Lesson_3.2- Data-link-Control Lesson_3.2- Data-link-Control.ppt

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Chapter 3 – Data Link Layer
3.2 Data Link Control

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Overview
Data Link Control has the following
functions:
 Framing
 Flow and Error Control
 Protocol

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FRAMING
FRAMING
•The data link layer needs to pack bits into
The data link layer needs to pack bits into frames.
frames.
•Each frame is distinguishable from another.
Each frame is distinguishable from another.
•Framing types:
Framing types:
•Fixed-Size Framing: Boundary between frames is not necessary
e.g. ATM networks.
•Variable-Size Framing: need methods to define the end and the
beginning of frames. We will discuss Character-oriented and Bit-
oriented methods.

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A frame in a character-oriented protocol
Byte stuffing and unstuffing

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A frame in a bit-oriented protocol
Bit stuffing and unstuffing

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FLOW AND ERROR CONTROL
FLOW AND ERROR CONTROL
The most important responsibilities of the data link
The most important responsibilities of the data link
layer are
layer are flow control
flow control and
and error control
error control.
.
These functions are known as
These functions are known as data link control
data link control.
.
Error control in the data link layer is based on
automatic repeat request, which is the retransmission
of data.
Flow control refers to a set of procedures used to
restrict the amount of data that the sender can send
before waiting for acknowledgment.

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PROTOCOLS
PROTOCOLS
-
- How the data link layer can combine framing, flow control, and
How the data link layer can combine framing, flow control, and
error control to achieve the delivery of data from one node to
error control to achieve the delivery of data from one node to
another?
another?
- The protocols are normally implemented in software.
- The protocols are normally implemented in software.

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NOISELESS CHANNELS
NOISELESS CHANNELS
Stop-and-Wait Protocol

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NOISY CHANNELS
NOISY CHANNELS
Three protocols in this section that use error control.
Three protocols in this section that use error control.
Stop-and-Wait Automatic Repeat Request
Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request

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Stop-and-Wait ARQ
•Keep a copy of the sent frame and retransmit the frame
when the timer expires.
•Use sequence numbers to number the frames.
•The acknowledgment number always announces the
sequence number of the next frame expected.
•Sequence numbers are based on modulo-2 arithmetic.

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Design of the Stop-and-Wait ARQ Protocol

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Example: 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.
If the system data frames are 1000 bits in length, what is the utilization
percentage of the link?
Solution: The bandwidth-delay product is
Link utilization
•The system can send 20,000 bits during the time it takes for the data
to go from the sender to the receiver and then back again.
•However, the system sends only 1000 bits i.e. the link utilization is
only 1000/20,000, or 5%.
•For a link with a high bandwidth or long delay, the use of Stop-and-
Wait ARQ wastes the capacity of the link.

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Go-Back-N Automatic Repeat Request
Selective Repeat Automatic Repeat Request
•Send several frames before receiving acknowledgments
•Keep a copy of sent frames before acknowledgements
arrive

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•In the Go-Back-N Protocol, the sequence numbers are
modulo 2m
, where m is the size of the sequence number
field in bits.
Go-Back-N Automatic Repeat Request

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Figure 3.2.12 Send window for Go-Back-N ARQ

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The send window is an abstract concept
defining an imaginary box of size 2m
− 1
with three variables: Sf, Sn, and Ssize.
Note

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The send window can slide one
or more slots when a valid
acknowledgment arrives.
Note

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Figure 3.2.13 Receive window for Go-Back-N ARQ

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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.
Note

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Figure 3.2.14 Design of Go-Back-N ARQ

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Figure 3.2.15 Window size for Go-Back-N ARQ

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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.
Note

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Algorithm 3.2.7 Go-Back-N sender algorithm
(continued)

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Algorithm 3.2.7 Go-Back-N sender algorithm (continued)

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Algorithm 3.2.8 Go-Back-N receiver algorithm

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Example 3.2.6
Figure 11.16 shows an example of Go-Back-N. This is an
example of a case where the forward channel is reliable,
but the reverse is not. No data frames are lost, but some
ACKs are delayed and one is lost. The example also
shows how cumulative acknowledgments can help if
acknowledgments are delayed or lost. After initialization,
there are seven sender events. Request events are
triggered by data from the network layer; arrival events
are triggered by acknowledgments from the physical
layer. There is no time-out event here because all
outstanding frames are acknowledged before the timer
expires. Note that although ACK 2 is lost, ACK 3 serves
as both ACK 2 and ACK 3.

