HDLC - 1 www.comnets.uni-bremen.de

Data Link Layer (cont.) (Sicherungsschicht)

Chapter 5 (part 2) [Wa0001 ]

Dr.-Ing. Koojana Kuladinithi (koo) (comnets.uni-bremen.de)

HDLC - 2 www.comnets.uni-bremen.de

LOGICAL LINK CONTROL

MEDIUM ACCESS CONTROL

PHYSICAL SIGNALING

PHYSICAL MEDIA ATTACHMENT

MEDIUM

DATA LINK LAYER

PHYSICAL LAYER

ACCESS UNIT INTERFACE

OSI-model for local networks with partitioning of the link layer in media access control and logical link control, as defined by IEEE 802.

HDLC - 3 www.comnets.uni-bremen.de

Bit-Oriented Protocols: HDLC – High Level Data Link Control

Low overhead Bit streams are transmitted without breaking it into

characters. Bit streams that are not integral multiples of character

lengths can be transmitted. Subsets:

High Level Data Link Control (ISO standard): HDLC Synchronous DLC: SDLC HDLC Subsets: CCITT's Link Access Protocol - Balanced (LAP-B) Link Access Protocol for the ISDN D-channel (LAP-D) Link Access Protocol for Modems (LAP-M)

HDLC - 4 www.comnets.uni-bremen.de

HDLC Stations

Two types of Stations Primary Secondary

Types of Commands Commands: sent from primary to secondary Responses: sent from secondary to primary

Only one primary can be active at any given time, while multiple secondaries can be active simultaneously

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HDLC Link Structures Unbalanced point-to-point link

One primary, one secondary

Unbalanced multi-point link One primary, multiple secondaries

secondary station A

secondary station B

primary station

"

One-way controlled channel commands

answers

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HDLC Link Structures (cont.)

Balanced point-to-point link Both stations have primary/secondary status. While one

station is transmitting, the other has to be a secondary.

primary station A

secondary station A

secondary station B

primary station B

symmetric channel

answers commands

answers commands

combined station

combined station

both-way controlled channel commands

answers

HDLC - 7 www.comnets.uni-bremen.de

HDLC Modes of Operation

Normal Response Mode (NRM) Used in unbalanced configurations In this unbalanced mode, secondaries can transmit

only when instructed by the primary station The link may be point-to-point or multi-point

primary station

secondary station

secondary station

primary station

1) Poll-command 2) information 3) receipt

1) select-command plus information 2) receipt

HDLC - 8 www.comnets.uni-bremen.de

HDLC Modes of Operation (cont.)

Asynchronous Response Mode (ARM) Used in unbalanced configurations. A secondary can transmit without being polled by the

primary. Used in point-to-point configurations and duplex links.

secondary station

primary station

answer command

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HDLC Modes of Operation (cont.) Asynchronous Balanced Mode (ABM)

Used in balanced configurations where all stations are "equal" Duplex point-to-point links

combined station

combined station

combined station

combined station

1) command 2) answer

1) command 2) answer

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closing flag 8 bits 01111110

frame check field 16 or 32 bits

information field variable

control field 8 or 16 bits

address field 8 x n bits (n = 1 to k)

opening flag 8 bits 01111110

F FCS I C A F

frame

span of frame check and zero insertion

HDLC Frame Format

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HDLC Frame Fields Flag Field

Start and End Flag pattern (01111110) Zero insertion required for data transparency

Address Field Contains the address of the secondary station Broadcast address (All 1's) Extended address option that can make the address as long as

necessary (in increments of 8 bits) The last extended address octet starts with a 1,

while the previous octets start with 0 Control Field

Identifies frame types Contains Send and Receive sequence numbers

Information Arbitrary length, arbitrary bit streams

FCS 16-bit CRC (CCITT-16) FCS calculated for Address-Information fields

without the inserted zeros (for data transparency)

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8 7 6 5 4 3 2 1

Address may be that of a secondary/combined station.

Global (all 1’s) or Null ( all 0’s)

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

Extended Address Field – 1 or more Octets

LSB transmitted first

LSB transmitted first

(LSB : Least Significant Bit)

Basic Address Field – 1 Octet

0 1 0 . . . Extend Address to next Octet Terminate

Address Field

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HDLC Frame Types Information Frames

Carry data In duplex links, can use piggy-backed acknowledgements Identified by a 0 in the first bit of the control field

Supervisory Frames Used for error and flow control Uses CRC-CCITT Contain sequence numbers Identified by a 10 in the first two bits of the control field

Unnumbered Frames Used for connection establishment and disconnection No acknowledgements Identified by a 11 in the first two bits of the control field

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0 SN P/F RN

1 Type P/F RN

1 Type P/F Type

0

1

Bits: 1 2 3 4 5 6 7 8

Information

Supervisory

Unnumbered

HDLC Control Field

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8 7 6 5 4 3 2 1

LSB TRANSMITTED FIRST

INFORMATION

SUPERVISORY

UNNUMBERED

N(R)

