FFrraammiinngg
DDaattaa lliinnkk llaayyeerr nneeeeddss ttoo ppaacckk bbiittss iinnttoo ffrraammeess,, ssoo
tthhaatt eeaacchh ffrraammee iiss ddiissttiinngguuiisshhaabbllee ffrroomm aannootthheerr
Separate a message from one source to a
destination, or from other messages to other
destinations, by adding a sender address and a
destination address
Fixed-size framing: ATM (Chapter 18)
Variable-size framing
Need a way to define the end of the frame and the
beginning of the next
Character-oriented approach and bit-oriented approach
Dr. Md. Abdur Razzaque
Green Networking Research Group
Dept. of Computer Science and Engineering, University of Dhaka

CChhaarraacctteerr--OOrriieenntteedd PPrroottooccoollss
FFrraammee ssttrruuccttuurree
• BByyttee ssttuuffffiinngg:: pprroocceessss ooff aaddddiinngg 11 eexxttrraa bbyyttee wwhheenneevveerr
tthheerree iiss aa ffllaagg oorr eessccaappee cchhaarraacctteerr iinn tthhee tteexxtt
Universal Coding!!!

BBiitt--OOrriieenntteedd PPrroottooccoollss
FFrraammee ssttrruuccttuurree
• Bit stuffing: process of adding one extra 0 whenever five consecutive
1s follow a 0 in the data

FFllooww aanndd EErrrroorr CCoonnttrrooll
Data link control = flow control + error control
Flow control refers to a set of procedures used to restrict the amount
of data that the sender can send before waiting for acknowledgement
Error control in the data link layer is based on automatic repeat
request (ARQ), which is the retransmission of data
ACK, NAK(Negative ACK), Piggybacking (ACKs and NAKs in data
frames)

NNooiisseelleessss CChhaannnneellss:: SSiimmpplleesstt
PPrroottooccooll
Simplest protocol with no flow or error control

SSiimmpplleesstt PPrroottooccooll
Sender-site algorithm
• Receiver-site algorithm

SSttoopp--aanndd--WWaaiitt PPrroottooccooll
Simple tokens of ACK and flow control added

SSttoopp--aanndd--WWaaiitt PPrroottooccooll
• Receiver-site algorithm

SSttoopp--aanndd--WWaaiitt PPrroottooccooll::
EExxaammppllee

NNooiissyy CChhaannnneellss:: SSttoopp--aanndd--WWaaiitt
AARRQQ
Stop-and-wait Automatic Repeat Request (ARQ)
Error correction in Stop-and-Wait ARQ is done by
keeping a copy of the sent frame and
retransmitting of the frame when the timer expires
In Stop-and-Wait ARQ, we use sequence numbers
to number the frames. The sequence numbers are
based on modulo-2 arithmetic
Acknowledgment number always announces in
modulo-2 arithmetic the sequence number of the
next frame expected.

SSttoopp--aanndd--WWaaiitt AARRQQ

Receiver-site algorithm

SSttoopp--aanndd--WWaaiitt AARRQQ:: EExxaammppllee

GGoo--BBaacckk--NN AARRQQ
Pipelining improves the efficiency of the transmission
In the Go-Back-N Protocol, the sequence numbers are modulo 2m,
where m is the size of the sequence number field in bits
The send window is an abstract concept defining an imaginary box of
size 2m − 1 with three variables: Sf, Sn, and Ssize
The send window can slide one or more slots when a valid
acknowledgment arrives.

Receive window for Go-Back-N ARQ
The receive window is an abstract concept defining an imaginary box
of size 1 with one single variable Rn. The window slides when a
correct frame has arrived; sliding occurs one slot at a time.

Sliding windows, Timers, ACK, Resending a frame

Go-Back-NN AARRQQ:: SSeenndd WWiinnddooww SSiizzee
In Go-Back-N ARQ, the size of the send window must be less than 2m;
the size of the receiver window is always 1
Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the
size of the send window is 1

GGoo--BBaacckk--NN AARRQQ:: SSeennddeerr AAllggoorriitthhmm

GGoo--BBaacckk--NN AARRQQ:: RReecceeiivveerr AAllggoorriitthhmm

GGoo--BBaacckk--NN AARRQQ:: EExxaammppllee 11

GGoo--BBaacckk--NN AARRQQ:: EExxaammppllee 22

SSeelleeccttiivvee RReeppeeaatt AARRQQ
Sender window size
• Receive window size

SSeelleeccttiivvee RReeppeeaatt AARRQQ

SSeelleeccttiivvee RReeppeeaatt AARRQQ:: WWiinnddooww
SSiizzee The size of the sender and receiver window must be at most one-half
of 2m

