UNIT-2• Design Issues in Data link Layer • Framing • Error Detection and Correction - Parity Check - CRC - Hamming Code• Flow Control - Sliding Window Protocols• HDLC • PPP

Data Link LayerDesign Issues :• Design Issues of Data link layer are :

1. Service Provided to the Network Layer

2. Framing

3. Error Detection and Correction

4. Flow Control

1. Service Provided to the Network Layer – Data link layer provides three services to the network layer .

i ) Un acknowledged Connectionless Service

ii ) Acknowledged Connection less Service

iii ) Acknowledged Connection Oriented Service

Data Link Layer i ) Un acknowledged Connectionless Service

• Source sends independent frames to the destination with out any acknowledgement .

• Not a reliable service .

• Used in the systems where error rate is low. Ex : LANS

ii ) Acknowledged Connectionless Service

• Source sends independent frames to the destination with an acknowledgement .

• Reliable service .

• Used in Unreliable channels . Ex : Wireless Systems

iii ) Acknowledged Connection – Oriented Service

• Here source and destination establish a connection before any data is transferred and send an acknowledgement to the source after receiving each frame in order .

• Most reliable service and Used in more noisy channels .Ex : Video

Data Link LayerFraming :• Grouping the bits together from the physical layer and constructing a

frame is called framing .

• The frames are created by the adding a header and trailer to the packet which specify the source and destination addresses .

• Frames are used to perform the error detection and correction operations .

• Framing is of two types :

1. Fixed size framing or static framing

- size of frame is fixed

- no need to specify the start of the frame .

2. Variable size framing or Dynamic

- Size of the frames changed .

- It is necessary to specify the start of each frame .

Data Link Layer• Methods to Break the Bit Stream into Frames .

1. Character Count

2. Character Stuffing or Byte Stuffing

3. Bit Stuffing

4. Physical Layer Coding Violations .

Character Count • Here a field in the header ,Specifies the number of characters in the

frame .

• Data link layer at the destination looks the character count and deside the end of frame .

• The main draw back of this method is if any count is garbled by a transmission error then the destination is unable to decide the frames .

Data Link Layer

Fig : Character Count a) With out Errors b) With Errors

Data Link LayerCharacter Stuffing or Byte Stuffing • In this method each frame starts with the ASCII character sequence

DLE STX and ends with DLE ETX .

• DLE STX and DLE ETX sequences are called frame boundaries .

• It is possible to occur DLESTX or DLEETX in the data ,which will interfere the frame .

• To solve this problem the data link layer at ender side stuff an ASCII DLE character just before each accidental DLE character in the data .

• The Data link layer at receiving end removes this DLE before giving the data to network layer .

• Disadvantage of this method is more overhead and each frame is tied with 8 bit characters .

Data Link Layer

Fig : Character Stuffing

Data Link LayerBit Stuffing • In this method each frame starts and ends with a special bit pattern

like 0111110 called a flag byte .

• When the sender data link layer encounters a five consecutive ones in the data it automatically stuffs a 0 bit into the out going bit stream .

• When the receiver sees five consecutive 1’s followed by a 0 bit it automatically destuffs the 0 bit and send data to network layer .

Fig: Bit Stuffing

Data Link LayerPhysical Layer Coding Violations :• This method is only applicable to network in which the encoding on

the physical medium contains some redundancy .

• 1 bit of data may encode using two physical bits like 0 and 1 .

• 1 bit represents high to low and 0 represents low to high .

• The combinations like high to high or low to low are not used for data .by using this the receiver easily locates bit boundaries .

Data Link LayerError Detection and Correction• A reliable system must have the mechanism to detect and correct the

errors .

Types of Errors • There are three types of Errors :

1. Single Bit

- one bit of a data unit is changed from 0 to 1 and 1 to 0.

Example :

Data Link Layer2. Multiple Bit

- Two or more non consecutive bits in data unit have changed from 1 to 0 or 0 to1 .

Example :

Fig : Multiple Bit Errors

Data Link Layer3. Burst Errors• Two or more consecutive bits in the data unit have changed from 1 to

0 and 0 to 1 .