N(R) P/F

P/F

N(S) 0

SS 0 1

MMM P/F MM 1 1

COMMENTS N(R) = RECEIVE SEQUENCE COUNT = 0 TO 7 N(S) = SEND SEQUENCE COUNT = 0 TO 7

SS specifies RR, RNR, REJ, SREJ

MMM and MM specify the U command response P/F = POLL for command, F for response

BASIC CONTROL FIELD – 1 OCTET

EXTENDED CONTROL FIELD – 2 OCTETS 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

INFORMATION

SUPERVISORY

UNNUMBERED

N(R)

N(R)

P/F

P/F

P/F 0 0 0 0 0 0 0

N(S) 0

0 0 0 0 SS 0 1

MMM 0 MM 1 1

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A N(S) = 3 N(R) = 5

I 3.5 P = 1 B N(R) = 3 RNR3

RR5

F = 1

P = 1

F = 1

P = 1

RNR3

RR5

RR3

?

?

A N(S) = 1 N(R) = 5

N(S) = 2 N(R) = 5

a)

b)

I 1.5

I 2.5

REJ1

I 1.5

I 2.5

FCS ERROR

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The Poll/Final bit Depends on whether the transmission is from the

primary or secondary If the frame is from a primary,

the P/F bit is interpreted as the Poll bit If the frame is from a secondary,

the P/F bit is interpreted as the Final bit, indicating the end of a sequence of frames

A primary cannot send another poll to a specific secondary, until it has received the final frame

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Supervisory Frames Used for positive/negative acknowledgements. 2 S bits in the control byte identifying

4 kinds of supervisory frames: Receive Ready (RR) Receive Not Ready (RNR) Reject (REJ) Selective Reject (SREJ)

RR Frame Used for acknowledging correctly received frames All frames with numbers less than N(R) have been received Also indicates that the receiver is ready to receive frames

RNR Frame Used to indicate that the receiver is temporarily not ready May be used for flow control as a

Wait-Acknowledgement (WACK) If a data frame is acknowledged by a WACK, the transmitter

cannot send any more frames, until it receives an ACK to the transmitter's ENQ

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Supervisory REJ & SREJ Frames Both REJ and SREJ act as positive acknowledgements

for all frames below number N(R) Frames with bad FCS are not necessarily rejected,

but ignored REJ Frame

Used as a negative acknowledgement The frame numbered N(R)

and all frames after N(R) are to be retransmitted Go-Back-N ARQ operation

SREJ Frame Used as a NAK for frame numbered N(R) Retransmission requested only for that frame Selective Repeat ARQ operation

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Unnumbered Frames Used for maintenance purposes

Link start-up and shut-down Set operational modes

No frame sequence numbers 5 Management bits in the control frame identify

32 different commands/responses X bits in the extended control field

are reserved (set to 0)

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Unnumbered Commands and Responses Commands

Set Asynchronous Response Mode (SARM) Set Asynchronous Response Mode Extended (SARME) Set Normal Response Mode (SNRM) Set Normal Response Mode Extended (SNRME) Set Asynchronous Balanced Mode (SABM) Set Asynchronous Balanced Mode Extended (SABME) Reset (RSET) Frame Reject (FRMR) Disconnect (DISC)

Responses Unnumbered Acknowledgement (UA) Command Reject (CMDR) Frame Reject (FRMR) Disconnect Mode (DM)

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Link Access Procedure Balanced (LAP-B) Used to connect a computer to the data network,

example: X.25 Balanced Mode, Connection-oriented,

Point-to-point duplex links SABM and SABME are the only two SET-commands allowed Addressing: DCE to DTE or DTE to DCE

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disconnect response

data indicate data

response

protocol actions

System A interaction between

System B interaction between

network layer

data link layer

signals on the channel

between A and B

data link layer

network layer

connect request

connect confirm data request

data confirm disconnect

request

disconnect confirm

SABM

UA

information frame(s)

acknowledge frame(s) DISC

connect indicate connect response

disconnect indicate UA

Example: Bit oriented protocol (HDLC)

HDLC - 24 www.comnets.uni-bremen.de

DLCA DLCB

B (1,1)

B (2,2)

B (3,3)

B (2,4)

A (1,1)

A (2,1) A demands erroneous

packet once more REJ (1)

RR (3)

B initializes data transmission SABM

UA A confirms

A (0,0) A sends data

RR (2) A has no data left to send

RR (4)

DM A confirms end of connection

Reliable data transmission with HDLC

B (0,0) B receives UA and sends data

A (3,1) B (1,3) B repeats starting with packet 1

B (3,4) B has no data left to send

DISC B demands end of connection

Formats: Sender (SN, RN), message (RN) Meaning of message: SABM Set Asynchronous Balanced Mode UA Unnumbered Acknowledge REJ Reject RR Receive Ready DISC Disconnect DM Disconnect Mode

Go-Back-N ARQ

HDLC - 25 www.comnets.uni-bremen.de

Logical Link Control (LLC) HDLC subset used with LANs Data link layer in LANs consists

of two sublayers Logical Link Control (LLC) sublayer Medium Access Control (MAC) sublayer

LLC supports connection-oriented and connectionless services.

