Lecture 2 data link layer 1 v1

1. Introduction
• In this lecture we look at algorithms for achieving reliable,
efficient communication of whole units of information called
frames (rather than individual bits, as in the physical layer)
between two adjacent machines.
• By adjacent, we mean that the two machines are connected
by a communication channel.
• The data link layer uses the services of the physical layer to
send and receive bits over communication channels.
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2. Data Link Layer Design Issues
• The following functions need to be incorporated during data
link layer protocol design:
– Providing a well-defined service interface to the network layer.
– Error control: Dealing with transmission errors.
– Flow control: Regulating the flow of data so that slow receivers are not
swamped by fast senders.
• To accomplish these goals, the data link layer takes the
packets it gets from the network layer and encapsulates them
into frames for transmission.
• Each frame contains a frame header, a payload field for
holding the packet, and a frame trailer.
3

2. Data Link Layer Design Issues
4

2.1Services Provided to the Network Layer
• The data link layer transfers data from the network layer on
the source machine to the network layer on the destination
machine.
5

2.1 Services Provided to the Network Layer
• The data link layer can be designed to offer different services.
• The actual services that are offered vary from protocol to
protocol.
• Three possibilities of services are:
– Unacknowledged connectionless service.
– Acknowledged connectionless service.
– Acknowledged connection-oriented service.
6

• Unacknowledged connectionless service.
– Unacknowledged connectionless service consists of having the source
machine send independent frames to the destination machine without
having the destination machine acknowledge them.
– Ethernet is a good example of a data link layer that provides this
class of service.
– No logical connection is established beforehand or released
afterward.
– If a frame is lost due to noise on the line, no attempt is made to
detect the loss or recover from it in the data link layer.
– This class of service is appropriate when the error rate is very low, so
recovery is left to higher layers.
– It is also appropriate for real-time traffic, such as voice, in which late
data are worse than bad data.
7

• Acknowledged connectionless service.
– When this service is offered, there are still no logical connections
used, but each frame sent is individually acknowledged.
– In this way, the sender knows whether a frame has arrived correctly or
been lost.
– If it has not arrived within a specified time interval, it can be sent
again.
– This service is useful over unreliable channels, such as wireless
systems. 802.11 (WiFi) is a good example of this class of service.
8

• Acknowledged connection-oriented
service.
– With this service, the source and destination machines establish a
connection before any data are transferred.
– Each frame sent over the connection is numbered, and the data link
layer guarantees that each frame sent is indeed received.
– Guarantees that all frames are received in the right order.
– It is appropriate over long, unreliable links such as a satellite channel
or a long-distance telephone circuit.
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2.2 Error Detection and Correction
• For unacknowledged connectionless service it might be fine if
the sender just kept outputting frames without regard to
whether they were arriving properly.
• But for reliable, connection-oriented service it would not be
fine at all.
• Sender can send either send:
– a positive or negative acknowledgement for correctly received frames
– indicates correctly received frames
– or negative acknowledgement for frames not received - indicates
incorrectly received frames – frames needs to be retransmitted
10

2.2 Error Detection and Correction
• There are two basic strategies for dealing with errors:
– Error correction:
• Include enough redundant information to enable the receiver to deduce what the
transmitted data must have been.
• This is also known as Forward Correction (FEC).
• Used on noisy channels such as wireless links that may make many errors
• FEC is used on unreliable/noisy channels because retransmissions are just as likely
to be in error as the first transmission e.g wireless channels
– Error detection:
• Include only enough redundancy to allow the receiver to deduce that an error has
occurred (but not which error) and have it request a retransmission.
• Used on channels that are highly reliable such as fiber
11

3.Elementary Data Link Protocols
• Data Link Layer protocols are responsible to ensure and
confirm that the bits and bytes that are received are identical
to the bits and bytes being transferred from the sender.
• Data Link Layer protocols refers a set of specifications that
are used for implementation of data link layer
• Elementary data link protocols refer to basic data link
protocols.
• We look at the basic data link protocols before we look at
more advanced ones.
• We will look at simplex protocols here
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3.Elementary Data Link Protocols
• Some of the data link layer process and the physical layer
process run on dedicated hardware called a NIC (Network
Interface Card).
• The rest of the link layer process and the network layer
process run on the main CPU as part of the operating system,
with the software for the link layer process often taking the
form of a device driver.
13

