Data Link layer in computer networks cse

Communication at the data-link layer

Nodes and Links
6 nodes- host, destination, routers
1,3,5 link- 3 LAN
2,4 link- WAN

Services
A communication with only three nodes

Services
• Framing
• Flow Control
• Error Control
• Congestion Control

Two Categories of Links
• point-to-point link
• Broadcast link.

Three Types of addresses
Unicast Address
•Unicasting means one-to-one communication.
Multicast Address
•Multicasting means one-to-many communication
Broadcast Address
•A frame with a destination broadcast address is sent to
all entities in the link.

Address Resolution Protocol (ARP)
Position of ARP in TCP/IP protocol suite

ARP operation

Packet Format

Error Detection and Correction
Types of Errors
•The central concept in detecting or correcting errors is
redundancy
•Redundancy is achieved through various coding
schemes

Block Coding
• Divide our message into blocks, each of k bits, called
datawords
• Add r redundant bits to each block to make the
length n = k + r. n-bit blocks are called codewords.
• create a combination of 2k
datawords and create a
combination of 2n
codewords
• This means that we have 2n
− 2k
codewords that are
not used, call these codewords invalid or illegal

Error Detection
• The following two conditions are met, the receiver
can detect a change in the original codeword.
• The receiver has (or can find) a list of valid
codewords. The original codeword has changed to an
invalid one.

Error Detection- Example
• Let us assume that k = 2 and n = 3. Table shows the list of
datawords and codewords.
• sender encodes the dataword 01 as 011 and sends it
• The receiver receives 011. It is a valid codeword.
• 111 is received . This is not a valid codeword and is discarded
• Even data corrupted, 000 is received, So error undetectable

Hamming Distance
• The Hamming distance between two words is the number of
differences between the corresponding bits
• Hamming distance between two words x and y as d(x, y)
• For example, if the codeword 00000 is sent and 01101 is
received, 3 bits are in error
• Hamming distance between the two is d(00000, 01101) = 3
• Hamming distance not zero, the codeword is corrupted, The
Hamming distance is found by apply the XOR operation ( )
⊕

Hamming Distance - Example
• Let us find the Hamming distance between two pairs of words
• The Hamming distance d(000, 011) is 2 because (000 011)
⊕
is 011 (two 1s).
• The Hamming distance d(10101, 11110) is 3 because (10101
11110) is 01011 (three 1s).
⊕

Minimum Hamming Distance for Error
Detection
• The minimum Hamming distance is the smallest Hamming
distance between all possible pairs of codewords.
• Now let us find the minimum Hamming distance in a code if
we want to be able to detect up to s errors.
• This means that dmin must be an integer greater than s or
dmin = s + 1

Linear Block Codes
• Subset of block codes called linear block codes.
• linear block code is a code in which the exclusive OR (addition
modulo-2) of two valid codewords creates another valid
codeword
• The minimum Hamming distance is the number of 1s in the
nonzero valid codeword with the smallest number of 1s
• In our first code table, the numbers of 1s in the nonzero
codewords are 2, 2, and 2. So the minimum Hamming
distance is dmin = 2.

Parity-Check Code
• n = k + 1
• r0 = a3 + a2 + a1 + a0 (modulo-2)
• s0 = b3 + b2 + b1 + b0 + q0 (modulo-2)
• syndrome is 0, there is no detectable error ,syndrome is 1, the
data portion of the received codeword is discarded

CYCLIC CODES
• In a cyclic code, if a codeword is cyclically shifted (rotated),
the result is another codeword
• For example, if 1011000 is a codeword and we cyclically left-
shift, then 0110001 is also a codeword

Cyclic Redundancy Check
• The dataword has k bits (4 here); the codeword has n bits (7).
• The size of the dataword is by adding n − k (3 here) 0s in right
• The n-bit result is fed into the generator.
• The generator uses a divisor of size n − k + 1 (4 here)
• The generator divides the dataword by the divisor (modulo-2)
• The quotient of the division is discarded, the remainder
(r2r1r0) is appended to the dataword to create the codeword.