Fig : Burst Errors

Data Link LayerError Detection

1. Redundency• To detect the errors we use a mechanism called redundancy .

• It involves the transmission of each data unit twice .

• Same data unit is not received twice in succession a transmission error occurred .

• The disadvantage of this system is slow and if an error occurred at the same position of both data units .

• To avoid the disadvantage of this method a shorter group of bits added to the end of each data unit ,this technique is called redundancy

• Data unit is passed through a generating unit and it adds an appropriate redundancy check bits .

• This data unit send to the receiver and receiver puts the entire stream into a checking function .

• If there is no error data is accepted other wise rejected .

Data Link Layer

Fig : Redundency

2. Exact –Count Encoding :

• The Number of 1’s in the data unit are same .

• In the destination side it count the number of 1’s received same or not to determine the transmission error .

Data Link LayerError Correction• Error correction can be handled in two ways :

1. When the error is discovered ,the receiver asks the sender to retransmit the same data unit .

2. Receiver can use an error correcting codes to Detect and Correct the errors .

• There are different types of Error Detecting and correcting Techniques :

1. Parity Checker

2. Hamming Code

3. Cyclic Redundancy Check

1. Parity Checker • It is the simplest technique for detecting and correcting errors.

• The MSB of an 8-bits word is used as the parity bit and the remaining 7 bits are used as data or message bits.

Data Link Layer• The parity of 8-bits transmitted word can be either even parity or odd

parity.

• Even parity  : Even parity means the number of 1's in the given word including the parity bit should be even (2,4,6,....).

• Odd parity  : Odd parity means the number of 1's in the given word including the parity bit should be odd (1,3,5,....).

Data Link LayerExample :

Drawbacks• It does not detect all types of errors .

• Not suitable for multiple bit errors .

Data Link Layer2. Hamming Code • Hamming code can be applied to data units of any length .

• Hamming code is used to detect and correct single bit errors .

• The distance between two words of the same size is called Hamming distance.

Example :

• 1. The Hamming distance d(000, 011) is 2 because 000 +011 is 011 (two 1’s).

• 2. The Hamming distance d(10101, 11110) is 3 because 10101 +11110 is 01011 (three 1’s).

• To calculate the number of redundancy bits (r) required to correct a given number of data bits (m ) it should satisfy a condition :

2r>=m+r+1

Data Link Layer

Number of Data Bits(m)

Number of Redundancy Bits (r)

Total Bits(m+r)

1 2 3

2 3 5

3 3 6

4 3 7

5 4 9

6 4 10

7 4 11

Table : Relation Between Data and Redundancy Bits

Data Link Layer• The redundancy bits in the original data are placed in powers of 2 like

1 ,2,4,8,16 etc .

Fig : Position of Redundancy bits in hamming code

• r1 is selected so as to establish even parity in bit positions : 1,3,5,7,9,11 .

• r2 is selected so as to establish even parity in bit positions : 2,3,6,7,10,11 .

• r1 is selected so as to establish even parity in bit positions : 4,5,6,7.

• r1 is selected so as to establish even parity in bit positions : 8,9,10,11

Data Link Layer• If any combination the number of 1’s even number ,assign the

corresponding r value as 0 else 1 .

• Like this after knowing the values of r1,r2,r4 and r8 the error bit location can be identified by r’s position ( r8 r4 r2 r1).

• All values of r’s zero indicates no error has occurred .Example 1: Consider the message 1001101 is transmitted through the

channel ,obtain the redundancy bits and transmitting unit needed. Assume bit number 8 has been changed .how to locate it.

Example 2: Consider the message 1010111 is transmitted through the channel ,obtain the redundancy bits and transmitting unit needed. Assume bit number 3 has been changed .how to locate it.

Example 3: Consider the message 1101100 is transmitted through the channel ,obtain the redundancy bits and transmitting unit needed. Assume bit number 5 has been changed .how to locate it.

Data Link LayerCRC (Cyclic Redundancy Check)

• Most power full redundancy technique based on the binary division .

• It uses polynomial codes for generating check bits in the form of cyclic redundancy check .