3. Elementary Data Link Protocols
Continued 
Some definitions needed in the protocols to follow. These are located in
the file protocol.h.
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(1.2)Protocol
Definitions
(ctd.)
Some definitions needed in
the protocols to follow. These
are located in the file
protocol.h.
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• A frame is composed of four fields: kind, seq, ack, and info
• The first three of which contain control information and the
last of which may contain actual data to be transferred. These
control fields are collectively called the frame header.
• The kind field tells whether there are any data in the frame,
because some of the protocols distinguish frames containing
only control information from those containing data as well.
• The seq and ack fields are used for sequence numbers
and acknowledgements, respectively; their use will be
described in more detail later.
• The info field of a data frame contains a single packet; the info
field of a control frame is not used.
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• Initially, the receiver has nothing to do. It just sits around
waiting for something to happen.
• In the example protocols we will indicate that the data link
layer is waiting for something to happen by the procedure call
wait for event(&event).
• This procedure only returns when something has happened
(e.g., a frame has arrived).
• Upon return, the variable event tells what happened.
• The set of possible events differs for the various protocols
depending on the complexity of the protocol
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• When a frame arrives at the receiver, the checksum is
recomputed.
• If the checksum in the frame is incorrect (i.e., there was a
transmission error), the data link layer is so informed (event =
cksum err).
• If the inbound frame arrived undamaged, the data link layer is
also informed (event = frame arrival) so that it can acquire the
frame for inspection using from physical layer.
• As soon as the receiving data link layer has acquired an
undamaged frame, it checks the control information in the
header, and, if everything is all right, passes the packet
portion to the network layer.
18

• The procedures to network layer and from network layer are
used by the data link layer to pass packets to the network
layer and accept packets from the network layer, respectively.
• Note that from physical layer and to physical layer pass
frames between the data link layer and the physical layer.
• In other words, to network layer and from network layer deal
with the interface between layers 2 and 3, whereas from
physical layer and to physical layer deal with the interface
between layers 1 and 2.
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• In most of the protocols, we assume that the channel is
unreliable and loses entire frames upon occasion.
• To be able to recover from such calamities, the sending data
link layer must start an internal timer or clock whenever it
sends a frame.
• If no reply has been received within a certain predetermined
time interval, the clock times out and the data link layer
receives an interrupt signal.
• In our protocols this is handled by allowing the procedure
wait for event to return event = timeout.
• The procedures start timer and stop timer turn the timer on
and off, respectively.
20

• Timeout events are possible only when the timer is running
and before stop timer is called.
• It is explicitly permitted to call start timer while the timer is
running; such a call simply resets the clock to cause the next
timeout after a full timer interval has elapsed (unless it is
reset or turned off).
21

• The procedures enable network layer and disable network
layer are used in the more sophisticated protocols.
• When the data link layer enables the network layer, the
network layer is then permitted to interrupt when it has a
packet to be sent.
• We indicate this with event = network layer ready.
• When the network layer is disabled, it may not cause such
events.
• By being careful about when it enables and disables its
network layer, the data link layer can prevent the network
layer from swamping it with packets for which it has no buffer
space.
22

• Frame sequence numbers are always in the range 0 to MAX
SEQ (inclusive), where MAX SEQ is different for the different
protocols.
• It is frequently necessary to advance a sequence number by 1
circularly (i.e., MAX SEQ is followed by 0).
• The macro inc performs this incrementing. It has been defined
as a macro because it is used in-line within the critical path.
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• There are three types of elementary data link protocols:
– An Unrestricted Simplex Protocol
– A Simplex Stop-and-Wait Protocol
– A Simplex Protocol for a Noisy Channel
• They are all simplex protocols - data is transmitted in one
direction only
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3.1 An Unrestricted Simplex Protocol
• This protocol assumes nothing ever goes wrong
• Data are transmitted in one direction only.
• Both the transmitting and receiving network layers are always
ready.
• Infinite buffer space is available.
• And best of all, the communication channel between the data
link layers never damages or loses frames.
• No sequence numbers or acknowledgements are used here,
so MAX SEQ is not needed.
• The only event type possible is frame arrival (i.e., the arrival
of an undamaged frame).
25

