DATA LINK CONTROL

Data Link Layer Design Issues

• Services Provided to the Network Layer
• Framing
• Error Control
• Flow Control
FRAMING

The data link layer needs to pack bits into frames, so

that each frame is distinguishable from another. Our
postal system practices a type of framing. The simple
act of inserting a letter into an envelope separates one
piece of information from another; the envelope serves
as the delimiter.
Topics discussed in this section:
Fixed-Size Framing
Variable-Size Framing
Types of Framing

◼ Fixed Size Framing

◼ Variable Size Framing--- Character
oriented protocols and Bit oriented
Protocols.
A frame in a character-oriented protocol
Byte stuffing and unstuffing
Note

Byte stuffing is the process of adding 1

extra byte whenever there is a flag or
escape character in the text.
A frame in a bit-oriented protocol
Note

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.
Bit stuffing and unstuffing
FLOW AND ERROR CONTROL

The most important responsibilities of the data link

layer are flow control and error control. Collectively,
these functions are known as data link control.

Topics discussed in this section:

Flow Control
Error Control
Note

Flow control refers to a set of procedures

used to restrict the amount of data
that the sender can send before
waiting for acknowledgment.
Note

Error control in the data link layer is

based on automatic repeat request,
which is the retransmission of data.
Taxonomy of protocols discussed in this chapter
NOISELESS CHANNELS

Let us first assume we have an ideal channel in which

no frames are lost, duplicated, or corrupted. We
introduce two protocols for this type of channel.

Topics discussed in this section:

Simplest Protocol
Stop-and-Wait Protocol
Simplest Protocol

◼ No flow and Error Control

The design of the simplest protocol with no flow or error control
Flow diagram for Simplest
Stop-and-Wait Protocol

◼ Sender sends one frame, stops until it

gets confirmation from receiver.
Design of Stop-and-Wait Protocol
Flow diagram for Stop and Wait
NOISY CHANNELS

Although the Stop-and-Wait Protocol gives us an idea

of how to add flow control to its predecessor, noiseless
channels are nonexistent. We discuss three protocols
in this section that use error control.

Topics discussed in this section:

Stop-and-Wait Automatic Repeat Request
Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request
Note

In Stop-and-Wait ARQ, we use sequence

numbers to number the frames.
The sequence numbers can be
1,0,1,0,1…
Note

In Stop-and-Wait ARQ, the

acknowledgment number is 0 if
sequence number is 1 and
acknowledgment number is 1 if
sequence number is 0
Design of the Stop-and-Wait ARQ Protocol
Flow diagram for Example 11.3
Disadvantage Stop-and-Wait ARQ
Protocol
◼ Inefficient---if channel is thick and long
◼ Thick means high bandwidth
◼ Long means roundtrip delay
◼ Product of both is bandwidth delay.
◼ Bandwidth delay is number of bits we can
send while waiting for news from receiver.
 Assume that, in a Stop-and-Wait ARQ system, the
bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to
make a round trip. What is the bandwidth-delay product?
If the system data frames are 1000 bits in length, what is
the utilization percentage of the link?

Solution
The bandwidth-delay product is

11.28
Pipelining

◼ Task begins before end of first task.

◼ Stop-and-Wait ARQ does not use pipelining
but other two techniques do.

◼ This improves efficiency.

Go-Back-N Protocol

◼ This sends multiple frames before

receiving acknowledgment from receiver.
Note

In the Go-Back-N Protocol, the sequence

numbers are modulo 2m,
where m is the size of the sequence
number field in bits.
Sliding Window

◼ Defines the range of sequence numbers

that is concern of sender and receiver.
◼ The range which is concern of sender is
called sender sliding window.
◼ The range which is concern of receiver is
called receiver sliding window.
Send window for Go-Back-N ARQ
Note

The send window is an abstract concept

defining an imaginary box of size 2m − 1
with three variables: Sf, Sn, and Ssize.
Note

The send window can slide one

or more slots when a valid
acknowledgment arrives.
Receive window for Go-Back-N ARQ
Note

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.
Design of Go-Back-N ARQ
Note

In Go-Back-N ARQ, the size of the send

window must be less than 2m;
the size of the receiver window
is always 1.
Flow diagram
Flow diagram
Note

Stop-and-Wait ARQ is a special case of

Go-Back-N ARQ in which the size of the
send window is 1.
Send window for Selective Repeat ARQ
Receive window for Selective Repeat ARQ
Design of Selective Repeat ARQ
Note

In Selective Repeat ARQ, the size of the

sender and receiver window
must be at most one-half of 2m.
Delivery of data in Selective Repeat ARQ
Flow diagram
Design of piggybacking in Go-Back-N ARQ