• crc bits are appended at the end of each data unit so that the resulting data unit becomes exactly divisible by a second ,predetermined number .

• At the receiver the incoming data unit is divided by the same number .

• If there is no reminder the data unit is intact and it is accepted .

• A reminder indicates that data unit has damaged .

Data Link Layer

Fig : CRC Generator and Checker

Data Link Layer

Fig : CRC Encoding and Decoding

Data Link Layer

Fig : Binary Division in CRC Generator

Data Link Layer

Fig : Binary Division in CRC Checker

Data Link LayerExample 1: What is the frame transmitted if the message i(x) is x7 + x4 + x3+x and

generator polynomial g(x) is x3 + x2 +1 .

1001

1101

1000

1101

1011

1101

1100

1101

1000

1101

1101

k + 1 bit check sequence c,

equivalent to a degree-k

polynomial

101

1101Remainder

m mod c

10011010000 Message plus k zeros

Result:

Transmit message followed by remainder:

10011010101

11111001

Data Link Layer

1001

1101

1000

1101

1011

1101

1100

1101

1101

1101

1101

k + 1 bit check sequence c,

equivalent to a degree-k

polynomial

0

1101Remainder

m mod c

10011010101 Received message, no

errors

1.1. ResultResult:

CRC test is passed

C(x) = x3 x2 1 = 1101 Generator P(x) = x10 x7 x6 x4 x2 1 = 10011010101 Received Message

11111011

Data Link Layer

Example 2: i(x) =10110111 g(x)=110011

Example 3: i(x)=1101011011 g(x) =x4+x+1

Example 4: i(x)=1010011010 g(x) =10101

Data Link LayerFlow Control • There are two main entities in communication process one is the

transmitter and the other is the receiver .

• The receiver must be able to receive all the frames transmitted by the transmitter .

• For this reason the data receiving rate of the receiver must be grater than that of the transmitter such that transmitter does not overwhelm the receiver .this mechanism is called flow control .

Piggybacking • When the data frames arrive at the receiver ,instead of immediately

sending separate control frame ,the receiver restores itself and waits until the network layer passes the next packet .

• The acknowledgement is attached to the out going data frame ,in effect the acknowledgement get a free ride on the next out going data frame .

Data Link Layer• The technique of temporarily delaying the out going

acknowledgement so that they can be hooked on to the next out going frame , which is known as “piggy backing “ .

Sliding Window Protocols • In this mechanism the sender can transmit several frames before

requiring an acknowledgement .

• Several frames can be sent at once the channel capacity can be used efficiently .

• The receiver acknowledges only some of the frames , using a single ACK to confirm the receipt of multiple data frames .

• There are three different types of Sliding window protocols .

1. One bit Sliding Window Protocol

2. Go – Back – n Automatic Repeat – Request (ARQ)

3. Selective Repeat – Automatic Repeat Request (ARQ)

Data Link Layer1. One bit Sliding Window Protocol

Fig : (a) At start. Receiver waits for 0. (b) Sender sends 0. (c) Receiver receives 0. Waits for 1. (d) Sender got ack for 0. Hasn't got 1 from its Network layer yet.

Data Link Layer

Fig : a. Normal( seq , Ack , Packet number)

b. Synchronization Problem

Data Link Layer2.Go – Back – n Automatic Repeat – Request (ARQ)• In this method if one frame is lost or damaged all frames sent since

the last frame acknowledged are retransmitted .

Fig : Go-Back-n ARQ

Data Link LayerSelective Repeat (ARQ)• In this method only the specified damaged or lost frame is

retransmitted .

• If a frame is corrupted in transit a NAK is returned and the frame is resent out of sequence .

• The receiving device must be able to sort the frames it has and insert the retransmitted frame into its proper place in the sequence .

• In this technique the sending device must contain searching and receiving machine contains sorting techniques .

Data Link Layer

Fig : Selective Repeat

Data Link LayerHigh Level Data Link Control ( HDLC)• HDLC is a most important data link control protocol .

• HDLC Defines three type of Stations

1. Primary Station : Responsible for controlling the operation of the link . Frames issued by the primary station are called commands .