• The protocol consists of two distinct procedures, a sender and
a receiver.
• The sender runs in the data link layer of the source machine,
and the receiver runs in the data link layer of the destination
machine.
26

Unrestricted
Simplex
Protocol
27

• The sender is in an infinite while loop just pumping data out
onto the line as fast as it can.
• The body of the loop consists of three actions:
– go fetch a packet from the (always obliging) network layer,
– construct an outbound frame using the variables,
– and send the frame on its way.
• Only the info field of the frame is used by this protocol,
because the other fields have to do with error and flow
control and there are no errors or flow control restrictions
here.
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• The receiver is equally simple.
• Initially, it waits for something to happen, the only possibility
being the arrival of an undamaged frame.
• Eventually, the frame arrives and the procedure wait for event
returns, with event set to frame arrival (which is ignored
anyway).
• The call to from physical layer removes the newly arrived
frame from the hardware buffer and puts it in the variable r,
where the receiver code can get at it.
• Finally, the data portion is passed on to the network layer, and
the data link layer settles back to wait for the next frame,
effectively suspending itself until the frame arrives.
29

3.2 A Simplex Stop-and-Wait Protocol
• Now we will tackle the problem of preventing the sender from
flooding the receiver with frames faster than the latter is able
to process them because of limited buffer space -> Flow
control.
• Solution to this problem is to have the receiver provide
feedback to the sender.
• After having passed a packet to its network layer, the receiver
sends a little dummy frame back to the sender which, in
effect, gives the sender permission to transmit the next
frame.
30

• After having sent a frame, the sender is required by the
protocol to bide its time until the little dummy (i.e.,
acknowledgement) frame arrives.
• This delay is a simple example of a flow control protocol.
• Protocols in which the sender sends one frame and then
waits for an acknowledgement before proceeding are called
stop-and-wait.
31

(3.2)
Simplex
Stop-and-
Wait
Protocol
32

• As in protocol 1, the sender starts out by fetching a packet
from the network layer, using it to construct a frame, and
sending it on its way.
• But now, unlike in protocol 1, the sender must wait until an
acknowledgement frame arrives before looping back and
fetching the next packet from the network layer.
• The sending data link layer need not even inspect the
incoming frame as there is only one possibility.
• The incoming frame is always an acknowledgement.
33

• The only difference between receiver1 and receiver2 is that
after delivering a packet to the network layer, receiver2 sends
an acknowledgement frame back to the sender before
entering the wait loop again.
34

3.3 A Simplex Protocol for a Noisy Channel
• Now let us consider the normal situation of a communication
channel that makes errors.
• Frames may be either damaged or lost completely.
• However, we assume that if a frame is damaged in transit, the
receiver hardware will detect this
• Protocols in which the sender waits for a positive
acknowledgement before advancing to the next data item are
often called ARQ (Automatic Repeat reQuest) or PAR
(Positive Acknowledgement with Retransmission).
35

A Simplex Protocol for a Noisy Channel
A positive
acknowledgement with
retransmission protocol.
Continued  36

A Simplex Protocol for a Noisy Channel
A positive acknowledgement with retransmission protocol.
37

• Protocol 3 differs from its predecessors in that both sender
and receiver have a variable whose value is remembered
while the data link layer is in the wait state.
• The sender remembers the sequence number of the next
frame to send in next frame to send; the receiver remembers
the sequence number of the next frame expected in frame
expected.
• After transmitting a frame, the sender starts the timer
running.
38