Polynomials
Degree of a Polynomial- x6 + x + 1 is 6
Adding and Subtracting Polynomials- adding x5 + x4 + x2 and x6 +
x4 + x2 gives just x6 + x5
Multiplying or Dividing Terms- x3 × x4 is x7, x5/x2 is x3

Polynomials
Multiplying Two Polynomials-
(x5 + x3 + x2 + x)(x2 + x + 1) = x7 + x6 + x5 + x5 + x4 + x3 + x4 + x3
+ x2 + x3 + x2 + x = x7 + x6 + x3 + x
Shifting-
Shifting left 3 bits:
10011 becomes 10011000
x4 + x + 1 becomes x7 + x4 + x3
Shifting right 3 bits:
10011 becomes 10
x4 + x + 1 becomes x

Cyclic Code Encoder Using Polynomials
• The dataword 1001 is represented as x3 + 1. The divisor 1011
is represented as x3 + x + 1.
• To find the augmented dataword, we have left-shifted the
dataword 3 bits (multiplying by x3).
• The result is x6 + x3 . Division is straightforward.
• We divide the first term of the dividend, x6 , by the first term
of the divisor, x3
• Then we multiply x3 by the divisor and subtract

• We define the following, where f(x) is a polynomial with binary
coefficients
• In a cyclic code,
1. If s(x) ¦ 0, one or more bits is corrupted.
2. If s(x) = 0, either
a. No bit is corrupted, or
b. Some bits are corrupted, but the decoder failed
to detect them.

• Dataword: d(x) Codeword: c(x) Generator: g(x) Syndrome: s(x)
Error: e(x)
• Received codeword = c(x) + e(x)
• The receiver divides the received codeword by g(x) to get the
syndrome.
• If this term does not have a remainder (syndrome = 0), either
e(x) is 0 or e(x) is divisible by g(x)

Single-Bit Error
•If the generator has more than one term and the coefficient of x0
is 1, all single-bit errors can be caught.
•If a generator cannot divide xt
+ 1 (t between 0 and n - 1), then
all isolated double errors can be detected.
•A generator that contains a factor of x + 1 can detect all odd-
numbered errors

Burst Errors
L is the length of the error
r is check bits

Checksum
For example, if the set of numbers is (7, 11, 12, 0, 6), we send (7,
11, 12, 0, 6, 36)

Forward Error Correction
Using Hamming Distance
•To detect s errors, the minimum Hamming distance should be
dmin = s + 1.
•For error detection, we definitely need more distance.
•It can be shown that to detect t errors, we need to have dmin=2t
+ 1

Using XOR
R = P1 P2 … Pi … PN → Pi = P1 P2 … R …
⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕
PN
•we apply the exclusive OR operation on N data items (P1 to PN),
•replacing the one to be created by the result of the previous operation (R).
•This means that we can divide a packet into N chunks, create the exclusive OR
of all the chunks and send N + 1 chunks.
•If any chunk is lost or corrupted, it can be created at the receiver site

Chunk Interleaving

DLC Services- data link control (DLC)
• Data link control functions include framing and flow and error
control
Framing
Character-Oriented Framing

Byte stuffing and unstuffing
Byte stuffing is the process of adding one extra byte whenever there is a flag
or escape character in the text.

Bit-Oriented Framing

Bit stuffing and unstuffing
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 mistake the pattern 0111110 for a flag.

Flow Control
Flow control in this case can be feedback from the receiving
node to the sending node to stop or slow down pushing frames

Buffers
•Flow control can be implemented in several ways, one of the
solutions is normally to use two buffers;
•one at the sending data-link layer and the other at the receiving
data-link layer
•A buffer is a set of memory locations that can hold packets at the
sender and receiver.
•When the buffer of the receiving data-link layer is full, it informs
the sending data-link layer to stop pushing frames

Error Control
A CRC is added to the frame header by the sender and checked
by the receiver.
Flow control is implemented using two methods
•In the first method, if the frame is corrupted, it is silently
discarded; if it is not corrupted, the packet is delivered to the
network layer
•In the second method, if the frame is corrupted, it is silently
discarded; if it is not corrupted, an acknowledgment is sent to the
sender

Data-link Layer Protocols
Traditionally four protocols have been defined for the data-link
layer to deal with flow and error control:
•Simple,
•Stop-and-Wait,
•Go-Back-N,
•Selective-Repeat

