9-HDLC-new

8/12/2019 9-HDLC-new

1/38

8/12/2019 9-HDLC-new

2/38

Data Link Protocols

Directly connected, wire-like Losses & errors, but no out-of-

sequence frames

Applications: Direct Links;LANs; Connections acrossWANs

Data Links Services Framing

Error control

Flow control

Multiplexing

Link Maintenance

Security: Authentication &Encryption

Examples

PPP

HDLC

Ethernet LAN

IEEE 802.11 (Wi Fi) LAN

Data link

layer

Physical

layer

Physical

layer

Data link

layerA B

Packets Packets

Frames

8/12/2019 9-HDLC-new

3/38

Framing

8/12/2019 9-HDLC-new

4/38

Framing

Mapping stream ofphysical layer bits into

frames

Mapping frames into

bit stream

Frame boundaries can

be determined using:

Character Counts Control Characters

Flags

CRC Checks

0

110110111

Framing

received

frames

0111110101

transmitted

frames

8/12/2019 9-HDLC-new

5/38

Data to be sent

A DLE B ETX DLE STX E

After stuffing and framing

DLE DLE B ETX DLE DLE STXDLE STX A E DLE ETX

Character-Oriented Framing

Frames consist of integer number of bytes Asynchronous transmission systems using ASCII to transmit printable

characters

Octets with HEX value

8/12/2019 9-HDLC-new

6/38

Byte stuffing is the process of adding 1 extra byte

whenever there is a flag or escape character in the text.

Note

8/12/2019 9-HDLC-new

7/38

Bit stuffing is the process of adding one extra 0 whenever

five consecutive 1s follow a 0 in the data, so that the

receiver does not mistakethe pattern 01111110 for a flag.

Note

8/12/2019 9-HDLC-new

8/38

Framing & Bit Stuffing

Frame delineated by flag character HDLC uses bit stuffingto prevent occurrence of flag

01111110 inside the frame

Transmitter inserts extra 0 after each consecutive

five 1s insidethe 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

Flag FlagAddress Control Information FCS

HDLC frame

any number of bits

8/12/2019 9-HDLC-new

9/38

11.9

Figure 11.4 Bit stuffing and unstuffing

8/12/2019 9-HDLC-new

10/38

0110111111111100

Data to be sent


0111111001101111101111100001111110

(a)

*000111011111-11111-110*

Data received

After destuffing and deframing

01111110000111011111011111011001111110

(b)

Example: Bit stuffing & de-

stuffing

8/12/2019 9-HDLC-new

11/38

PPP Frame

PPP uses similar frame structure as HDLC, except

Protocol type field

Payload contains an integernumber 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

Malicious users can inflate bandwidth by inserting 7D & 7E

Flag FlagAddress Control Information CRCProtocol01111110 011111101111111 00000011

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

8/12/2019 9-HDLC-new

12/38

PPP is character-oriented version of HDLC Flag is 0x7E (01111110)

Control escape 0x7D (01111101)

Any occurrence of flag or control escape inside of frame isreplaced with 0x7D followed by

original octet XORed with 0x20 (00100000)

Byte-Stuffing in PPP

Data to be sent

41 7D 42 7E 50 70 46


5D 42 7D 5E 50 70 467E 41 7D 7E

8/12/2019 9-HDLC-new

13/38

Receiver removes flags.

In the frame, when it comes across a control

character ( 7D),it is immediately dropped and

the next octet is Ex-Or ed with 0x20 and theresult is the data byte.