• If it was already running, it will be reset to allow another full
timer interval.
• Only when that interval has elapsed is it safe to assume that
either the transmitted frame or its acknowledgement has
been lost, and to send a duplicate frame.
• After transmitting a frame and starting the timer, the sender
waits for something exciting to happen.
• Only three possibilities exist:
– an acknowledgement frame arrives undamaged,
– a damaged acknowledgement frame staggers in,
– or the timer expires.
39

• If a valid acknowledgement comes in, the sender fetches the
next packet from its network layer and puts it in the buffer,
overwriting the previous packet.
• It also advances the sequence number.
• If a damaged frame arrives or the timer expires, neither the
buffer nor the sequence number is changed so that a
duplicate can be sent.
• In all cases, the contents of the buffer (either the next packet
or a duplicate) are then sent.
40

• When a valid frame arrives at the receiver, its sequence
number is checked to see if it is a duplicate
• If not, it is accepted, passed to the network layer, and an
acknowledgement is generated.
41

Sliding Window Protocols
• In the previous protocols, data frames were transmitted in
one direction only.
• In most practical situations, there is a need to transmit data in
both directions.
• One way of achieving full-duplex data transmission is to run
two instances of one of the previous protocols, each using a
separate link for simplex data traffic (in different directions).
• Each link is then comprised of a ‘‘forward’’ channel (for
data) and a ‘‘reverse’’ channel (for acknowledgements).
• In both cases the capacity of the reverse channel is almost
entirely wasted – you only use it to send ack frames
42

4. Sliding Window Protocols
• A better idea is to use the same link for data in both
directions.
• Sliding window protocols allow sending of data in both
directions
• After all, in protocols 2 and 3 it was already being used to
transmit frames both ways, and the reverse channel normally
has the same capacity as the forward channel.
• In this model the data frames from A to B are intermixed with
the acknowledgement frames from A to B - piggybaking.
• By looking at the kind field in the header of an incoming
frame, the receiver can tell whether the frame is data or an
acknowledgement.
43

• A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.
44

• We will look at three slisding window protocols:
– One bit sliding window protocol
– Stop and wait protocol
– Selective repeat protocol
• The are all bidirectional protocols
• In these, as in all sliding window protocols, each outbound frame
contains a sequence number, ranging from 0 up to some maximum.
• The stop-and-wait sliding window protocol uses n = 1, restricting
the sequence numbers to 0 and 1, but more sophisticated versions
can use an arbitrary n.
45

• The essence of all sliding window protocols is that at any
instant of time, the sender maintains a set of sequence
numbers corresponding to frames it is permitted to send.
• These frames are said to fall within the sending window.
• Similarly, the receiver also maintains a receiving window
corresponding to the set of frames it is permitted to accept.
• The sequence numbers within the sender’s window represent
frames that have been sent or can be sent but are as yet not
acknowledged.
46

• Since frames currently within the sender’s window may
ultimately be lost or damaged in transit, the sender must keep
all of these frames in its memory for possible retransmission.
• Thus, if the maximum window size is n, the sender needs
n buffers to hold the unacknowledged frames.
• If the window ever grows to its maximum size, the sending
data link layer must forcibly shut off the network layer until
another buffer becomes free.
• The receiving data link layer’s window corresponds to the
frames it may accept.
• Any frame falling within the window is put in the receiver’s
buffer.
47

4.1 One Bit Sliding Window
• Before tackling the general case, let us examine a sliding
window protocol with a window size of 1.
• Such a protocol uses stop-and-wait since the sender transmits
a frame and waits for its acknowledgement before sending
the next one.
• Under normal circumstances, one of the two data link layers
goes first and transmits the first frame.
48

• The starting machine fetches the first packet from its network
layer, builds a frame from it, and sends it.
• When this (or any) frame arrives, the receiving data link layer
checks to see if it is a duplicate, just as in protocol 3.
• If the frame is the one expected, it is passed to the network
layer and the receiver’s window is slid up.
• The acknowledgement field contains the number of the last
frame received without error.
49