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Figure 3.2.16 Flow diagram for Example 11.6

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Figure 11.17 shows what happens when a frame is lost.
Frames 0, 1, 2, and 3 are sent. However, frame 1 is lost.
The receiver receives frames 2 and 3, but they are
discarded because they are received out of order. The
sender receives no acknowledgment about frames 1, 2, or
3. Its timer finally expires. The sender sends all
outstanding frames (1, 2, and 3) because it does not know
what is wrong. Note that the resending of frames 1, 2, and
3 is the response to one single event. When the sender is
responding to this event, it cannot accept the triggering of
other events. This means that when ACK 2 arrives, the
sender is still busy with sending frame 3.
Example 3.2.7

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The physical layer must wait until this event is completed
and the data link layer goes back to its sleeping state. We
have shown a vertical line to indicate the delay. It is the
same story with ACK 3; but when ACK 3 arrives, the
sender is busy responding to ACK 2. It happens again
when ACK 4 arrives. Note that before the second timer
expires, all outstanding frames have been sent and the
timer is stopped.
Example 3.2.7 (continued)

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Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note

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Figure 3.2.18 Send window for Selective Repeat ARQ

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Figure 3.2.19 Receive window for Selective Repeat ARQ

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Figure 3.2.20 Design of Selective Repeat ARQ

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Figure 3.2.21 Selective Repeat ARQ, window size

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In Selective Repeat ARQ, the size of the
sender and receiver window
must be at most one-half of 2m
.
Note

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Algorithm 3.2.9 Sender-site Selective Repeat algorithm
(continued)

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Algorithm 3.2.9 Sender-site Selective Repeat algorithm (continued)
(continued)

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Algorithm 3.2.9 Sender-site Selective Repeat algorithm (continued)

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Algorithm 3.2.10 Receiver-site Selective Repeat algorithm

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Algorithm 3.2.10 Receiver-site Selective Repeat algorithm

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Figure 3.2.22 Delivery of data in Selective Repeat ARQ

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This example is similar to Example 11.3 in which frame 1
is lost. We show how Selective Repeat behaves in this
case. Figure 11.23 shows the situation. One main
difference is the number of timers. Here, each frame sent
or resent needs a timer, which means that the timers need
to be numbered (0, 1, 2, and 3). The timer for frame 0
starts at the first request, but stops when the ACK for this
frame arrives. The timer for frame 1 starts at the second
request, restarts when a NAK arrives, and finally stops
when the last ACK arrives. The other two timers start
when the corresponding frames are sent and stop at the
last arrival event.
Example 3.2.8

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At the receiver site we need to distinguish between the
acceptance of a frame and its delivery to the network
layer. At the second arrival, frame 2 arrives and is stored
and marked, but it cannot be delivered because frame 1 is
missing. At the next arrival, frame 3 arrives and is
marked and stored, but still none of the frames can be
delivered. Only at the last arrival, when finally a copy of
frame 1 arrives, can frames 1, 2, and 3 be delivered to the
network layer. There are two conditions for the delivery
of frames to the network layer: First, a set of consecutive
frames must have arrived. Second, the set starts from the
beginning of the window.

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Another important point is that a NAK is sent after the
second arrival, but not after the third, although both
situations look the same. The reason is that the protocol
does not want to crowd the network with unnecessary
NAKs and unnecessary resent frames. The second NAK
would still be NAK1 to inform the sender to resend frame
1 again; this has already been done. The first NAK sent is
remembered (using the nakSent variable) and is not sent
again until the frame slides. A NAK is sent once for each
window position and defines the first slot in the window.

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The next point is about the ACKs. Notice that only two
ACKs are sent here. The first one acknowledges only the
first frame; the second one acknowledges three frames. In
Selective Repeat, ACKs are sent when data are delivered to
the network layer. If the data belonging to n frames are
delivered in one shot, only one ACK is sent for all of them.

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Figure 3.2.24 Design of piggybacking in Go-Back-N ARQ

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3.2-6 HDLC
3.2-6 HDLC
High-level Data Link Control (HDLC)
High-level Data Link Control (HDLC) is a
is a bit-oriented
bit-oriented
protocol for communication over point-to-point and
protocol for communication over point-to-point and
multipoint links. It implements the ARQ mechanisms
multipoint links. It implements the ARQ mechanisms
we discussed in this chapter.
we discussed in this chapter.
Configurations and Transfer Modes
Frames
Control Field
Topics discussed in this section:

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Figure 3.2.25 Normal response mode

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Figure 3.2.26 Asynchronous balanced mode

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Figure 3.2.28 Control field format for the different frame types

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Table 3.2.1 U-frame control command and response

