moment? How can be piggyback? Do we wait for info frame to come? Does a timer help here?

47. Required Timeout & Window SizeTf
Tproc
Tprop
Tout
Timeout value: after which window is retransmitted
Ws should be large enough to keep channel busy for at least Tout

48. Applications of Go-Back-N ARQ
HDLC (High-Level Data Link Control): bit-oriented data link control
V.42 modem: error control over telephone modem links

49. Required Window Size for Delay-Bandwidth Product
Frame = 1250 bytes =10,000 bits, R = 1 Mbps
2(tprop + tproc)
2 x Delay x BW
Window
1 ms
1000 bits 1
10 ms
10,000 bits 2
100 ms
100,000 bits 11
1 second
1,000,000 bits
101

50. Efficiency of Go-Back-N
What affects the efficiency of GBN?
Errors on link
Delays in transmitting ack on reverse link (How do you make delays shorter?)
GBN is very efficient when Ws is large enough to keep channel busy, and if channel is error-free
Using Pf frame loss probability, the time to deliver a frame is:
tf if first frame transmission succeeds (1 – Pf )
tf + Wstf /(1-Pf) if the first transmission does not succeed (Pf)
Delay-bandwidth product determines Ws

51. Selective Repeat ARQ
Go-Back-N ARQ inefficient because multiple frames are resent when errors or losses occur
Modify GBN in two ways
Make receiver window lager than 1 to allow for frames out of order
Modify retransmission so that only indiv. frames are retransmitted
Selective Repeat retransmits only an individual frame
Timeout causes individual corresponding frame to be resent
NAK causes retransmission of oldest un-acked frame
Transmit window remains same as in GBN
Receiver maintains a receive window of sequence numbers that can be accepted (Rnext to Rnext + W_R -1),
Error-free, but out-of-sequence frames with sequence numbers within the receive window are buffered
Arrival of frame with Rnext causes window to slide forward by 1 or more

52. What is the value of Rnext at each time instant?
Selective Repeat ARQ A B fr
Time 1 2 3 4 5 6
ACK1 8 9 7 10 11 12
ACK2
NAK2
ACK7
ACK8
ACK9
ACK10
ACK11
ACK12
What is the value of Rnext at each time instant?

53. 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
When timer expires
only corresponding
frame is transmitted
NAK is sent whenever
out of seq. frame is received
NAK carries the value of R_next

54. 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

55. What Ws and Wr are 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

56. 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

57. Why Ws + Wr = 2m works 2m-1 1 2m-1 1 Ws +Wr-1 Slast 2 2 receive window
Transmitter sends frames 0 to Ws-1; send window empty
All arrive at receiver
All ACKs lost
Receiver window starts at {0, …, Wr -1}
Window slides forward to {Ws,…,Ws+Wr-1}
Receiver rejects frame 0 because it is outside receive window
Transmitter resends frame 0
2m-1 1
2m-1 1
Ws +Wr-1
Slast 2 2
receive
window
Rnext Ws
send
window
Ws-1

58. Efficiency of Selective Repeat
Assume Pf frame loss probability, then number of transmissions required to deliver a frame is:
tf /(1-Pf)

59. Example: Impact Bit Error Rate on Selective Repeat
nf=1250 bytes = bits, na=no=25 bytes = 200 bits
Compare S&W, GBN & SR efficiency for random bit errors with p=0, 10-6, 10-5, 10-4 and R= 1 Mbps & 100 ms
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%
Selective Repeat outperforms GBN and S&W, but efficiency drops as error rate increases

60. Summary of ARQ Efficiencies
Assume na and no are negligible relative to nf, and
L = 2(tprop+tproc)R/nf =(Ws-1), then
Selective-Repeat:
Go-Back-N:
Stop-and-Wait:

61. Chapter 5 Peer-to-Peer Protocols and Data Link Layer
Framing

62. Framing Identify beginning an end of a block
Presupposes presence of enough synch. At physical layer to identify bit or byte
Two levels of synch. Accuracy:
Asynch data tx
Example: RS-232 for serial line interface
Tx does not occur at regular intervals
Rx resynch’s at start and end of each 8-bit charcter
Uses start and stop bits
Synch data transmission
Bits tx at regular interval
Phase locked loop to track bits
Synch facilitated by having enough transitions in bit stream

63. Types of Frames
Framing means Delineating (outlining) boundaries between frames
Frames could
Have fixed length
Have variable length
Contain any number of bits
Integer number of characters of certain length (e.g., octets or 32-bit words)

64. Character-based synch
Used when info in frame consists of integer no. of characters
E.g. tx a sequence of printable characters using 8-bit ASCII code
Use special 8 bit characters that don’t correspond to printable characters
All HEX values that are less than 20 correspond to non-printable characters
STX (start of text) has HEX value 02, to begin frame
ETX (end of text) has HEX value 03, to end frame
When would this method fail?