• If this number agrees with the sequence number of the
frame the sender is trying to send, the sender knows it is
done with the frame stored in buffer and can fetch the next
packet from its network layer.
• If the sequence number disagrees, it must continue trying to
send the same frame.
• Whenever a frame is received, a frame is also sent back.
50

4.1 A One-Bit Sliding Window Protocol
• The notation is (seq, ack, packet number).
51

4.1 A One-Bit Sliding Window Protocol
Continued  52

4.1 A One-Bit Sliding Window Protocol (ctd.)
53

4.2 A Protocol Using Go-Back-N
• Until now we have made the tacit assumption that the
transmission time required for a frame to arrive at the
receiver plus the transmission time for the acknowledgement
to come back is negligible.
• Sometimes this assumption is clearly false.
• In these situations the long round-trip time can have
important implications for the efficiency of the bandwidth
utilization.
• As an example, consider a 50-kbps satellite channel with a
500-msec round-trip propagation delay.
• Let us imagine trying to use protocol 4 to send 1000-bit
frames via the satellite. At t = 0 the sender starts sending the
first frame.
54

• At t = 20 msec the frame has been completely sent.
• Not until t = 270 msec has the frame fully arrived at the
receiver, and not until t = 520 msec has the acknowledgement
arrived back at the sender, under the best of circumstances
(of no waiting in the receiver and a short acknowledgement
frame).
• This means that the sender was blocked 500/520 or 96%
of the time.
• In other words, only 4% of the available bandwidth was used.
• Clearly, the combination of a long transit time, high
bandwidth, and short frame length is disastrous in terms of
efficiency.
55

• The problem described here can be viewed as a consequence
of the rule requiring a sender to wait for an acknowledgement
before sending another frame.
• If we relax that restriction, much better efficiency can be
achieved.
• Basically, the solution lies in allowing the sender to transmit
up to w frames before blocking, instead of just 1.
• With a large enough choice of w the sender will be able to
continuously transmit frames since the acknowledgements
will arrive for previous frames before the window becomes
full, preventing the sender from blocking.
56

4.2 A Protocol Using Go Back N
57

4.2 A Protocol Using Go Back N
58

• This technique of keeping multiple frames in flight is an
example of pipelining. Pipelining frames over an unreliable
communication channel raises some serious issues.
• First, what happens if a frame in the middle of a long stream
is damaged or lost?
• Large numbers of succeeding frames will arrive at the receiver
before the sender even finds out that anything is wrong.
• When a damaged frame arrives at the receiver, it obviously
should be discarded, but what should the receiver do with all
the correct frames following it?
• Remember that the receiving data link layer is obligated to
hand packets to the network layer in sequence.
59

• One option, called go-back-n, is for the receiver simply to
discard all subsequent frames, sending no acknowledgements
for the discarded frames.
• This strategy corresponds to a receive window of size 1.
• In other words, the data link layer refuses to accept any frame
except the next one it must give to the network layer.
• Eventually, the sender will time out and retransmit all
unacknowledged frames in order, starting with the damaged
or lost one.
• This approach can waste a lot of bandwidth if the error rate is
high.
60

4.3 A Protocol Using Selective Repeat
• The go-back-n protocol works well if errors are rare, but if the
line is poor it wastes a lot of bandwidth on retransmitted
frames.
• An alternative strategy, the selective repeat protocol, is to
allow the receiver to accept and buffer the frames following a
damaged or lost one.
• In this protocol, both sender and receiver maintain a window
of outstanding and acceptable sequence numbers,
respectively.
• The sender’s window size starts out at 0 and grows to some
predefined maximum.
61

4.3 A Protocol Using Selective Repeat
• When it is used, a bad frame that is received is discarded, but
any good frames received after it are accepted and buffered.
• When the sender times out, only the oldest unacknowledged
frame is retransmitted.
• If that frame arrives correctly, the receiver can deliver to the
network layer, in sequence, all the
frames it has buffered.
• Selective repeat corresponds to a receiver window larger
than 1.
62

4.3 Sliding Window Protocol Using Selective
Repeat
63

Lecture 2 data link layer 1 v1

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