Data-link Layer Protocols
The behavior of a data-link-layer protocol can be better shown as
a finite state machine (FSM)

Simple Protocol
simple protocol with neither flow nor error control

Simple Protocol
FSMs for the simple protocol

Stop-and-Wait Protocol
Stop-and-Wait protocol, which uses both flow and error control

Stop-and-Wait Protocol
FSM for the Stop-and-Wait protocol

High-level Data Link Control (HDLC)
• It is a bit-oriented protocol for communication over point-to-
point and multipoint links.
• It implements the Stop-and-Wait protocol
• HDLC provides two common transfer modes that can be used
in different configurations:
 Normal response mode (NRM)
 Asynchronous balanced mode (ABM)

High-level Data Link Control (HDLC)
Normal response mode
Asynchronous balanced mode

HDLC frames
Information frames (I-frames)- used to data-link user data and
control information relating to user data
supervisory frames (S-frames)- used only to transport control
information
unnumbered frames (U-frames)- reserved for system
management

HDLC frames
Flag field- synchronization pattern 01111110
Address field- the secondary station
Control field- one or two bytes used for flow and error control
Information field- the user’s data from the network layer
FCS field- The frame check sequence (FCS) error detection field, It
can contain either a 2- or 4-byte CRC

Control field formats
N(S)- define the sequence number of the frame
N(R)- correspond to the acknowledgment number
P/F(Poll/Final)- 1- Poll, sender to receiver ( Receiver address)
0-Final Receiver to sender (sender address)
Control Field for S-Frames

Code 2 bits- used to define type of S frame
Code 2 bits- used to define type of S frame
The last 3 bits, called N(R), correspond to the (ACK) or (NAK),
depending on the type of S-frame
The 2 bits called code represents type of S frame
Receive ready (RR). If the value of the code subfield is 00
Receive not ready (RNR). If the value of the code subfield is 10
Reject (REJ). If the value of the code subfield is 01
Selective reject (SREJ). If the value of the code subfield is 11

POINT-TO-POINT PROTOCOL (PPP)
• PPP defines the format of the frame to be exchanged between
devices.
• It also defines how two devices can negotiate the
establishment of the link and the exchange of data.
• PPP is designed to accept payloads from several network
layers (not only IP).
• The new version of PPP, called Multilink PPP, provides
connections over multiple links

Framing
Address. The address field in this protocol is a constant value and
set to 11111111 (broadcast address)
Control. This field is set to the constant value 00000011
Protocol. The protocol field defines what is being carried in the
data field: either user data or other information
Payload field. This field carries either the user data or other
information

Wired LANs: Ethernet
IEEE standard for LANs

Implementation of standard Etherne

Efficiency of Standard Ethernet
• The efficiency of the Ethernet is defined as the ratio of the
time used by a station to send data to the time the medium is
occupied by this station.
• The practical efficiency of standard Ethernet has been
measured to be
• in which the parameter “a” is the number of frames that can
fit on the medium
• It can be calculated as a = (propagation delay)/(transmission
delay)

Efficiency of Standard Ethernet
Example
In the Standard Ethernet with the transmission rate of 10 Mbps,
we assume that the length of the medium is 2500 m and the size
of the frame is 512 bits. The propagation speed of a signal in a
cable is normally 2 × 108 m/s.

Implementation
10Base5: Thick Ethernet
10Base2: Thin Ethernet

Implementation
10Base-T: Twisted-Pair Ethernet
10Base-F: Fiber Ethernet

Characteristics
• Attenuation
• Interference
• Multipath Propagation
• Error

IEEE 802.11 PROJECT
The standard defines two kinds of services:
• the basic service set (BSS)
• the extended service set (ESS)
Basic service sets (BSSs)

IEEE 802.11 PROJECT
Extended service set (ESS)

MAC Sublayer
IEEE 802.11 defines two MAC sublayers:
•the distributed coordination function (DCF)
•point coordination function (PCF)

Distributed Coordination Function
DIFS- distributed interframe space, SIFS- short interframe
space

Point Coordination Function (PCF)

Bluetooth Layers
L2CAP data packet format

Data Link layer in computer networks cse

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