HDLC - 26 www.comnets.uni-bremen.de

Limitations of HDLC

Too many options Communicating HDLC devices

will need to have the same options Only a single address field works fine with a single primary station HDLC came before OSI (layering is not totally aligned) Flags may have errors and synchronization may be lost Only one FCS for the entire frame NAKs are not sent immediately causing a lower

transmission efficiency under high error rate situations

HDLC - 27 www.comnets.uni-bremen.de

Example protocol: PPP PPP is a form of HDLC, which is commonly used today When you dial up using your modem and connect your PC

to your ISP's router/remote access server, the data link layer protocol run on the link is PPP

PPP uses the ABM mode of HDLC and provides a connectionless unacknowledged service

Since it is ABM, multipoint is not used; this means the address byte does not change. It is set to all 1's. This means all stations accept the frame. But since PPP has only two nodes, one node accepts any frame sent from the other node.

Since it is unacknowledged, no supervisory frames are sent. So the control field is always 0000 0011. First two bits are ’11’ to represent unnumbered frames. Unnumbered frames must be allowed to carry data in addition to carrying SABM (Set ABM mode - open connection), SABME (E: Extended - control field is 16 bits and sequence numbers are 7 bits), unnumbered acks, etc. There are no "numbers" meaning no sequence numbers. The M bits of 0 must indicate a data type of the unnumbered frame. In PPP the header control field is shown as 0000 0011.

PPP has one additional field over HDLC - it has a protocol type field to indicate what the type of higher layer protocols carried in the PPP frame.

Also, PPP is byte-oriented, unlike HDLC, which is bit-oriented; therefore bit stuffing is not used when the flag pattern appears in the payload; instead an escape character is used.

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Example protocol: PPP (cont..)

www.comnets.uni-bremen.de

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Assumptions: Symmetric bit errors: equal probability of 0 1, 1 0 errors Independence assumption

Bit error probability p Probability of correct bit transmission? Block of n bits (frame/packet) is corrupted,

if one or more bits are corrupted, p(n) also known as PER

Calculation of Error Probabilities

P [i specific bits corrupted] =

P [i bits corrupted] = pi =

1 - p

ini pp −− )1(

ini ppin −−

)1(

HDLC - 30 www.comnets.uni-bremen.de

Calculation of Error Probabilities (cont.)

p(n) = P [≥ 1 bit corrupted] =

Probability that a block of n bits arrived with no errors,

p0 = P [0 bit corrupted] = correct transmission

p(n) =

Approximation for small values of p:

∑=

n

iip

1

npp )1(11 0 −−=−

1)1(1)1(1 <<=−−≈−− pnpnpp n

np)1( −

HDLC - 31 www.comnets.uni-bremen.de

Calculation of Error Probabilities (cont.)

Size of a packet F = 500 bytes Bit error probability P = 10-6 Compute the probability of PER, p(n) ? How many packets (frames) will be corrupted

at 100 kbps?

HDLC - 32 www.comnets.uni-bremen.de

Calculation of Error Probabilities (cont.)

Size of a frame F = 500 bytes (4000 bits) Bit error probability P = 10-6 P(0)= (1- 10-6)4000 = 0.9960 P(n)= 1-(1- 10-6) 4000 = 0.0040 4 out of every 1000 packets (frames) have

errors At 100 kbps, this means 6 packets (frames)

per minute are in error

HDLC - 33 www.comnets.uni-bremen.de

Error Handling Methods ARQ: Automatic Repeat Request

3 Schemes Stop-and-Wait Go Back N Selective Reject

Packets contain sequence number NS Correctly received packet: Receiver sends positive Acknowledgement ACK[NS] Erroneous packet: Receiver sends negative Acknowledgement NAK[NS] Sender must keep copy of packet until an ACK is received Error Detection with FCS, e.g. CRC (Cyclic Redundancy Check)

Checking Frame Sequence

HDLC - 34 www.comnets.uni-bremen.de

time of transmission

sending window

receiving window

5 5 6

5 5 6

Round-Trip-Delay

Stop-and-Wait Automatic Repeat Request

Sequence: Packet, Acknowledgement, Packet, Ackknowledgement, ...