8/12/2019 9-HDLC-new

14/38

High -Level Data Lin k Con tro l

8/12/2019 9-HDLC-new

15/38

High-Level Data Link Control

(HDLC)

Bit-oriented data link control

Derived from IBM Synchronous Data Link

Control (SDLC)

Related to Link Access Procedure Balanced(LAPB)

LAPD in ISDN

LAPM in cellular telephone signaling

8/12/2019 9-HDLC-new

16/38

Physical

layer

Data link

layerData link

layer

Network

layer

DLSDU DLSDU

Network

layer

Physical

layer

DLPDU

NLPDU

Packet

Frame

DLSAP DLSAP

8/12/2019 9-HDLC-new

17/38

Normal Response Mode

Used in polling multidrop lines

Asynchronous Balanced Mode

Used in full-duplex point-to-point links

HDLC Data Transfer Modes

Primary

Commands

Responses

Secondary Secondary Secondary

Primary

Secondary

Commands Responses

Primary

Secondary

CommandsResponses

Mode is selected during connection establishment

8/12/2019 9-HDLC-new

18/38

HDLC Frame Format

Control field gives HDLC its functionality

Codes in fields have specific meanings and uses Flag: delineate frame boundaries

Address: identify secondarystation (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 FlagAddress Control Information FCS

8/12/2019 9-HDLC-new

19/38

8/12/2019 9-HDLC-new

20/38

Control Field Format

0 N(S) N(R)P/F

1 2-4 5 6-8

Information Frame

N(R)P/F

Supervisory Frame

1 0 S S

Unnumbered Frame

P/F1 1 M M M M 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 interactionbetween primary and secondary

8/12/2019 9-HDLC-new

21/38

Information frames

Each I-frame contains sequence number N(S)

Positive ACK piggybacked

N(R)=Sequence number of nextframe expected

acknowledges all frames up to and including N(R)-1

3 or 7 bit sequence numbering

Maximum window sizes 7 or 127

Poll/Final Bit

NRM: Primary polls station by setting P=1; Secondary

sets F=1 in lastI-frame in response

Primaries and secondaries always interact viapairedP/F

bits

8/12/2019 9-HDLC-new

22/38

8/12/2019 9-HDLC-new

23/38


0 N(S) N(R)P/F

1 2-4 5 6-8

Information Frame

N(R)P/F

Supervisory Frame

1 0 S S

Unnumbered Frame

P/F1 1 M M M M M






8/12/2019 9-HDLC-new

24/38

Supervisory frames

Used for error (ACK, NAK) and flow control (Dont Send): Receive Ready(RR), SS=00

ACKs frames up to N(R)-1 when piggyback not available

REJECT(REJ), SS=01

Negative ACK indicating N(R) is first frame not receivedcorrectly. Transmitter must resend N(R) and later frames

Receive Not Ready(RNR), SS=10 ACKs frame N(R)-1 & requests that no more I-frames be sent

Selective REJECT(SREJ), SS=11 Negative ACK for N(R) requesting that N(R) be selectively

retransmitted

8/12/2019 9-HDLC-new

25/38


0 N(S) N(R)P/F

1 2-4 5 6-8

Information Frame

N(R)P/F

Supervisory Frame

1 0 S S

Unnumbered Frame

P/F1 1 M M M M M






8/12/2019 9-HDLC-new

26/38

Unnumbered Frames

Setting of Modes: SABM: Set Asynchronous Balanced Mode

UA: acknowledges acceptance of mode setting commands

DISC: terminates logical link connectio

Information Transfer between stations

UI: Unnumbered information

Recovery used when normal error/flow control fails

FRMR: frame with correct FCS but impossible semantics

RSET: indicates sending station is resetting sequence

numbers

XID: exchange station id and characteristics

8/12/2019 9-HDLC-new

27/38

11.27

Table 11.1 U-frame control command and response

Example 11 9

8/12/2019 9-HDLC-new

28/38

11.28

Figure 11.29 shows how U-frames can be used for

connection establishment and connection release. Node A

asks for a connection with a set asynchronous balanced

mode (SABM) frame; node B gives a positive response with

an unnumbered acknowledgment (UA) frame. After thesetwo exchanges, data can be transferred between the two

nodes (not shown in the figure). After data transfer, node A

sends a DISC (disconnect) frame to release the connection;

it is confirmed by node B responding with a UA(unnumbered acknowledgment).

