Different protocols for data communication networks

Different protocols for data
communication network
Chapter 11
Data communication and networks

Framing
• The physical layer provides bit synchronization to ensure that the sender
and receiver use the same bit durations and timing.
• Data link layer needs to pack bits into frames, so that each frame is
distinguishable from another
• 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: There is no need for defining the boundaries of the
frames . ( example : ATM )
• Variable-size framing : Need a way to define the end of the frame and the
beginning of the next .
1. Character-oriented approach
2. bit-oriented approach

Character-Oriented Protocols
• To separate one frame from the next, an 8-bit (1-byte) flag is added at the
beginning and the end of a frame.
• Any pattern used for the flag could also be part of the information. If this
happens, the receiver, when it encounters this pattern in the middle of the data,
thinks it has reached the end of the frame.
• The data section is stuffed with an extra byte “byte stuffing” ,This byte is usually
called the escape character (ESC), which has a predefined bit pattern. Whenever
the receiver encounters the ESC character, it removes it from the data section and
treats the next character as data .
• If the ESC characters that are part of
the text must also be marked by another
ESC character. In other words, if the ESC
character is part of the text, an extra one
is added to show that the second one
is part of the text

Bit-Oriented Protocols
• Bit stuffing is the process of adding one extra “0” whenever five
consecutive 1’s follow a “0” in the data, so that the receiver does not
mistake the pattern “01111110” for a flag.
• This extra stuffed bit is eventually removed from the data by the receiver
Example :
Data : 01111110
Stuffed data : 011111010
Stuffed bit will be removed by receiver

flow control & error control
• 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 acknowledgment.
• Error control : It allows the receiver to inform the sender of any frames
lost or damaged in transmission and coordinates the retransmission of
those frames by the sender. This process is called automatic repeat
request (ARQ).

Simplest Protocol ( no flow control and
no error control )
• There is no action until there is a request from the network layer.
• The sender site cannot send a frame until its network layer has a data packet to
send. The receiver site cannot deliver a data packet to its network layer until a
frame arrives

Simplest Protocol
• Sender-site algorithm
• Receiver-site algorithm

Stop-and-Wait Protocol
• sender sends one frame, stops until it receives confirmation from the
receiver (okay to go ahead), and then sends the next frame .
• Adds flow control to simplest protocol
• At any time, there is either one data frame on the forward channel or one
ACK frame on the reverse channel (half-duplex link ) .

Stop-and-Wait Protocol

Stop-and-Wait ARQ
• Adds a simple error control mechanism to the Stop-and-Wait Protocol .
• When the frame arrives at the receiver site, it is checked and if it is corrupted, it is
silently discarded , same process is done vice-versa .
• The sender keeps a copy of the sent frame. At the same time, it starts a timer. If
the timer expires and there is no ACK for the sent frame, the frame is resent, the
copy is held, and the timer is restarted .
• Different cases :
1. The frame arrives safe and sound at the receiver site , the receiver sends an
acknowledgment. The acknowledgment arrives at the sender site, causing the
sender to send the next frame numbered x + 1.
2. The frame arrives safe and sound at the receiver site; the receiver sends an
acknowledgment, but the acknowledgment is corrupted or lost. The sender
resends the frame (numbered x) after the time-out.
3. The frame is corrupted or never arrives at the receiver site; the sender resends
the frame (numbered x) after the time-out
• sequence numbering :
Sender to receiver frame’s
1st frame  0 : 2nd frame  1 : 3rd frame  0 : 4th frame  1 and so on
Receiver to sender frame’s
if frame 0 has arrived safe and sound, the receiver sends an ACK frame with
acknowledgment 1 (meaning frame 1 is expected next).

Stop-and-Wait ARQ

Go-Back-N ARQ
• In this protocol we can send several frames before receiving
acknowledgments; we keep a copy of these frames until the ACK
arrive.
• If the header of the frame allows m bits for the sequence number,
the sequence numbers range from 0 to 2m - 1.
• 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.
• 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.
• The receiver sends a positive acknowledgment if a frame has
arrived safe and sound and in order. If a frame is damaged or is
received out of order, the receiver is silent and will discard all
subsequent frames until it receives the one it is expecting

Selective Repeat ARQ
• In a noisy link a frame has a higher probability
of damage, which means the resending of
multiple frames. This resending uses up the
bandwidth and slows down the transmission .
• another mechanism that does not resend “N”
frames when just 1 frame is damaged; only
the damaged frame is resent. This mechanism
is called Selective Repeat ARQ. It is more
efficient for noisy links .

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

HDLC
• High-level Data Link Control : bit-oriented protocol for
communication
• Two common transfer mode: normal response mode (NRM) and
asynchronous balanced mode (ABM)

HDLC: Frames
• I(information)-frames are used to transport user data and control
information relating to user data (piggybacking) .
• S(supervisory)-frames are used only to transport control information.
• U(unnumbered)-frames are reserved for system management.
• In multiple-frame transmissions, the ending flag of one frame can serve as
the beginning flag of the next frame.

HDLC: Frames fields
• Flag field. The flag field of an HDLC frame is an 8-bit sequence with the bit pattern
01111110
• Address field. The second field of an HDLC frame contains the address of the
secondary station. If a primary station created the frame, it contains a to address.
If a secondary creates the frame, it contains a from address. An address field can
be 1 byte or several bytes long, depending on the needs of the network. One byte
can identify up to 128 stations (l bit is used for another purpose).
• Control field. The control field is a 1- or 2-byte segment of the frame used for flow
and error control. The interpretation of bits in this field depends on the frame type
and its functionality
• Information field. The information field contains the user's data from the network
layer or management information. Its length can vary from one network to
another.
• FCS field. The frame check sequence (FCS) is the
HDLC error detection field. It can contain either a
2- or 4-byte ITU-T CRC.

Control fields
• Control Field for I-Frames :
1. If the first bit of the control field is 0, this means the frame is an I-frame.
2. The next 3 bits, called N(S), define the sequence number of the frame.
3. The last 3 bits, called N(R), correspond to the acknowledgment number
when piggybacking is used.
4. The single bit between N(S) and N(R) is called the P/F ( poll/final) bit. It has
meaning only when it is set (bit = 1) and can mean poll or final. It means poll
when the frame is sent by a primary station to a secondary. It means final
when the frame is sent by a secondary to a primary .
• Control Field for S-Frames :
1. If the first 2 bits of the control field is 10, frame is an S-frame.
2. The last 3 bits, called N(R), corresponds to the acknowledgment number
(ACK) or negative acknowledgment number (NAK) .
3. The 2 bits called code is used to define the type of S-frame
00 Receive ready n(r) = ACK number
01 Reject (REJ). n(r) = NAK number
10 Receive not ready (RNR) n(r) = ACK number
11 Selective reject (SREJ) n(r) = NAK number

Point-to-Point Protocol: PPP
• Flag: 01111110 the same as HDLC, but it treated as a byte
because of PPP is a byte-oriented protocol
• Address: constant value ie 11111111 (broadcast address)
• Control: This field is set to the constant value 11000000 , No
need because PPP has no flow control and limited error control
• Protocol. The protocol field defines what is being carried in
the data field: either user data or other information.
• This field carries either the user data or other information
that we will discuss shortly. The data field is a sequence of
bytes with the default of a maximum of 1500 bytes;
• This field is set to the constant value 11000000

Different protocols for data communication networks

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