2. Secondary Station : Operate under the control of primary station . Frames issued by a secondary station are called responses .The primary station maintains a separate logical link with each secondary station on the line .

3. Combined Station : Combines the features of Primary and Secondary station .it issue both commands and responses .

• There are two link configurations :

1. Unbalanced Configuration

2. Balanced Configuration

Data Link Layer1. Unbalanced Configuration : consists of one primary and one or more

secondary stations and supports both full duplex and half- duplex transmission.

2. Balanced Configuration : Consists of two combined stations and supports both full-duplex and half duplex transmission .

There are three data transfer modes :

1.Normal Response Mode (NRM):

• Used with unbalanced configuration .

• Primary station may initiate data transfer to a secondary station ,but a secondary station may only transmit data in response to a command from the primary station .

• Used on multidrop lines .

Data Link Layer2. Asynchronous Balanced Mode(AMB) :

• Used with a balanced configuration .

• Either combined station may initiate transmission with out receiving permission from the other combined station .

• It is most widely used in three modes .

3. Asynchronous Response Mode (ARM) :

• Used with an unbalanced configuration .

• The secondary station may initiate transmission without explicit permission of the primary station .

• The primary station still retains responsibility for the line ,including initialization ,error recovery and logical disconnection .

• It is rarely used .where the secondary station may need to initiate transmission .

Data Link LayerFrame Structure :

Data Link Layer

Data Link Layer• HDLC uses synchronous transmission .

• All transmissions are in the form of frames and single frame format suffices for all types of data and control exchanges .

• Flag ,address and control fields are known as header.

• FCS and flag fields are known as trailer .• Flag: 0111110- start and ending delimiter. Bits are stuffed for flags in

data frames .• FCS: 16-bit CRC using generating polynomial • Address field:

– mainly used in multidrop link configuration, and not used in point-to-point

– In unbalanced configuration, every secondary is assigned a unique address. Contains address of secondary station in both command and response frames

– In balanced mode, command frame has destination address and response frame has sending node’s address .

– Group addresses are also possible. E.g., One command sent to all the secondaries .

Data Link Layer• In I-frames, N(s) is the sequence number of the frame being sent, and

R(s) is the sequence number of the frame being expected.• The P/F bit, known as the poll/final bit, is used with different meaning

in different contexts. – It is used to indicate polling, to indicate the final I-frame, etc

• There are three different classes of frames used in HDLC

– Unnumbered frames, used in link setup and disconnection, and hence do not contain ACK.

– Information frames, which carry actual information. Such frames can piggyback ACK in case of ABM

– Supervisory frames, which are used for error and flow control purposes and hence contain send and receive sequence numbers

Data Link LayerPoint to Point Protocol (PPP)

• One sender, one receiver, one link: easier than broadcast link:

• No Media Access Control

• No need for explicit MAC addressing

• E.g., dialup link, ISDN line• Popular point-to-point and high-level DLC protocols:

• PPP (point-to-point protocol)

• HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack). HDLC is also used in multi-point links (one station many receivers)

• These protocols can often be run over other data link technologies providing best of both worlds

– E.g., PPPoE, HDLC encapsulation by Ethernet

Data Link LayerFunctionality : (similar to link layer services + extra management

functions) • Packet framing - encapsulation of network-layer datagram in data

link frame – Multi-protocol - carry network layer data of any network layer

protocol (not just IP) at same time ability to demultiplex upwards• Bit transparency - must carry any bit pattern in the data field (even if

underlying channel can't)• Error detection - not correction

PPP Data Frame Format

Data Link Layer• Flag: delimiter (framing)

• Address: ignored. (historical)

• Control: ignored. (historical)

• Protocol: upper layer protocol to which frame delivered (e.g., PPP-LCP, IP, IPCP, etc)

• info: upper layer data being carried

• check: cyclic redundancy check for error detection

Where does PPP get used?

• Dial-up – PPP over async serial, over modem

• ADSL – PPP over Ethernet

• Backbone – Packet over SONET (POS)

• Why?

– Framing (dialup, POS)– Efficiency (POS)

– Authentication, address negotiation (PPPoE)

Data Link Layer• PPP Data Control Protocol