Example 11.9

C ti E t bli h t &

8/12/2019 9-HDLC-new

29/38

Connection Establishment &

Release

Supervisory frames used to establish and releasedata link connection

In HDLC

Set Asynchronous Balanced Mode (SABM)

Disconnect (DISC)

Unnumbered Acknowledgment (UA)

SABM UAUA DISCData

transfer

8/12/2019 9-HDLC-new

30/38

11.30

Figure 11.29 Example of connection and disconnection

Example: HDLC using NRM

8/12/2019 9-HDLC-new

31/38

Primary A Secondaries B, C

B, RR, 0, P

B, I, 0, 0

B, I, 1, 0B, I, 2, 0,F

X

B, SREJ, 1C, RR, 0, P

C, RR, 0, F

B, SREJ, 1,P

B, I, 1, 0B, I, 3, 0B, I, 4, 0, F

B, I, 0, 5

Time

Example: HDLC using NRM

(polling)

A polls B

B sends 3 info

frames

A rejects fr1A polls C

C has no

Infmn. frames

to sendA polls B,

requests

selective

retrans. of fr1B resends fr1

Then fr 3 & 4

A send info fr0

to B, ACKs up to 4

N(R)

N(S) N(R)

Address of secondary

Frame Exchange using

8/12/2019 9-HDLC-new

32/38

Combined Station A Combined Station B

B, I, 0, 0 A, I, 0, 0

B, I, 1, 0

B, I, 2, 1

A, I, 1, 1

A, I, 2, 1

X

B, REJ, 1B, I, 3, 2

B, I, 4, 3

B, I, 1, 3

B, I, 2, 4

B, I, 3, 4

A, I, 3, 1

B, RR, 2

B, RR, 3

Frame Exchange using

Asynchronous Balanced Mode

A sends 5

frames to

B.

B ACKs fr0

B rejects

fr2

A goes

back totxing fr1 to

fr4.B ACKs

fr1

B ACKs

fr2

Example 11 10

8/12/2019 9-HDLC-new

33/38

Figure 11.30 shows an exchange using piggybacking. Node

A begins the exchange of information with anI-frame numbered 0 followed by another I-frame numbered

1. Node B piggybacks its acknowledgment of both frames

onto an I-frame of its own. Node Bs first

I-frame is also numbered 0 [N(S) field] and contains a 2 inits N(R) field, acknowledging the receipt of As frames 1

and 0 and indicating that it expects frame 2 to arrive next.

Node B transmits its second and third I-frames (numbered

1 and 2) before accepting further frames from node A.

Example 11.10

Example 11 10 (continued)

8/12/2019 9-HDLC-new

34/38

Its N(R) information, therefore, has not changed: B frames

1 and 2 indicate that node B is still expecting Asframe 2 to

arrive next. Node A has sent all its data. Therefore, it

cannot piggyback an acknowledgment onto an I-frame and

sends an S-frame instead. The RR code indicates that A isstill ready to receive. The number 3 in the N(R) field tells B

that frames 0, 1, and 2 have all been accepted and that A is

now expecting frame number 3.

Example 11.10 (continued)

8/12/2019 9-HDLC-new

35/38

11.35

Figure 11.30 Example of piggybacking without error

Example 11 11

8/12/2019 9-HDLC-new

36/38

Figure 11.31 shows an exchange in which a frame is lost.

Node B sends three data frames (0, 1, and 2), but frame 1 is

lost. When node A receives frame 2, it discards it and sends

a REJ frame for frame 1. Note that the protocol being used

is Go-Back-N with the special use of an REJ frame as aNAK frame. The NAK frame does two things here: It

confirms the receipt of frame 0 and declares that frame 1

and any following frames must be resent. Node B, after

receiving the REJ frame, resends frames 1 and 2. Node Aacknowledges the receipt by sending an RR frame (ACK)

with acknowledgment number 3.

Example 11.11

8/12/2019 9-HDLC-new

37/38

Figure 11.31 Example of piggybacking with error

8/12/2019 9-HDLC-new

38/38

I3 I4 I5 I6RNR5 RR6

Flow Control

Flow control is required to prevent transmitter fromoverrunning receiver buffers

Receiver can control flow by delayingacknowledgement messages

Receiver can also use supervisory frames toexplicitly control transmitter Receive Not Ready (RNR) & Receive Ready (RR)

Documents

9-HDLC-new