HDLC - 35 www.comnets.uni-bremen.de

Throughput

Throughput (Durchsatz): the amount of work that can be performed by a computer system or component in a given period of time [IEEE 90]. [IEEE 90] Institute of Electrical and Electronics Engineers. IEEE Standard Computer Dictionary: A Compilation of IEEE Standard Computer Glossaries. New York, NY: 1990.

Throughput: The amount of data transferred from one place to another or processed in a specified amount of time. Data transfer rates for disk drives and networks are measured in terms of throughput. Typically, throughputs are measured in kbps, Mbps and Gbps.

Often S (English) or D (German) is used as an abbreviation for throughput.

HDLC - 36 www.comnets.uni-bremen.de

Throughput (cont.) However, there is no standard, universally recognized definition of throughput and it is important to understand the definition of throughput that is being used in each situation. The throughput is often given in normalized form; that is, it is the rate at which user data is carried over the channel divided by the channel capacity. Sometimes this is referred to as normalized throughput, but often it is not. Another way of describing normalized throughput: portion of time used for transmitting “useful” information. Sometimes this is also called “goodput”.

HDLC - 37 www.comnets.uni-bremen.de

Stop-and-Wait-ARQ

Formula? n: packet length in bits PER: Packet Error probability c: round trip delay (sec), delay between the end of the last packet and the start of the

next one v: transmission speed in bit/sec Throughput? Low, especially for small packets and long propagation delays!

e.g. not suitable for satellite links

vcnPERnD*

)1(*+−

=

HDLC - 38 www.comnets.uni-bremen.de

Normalized Throughput = Useful load of the channel (portion of time used for the transmission of information over

some time T) ndata: packet length in bits nack: ACK lenghth in bits v: transmission speed in bit/sec tn: change over time /processing time td: Propagation delay Transmission delay vs Propagation delay ?

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Error Handling Methods ARQ: Automatic Repeat Request

Go-Back-N-ARQ Continuous data flow is possible Data is sent until a NAK[NSe] is received Sender restarts sending beginning with packet NSe

Receiver ignores all packets until packet NSe

Sufficient buffer space is needed A positive acknowledgement ACK[NS] acknowledges all

packets up to packet NS

HDLC - 42 www.comnets.uni-bremen.de

transmission time

sending window

receiving window

4 5 6 7 8 9 5 6 7 8 9

4 5 5 6

Round-Trip-Delay

for packet 5

Go-Back-N- Automatic Repeat Request

HDLC - 43 www.comnets.uni-bremen.de

Throughput? Formula? n: packet length in bits PER: Packet Error Rate c: round trip delay, delay between end of last

packet and the start of next one v: transmission speed in bit/sec

Go-Back-N-ARQ

Higher, because data can be sent continuously

vcPERnPERnD

**)1(*

+−

=

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Error Handling Methods ARQ: Automatic Repeat Request

Selective Reject Data can be sent continuously All correctly received packets are acknowledged

and if necessary buffered Erroneous packets are selectively requested by NAKs Buffering is needed to guarantee the right sequence of

packets to higher layers

HDLC - 47 www.comnets.uni-bremen.de

transmission time

sending window

receiving window

4 5 6 7 8 9 5 10 11 8 12

4 5 6 7 8 9 5 10 11

Round-Trip-Delay

Selective-Reject- Automatic Repeat Request

HDLC - 48 www.comnets.uni-bremen.de

Selective-Reject-ARQ

Throughput? Formula? n: packet length in bits PER: Packet Error probability c: round trip delay, delay between end of last

packet and the start of the next one v: transmission speed in bit/sec

Highest possible throughput, because data can be sent continuously and only erroneous packets have to be retransmitted

PERD −=1

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ARQ methods

Sender Buffer

Receiver Buffer

Stop & Wait ARQ

1 PDU

1 PDU

Go-Back-N ARQ

W PDUs

1 PDU

Selective Reject ARQ

W PDUs

W PDUs

HDLC - 51 www.comnets.uni-bremen.de

PACKET ERROR RATE vs THROUGHPUT

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ROUND TRIP DELAY vs THROUGHPUT

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TRANSMISSION SPEED vs THROUGHPUT

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Comparison FEC and ARQ

ARQ: Automatic Repeat Request Pros:

• High throughput for good channel quality • Guarantee for very low rest bit error rate • No complex decoding

Cons: • Return channel necessary • Throughput fluctuates with the channel quality • Varying delay, not suitable for real time applications • Additional buffers in sender and receiver necessary

FEC: Forward Error Correction Pros:

• No return channel necessary • Constant throughput independent of channel quality • Constant delay between sender and receiver

Cons: • Low Throughput because of high redundancy • Rest bit error rate depends on channel quality • Coding and Decoding algorithms highly complex

HDLC - 55 www.comnets.uni-bremen.de

no

yes error

cluster b1 cluster b2

bit timing

definition of a cluster error

Error Clusters

Errors are usually not independent Errors are correlated Clusters are one possible description