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Figure 11.29 shows how U-frames can be used for
connection establishment and connection release. Node A
asks for a connection with a set asynchronous balanced
mode (SABM) frame; node B gives a positive response
with an unnumbered acknowledgment (UA) frame. After
these two exchanges, data can be transferred between the
two nodes (not shown in the figure). After data transfer,
node A sends a DISC (disconnect) frame to release the
connection; it is confirmed by node B responding with a
UA (unnumbered acknowledgment).
Example 3.2.9

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Figure 3.2.29 Example of connection and disconnection

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Figure 11.30 shows an exchange using piggybacking.
Node A begins the exchange of information with an
I-frame numbered 0 followed by another I-frame
numbered 1. Node B piggybacks its acknowledgment of
both frames onto an I-frame of its own. Node B’s first
I-frame is also numbered 0 [N(S) field] and contains a 2
in its N(R) field, acknowledging the receipt of A’s frames
1 and 0 and indicating that it expects frame 2 to arrive
next. Node B transmits its second and third I-frames
(numbered 1 and 2) before accepting further frames from
node A.
Example 3.2.10

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Its N(R) information, therefore, has not changed: B
frames 1 and 2 indicate that node B is still expecting A’s
frame 2 to arrive next. Node A has sent all its data.
Therefore, it cannot piggyback an acknowledgment onto
an I-frame and sends an S-frame instead. The RR code
indicates that A is still ready to receive. The number 3 in
the N(R) field tells B that frames 0, 1, and 2 have all been
accepted and that A is now expecting frame number 3.

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Figure 3.2.30 Example of piggybacking without error

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Figure 3.2.31 shows an exchange in which a frame is
lost. Node B sends three data frames (0, 1, and 2), but
frame 1 is lost. When node A receives frame 2, it discards
it and sends a REJ frame for frame 1. Note that the
protocol being used is Go-Back-N with the special use of
an REJ frame as a NAK frame. The NAK frame does two
things here: It confirms the receipt of frame 0 and
declares that frame 1 and any following frames must be
resent. Node B, after receiving the REJ frame, resends
frames 1 and 2. Node A acknowledges the receipt by
sending an RR frame (ACK) with acknowledgment
number 3.
Example 3.2.11

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Figure 3.2.31 Example of piggybacking with error

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3.2-7 POINT-TO-POINT PROTOCOL
3.2-7 POINT-TO-POINT PROTOCOL
Although HDLC is a general protocol that can be used
Although HDLC is a general protocol that can be used
for both point-to-point and multipoint configurations,
for both point-to-point and multipoint configurations,
one of the most common protocols for point-to-point
one of the most common protocols for point-to-point
access is the
access is the Point-to-Point Protocol (PPP).
Point-to-Point Protocol (PPP). PPP is a
PPP is a
byte-oriented
byte-oriented protocol.
protocol.
Framing
Transition Phases
Multiplexing
Multilink PPP

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Figure 11.32 PPP frame format

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PPP is a byte-oriented protocol using
byte stuffing with the escape byte
01111101.
Note

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Figure 3.2.33 Transition phases

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Figure 3.2.34 Multiplexing in PPP

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Figure 3.2.35 LCP packet encapsulated in a frame

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Figure 3.2.36 PAP packets encapsulated in a PPP frame

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Figure 3.2.37 CHAP packets encapsulated in a PPP frame

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Figure 3.2.38 IPCP packet encapsulated in PPP frame

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Table 3.2.4 Code value for IPCP packets

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Figure 3.2.39 IP datagram encapsulated in a PPP frame

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Figure 3.2.40 Multilink PPP

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Let us go through the phases followed by a network layer
packet as it is transmitted through a PPP connection.
Figure 3.2.41 shows the steps. For simplicity, we assume
unidirectional movement of data from the user site to the
system site (such as sending an e-mail through an ISP).
The first two frames show link establishment. We have
chosen two options (not shown in the figure): using PAP
for authentication and suppressing the address control
fields. Frames 3 and 4 are for authentication. Frames 5
and 6 establish the network layer connection using IPCP.
Example 3.2.12

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The next several frames show that some IP packets are
encapsulated in the PPP frame. The system (receiver)
may have been running several network layer protocols,
but it knows that the incoming data must be delivered to
the IP protocol because the NCP protocol used before the
data transfer was IPCP.
After data transfer, the user then terminates the data link
connection, which is acknowledged by the system. Of
course the user or the system could have chosen to
terminate the network layer IPCP and keep the data link
layer running if it wanted to run another NCP protocol.

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Figure 3.2.41 An example (continued)

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