65. Method works if frame contains printable characters only
It fails if frame contains computer data
Frame might contain ETX causing frame to be truncated
To solve this, use byte stuffing
Introduce a new character DLE (data link escapae) with HEX value 10
Use DLE STX for beginning
Use DLE ETX for end
To deal with possible occurrence of DLE STX or DEL ETX inside frame, an extra DLE is stuffed before occurrence of DLE
So DLE’s inside the data will actually have 2 DLE’s
Only case when there is only one DLE is at beginning or end of frame

66. What are the problems with this Character-based sync.?
Variable no. of characters
Possibility for user to inflate BW he is using

67. Flag-Based Synch Used to transfer an arbitrary no of bits in a frame
Flag: delineate frame boundaries
Address: identify secondary station (1 or more octets)
In ABM mode, a station can act as primary or secondary so address changes accordingly
Control: purpose & functions of frame (1 or 2 octets)
Information: contains user data; length not standardized, but implementations impose maximum
Frame Check Sequence: 16- or 32-bit CRC
Flag
Address
Control
Information
FCS

68. Beginning and end specified by 0111110 flag (HEX 7E)
Address
Control
Information
CRC
Protocol
Unnumbered frame
Specifies what kind of packet is contained in the payload, e.g., LCP, NCP, IP, OSI CLNP, IPX
All stations are to
accept the frame
integer # of bytes
Beginning and end specified by flag (HEX 7E)
How to deal with occurrence of flag inside frame?
Use bit stuffing
Look for any 5 consecutive 1’s and insert 0
Rx does the opposite:
indicated bit stuffing
indicate a flag

69. PPP Frame PPP uses similar frame structure as HDLC, except
Flag
Address
Control
Information
CRC
Protocol
Unnumbered frame
Specifies what kind of packet is contained in the payload, e.g., LCP, NCP, IP, OSI CLNP, IPX
All stations are to
accept the frame
integer # of bytes
PPP uses similar frame structure as HDLC, except
Protocol type field
Payload contains an integer number of bytes
PPP uses the same flag, but uses byte stuffing
Problems with PPP byte stuffing
Size of frame varies unpredictably due to byte insertion

70. Byte-Stuffing in PPP Data to be sent 41 7D 42 7E 50 70 46
PPP is character-oriented version of HDLC
Flag is 0x7E ( )
Control escape 0x7D ( )
Any occurrence of flag or control escape inside of frame is replaced with 0x7D followed by
original octet XORed with 0x20 ( )
Data to be sent 41 7D 42 7E 50 70 46
After stuffing and framing 5D 5E

71. CRC-Based Framing Generic Framing Procedure (GFP)
Problem with byte an bit stuffing is that you can not know length of frame ahead of time
Opportunity for malicious users to inflate BW by inserting patterns in a frame
GFP combines frame length with Header Error Control
Idea: We need just to know the beginning of a frame and its length
To know length, look at length indication field
Count number of bytes to know beginning of new frame
Works if there are no errors
To deal with errors, use HEC for error control

72. PLI cHEC Type tHEC GEH GFP payload PLI: payload length indicator
PLI and cHEC are used to delineate a frame
PLI gives length of payload area (and so indicates beginning of next frame)
cHEC contains CRC-16 redundancy check bits for PLI and cHEC
cHEC can correct single errors and detect multiple errors

73. GFP receiver synchronizes to boundary through a 3-state process
Hunt state:
examines 4 bytes at a time to see if they constitute a valid code-word (how to do that?)
If no match, receiver moves by one octet
If it finds a match, it moves to next state
Pre-sync state:
Use the PLI info to determine location of next frame boundary
If target number N of successful frame detections is achieved, receiver moves to sync state
Sync. State: Normal state where
receiver examines PLI
validates using cHEC
Extracts payload and moves to next frame
Single error correcting capability of CRC-16 is activated (can we activate it before?)