SSeelleeccttiivvee RReeppeeaatt AARRQQ:: SSeennddeerr--SSiittee
AAllggoorriitthhmm

Selective RReeppeeaatt AARRQQ:: RReecceeiivveerr--SSiittee AAllggoorriitthhmm

SSeelleeccttiivvee RReeppeeaatt AARRQQ:: EExxaammppllee

PPiiggggyybbaacckkiinngg
To improve the efficiency of the bidirectional protocols
Piggybacking in Go-Back-N ARQ

HHDDLLCC
• High-level Data Link Control
• Two common transfer mode: normal response mode (NRM) and
asynchronous balanced mode (ABM)

HHDDLLCC:: FFrraammeess
• I(information)-frames, S(supervisory)-frames, U(unnumbered frame)-
frames
• Flag field: 01111110 to identify both the beginning and the end of a
frame and serve as synchronization pattern for receiver
• FCS field: 2- or 4-byte ITU-T CRC for error detection

HHDDLLCC:: FFrraammeess • Control Field: 1- or 2-byte segment of the frame used for flow and
error control
• Determine the type of frame and define its functionality
• Control field for I-frame: P/F (poll/final bit for primary/secondary)

• Control field for S-frame
• Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective
reject (SREJ)

• Control field for U-frame

HHDDLLCC:: EExxaammppllee 11
• Connection and disconnection

HHDDLLCC:: EExxaammppllee 22 • Piggybacking without error

HHDDLLCC:: EExxaammppllee 33
• Piggybacking with error

HHDDLLCC:: BBiitt SSttuuffffiinngg aanndd
UUnnssttuuffffiinngg

PPooiinntt--ttoo--PPooiinntt PPrroottooccooll:: PPPPPP
One of the most common protocols for point-to-point access
Many Internet users who need to connect their home computer to the
server of an Internet service provider use PPP
A point-to-point link protocol is required to control and manage the
transfer of data
PPP defines/provides
the format of the frame to be exchanged between devices
how two devices negotiate the establishment of the link and the exchange of data
how network layer data are encapsulated in the data link frame
how two devices can authenticate each other
multiple network layer services
connection over multiple links
Network address configuration
But, several services are missing for simplicity
no flow control, simple error control (detection and discard), no sophisticate
addressing for multipoint configuration

PPPPPP FFrraammee • Flag: 01111110 the same as HDLC, but it treated as a byte because of
PPP is a byte-oriented protocol
• Address: 11111111 (broadcast address)
• Control: No need because PPP has no flow control and limited error
control
• PPP is a byte-oriented protocol using byte stuffing with the escape byte
01111101

PPPPPP:: TTrraannssiittiioonn SSttaatteess

PPPPPP:: MMuullttiipplleexxiinngg
• PPP uses another set of other protocols to establish the link, authenticate
the parties, and carry the network layer data
• Three sets of protocols defined for powerful PPP: LCP, two APs, several
NCPs

LLCCPP:: EEnnccaappssuullaatteedd iinn aa FFrraammee

LLCCPP:: CCoommmmoonn OOppttiioonnss
Options are inserted in the information field
of the configuration packets

AAuutthheennttiiccaattiioonn
Authentication means validating the
identity of a user who needs to access
PPP is designed for use over dial-up links
Þ User authentication is necessary
PPP has two protocols for authentication
Password Authentication Protocol (PAP)
Challenge Handshake Authentication Protocol
(CHAP)

PPaasssswwoorrdd AAuutthheennttiiccaattiioonn
PPrroottooccooll (PPAAPP)

CChhaalllleennggee HHaannddsshhaakkee
AAuutthheennttiiccaattiioonn PPrroottooccooll (CCHHAAPP)
• Three-way hand-shaking authentication protocol with greater security
than PAP

NNeettwwoorrkk CCoonnttrrooll PPrroottooccooll:: NNCCPP
PPP is a multiple-network layer protocol.
It can carry a network data packet from protocols defined by the
Internet, OSI, Xerox, DECnet, AppleTalk, Novel
IPCP (IP Control Protocol)
Configures the link used to carry IP packets in the Internet

IIPPCCPP PPaacckkeett
IP Datagram in a PPP frame

MMuullttiippllee PPPPPP

EExxaammppllee (11)

EExxaammppllee (22)

Data link control

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