Dcn data link_layer

DATA COMMUNICATION NETWORK
Mangal Das
Assistant Professor
Electronics And Communication
Department
1

PHYSICAL LAYER: TRANSMISSION MEDIA
 The physical layer is the layer that actually interacts with
the transmission media, the physical part of the network
that connects network components together. This layer
is involved in physically carrying information from one
node in the network to the next.
 The physical layer has complex tasks to perform. One
major task is to provide services for the data link layer.
The data in the data link layer consists of 0s and 1s
organized into frames that are ready to be sent across
the transmission medium. This stream of Os and I s
must first be converted into another entity: signals. One
of the services provided by the physical layer is to
create a signal that represents this stream of bits. 4

PHYSICAL LAYER: TRANSMISSION MEDIA
 The physical layer must also take care of the
physical network, the transmission medium. The
transmission medium is a passive entity; it has no
internal program or logic for control like other
layers. The transmission medium must be
controlled by the physical layer. The physical layer
decides on the directions of data flow. The physical
layer decides on the number of logical channels for
transporting data coming from different sources.
5

GUIDED MEDIA
 Guided media, which are those that provide a
conduit from one device to another, include twisted-
pair cable, coaxial cable, and fiber-optic cable. A
signal traveling along any of these media is directed
and contained by the physical limits of the medium.
Twisted-pair and coaxial cable use metallic (copper)
conductors that accept and transport signals in the
form of electric current. Optical fiber is a cable that
accepts and transports signals in the form of light.
6

TWISTED-PAIR CABLE
 A twisted pair consists of two conductors (normally copper),
each with its own plastic insulation, twisted together, as shown
in Figure.
 One of the wires is used to carry signals to the receiver, and
the other is used only as a ground reference. The receiver
uses the difference between the two.
 In addition to the signal sent by the sender on one of the
wires, interference (noise) and crosstalk may affect both wires
and create unwanted signals.
7

TWISTED-PAIR CABLE
 If the two wires are parallel, the effect of these unwanted
signals is not the same in both wires because they are
at different locations relative to the noise or crosstalk
sources(e,g., one is closer and the other is farther). This
results in a difference at the receiver. By twisting the
pairs, a balance is maintained. For example, suppose in
one twist, one wire is closer to the noise source and the
other is farther; in the next twist, the reverse is true.
 Twisting makes it probable that both wires are equally
affected by external influences (noise or crosstalk). This
means that the receiver, which calculates the difference
between the two, receives no unwanted signals.
8

COAXIAL CABLE
 Coaxial cable (or coax) carries signals of higher
frequency ranges than those in twisted pair cable,
in part because the two media are constructed quite
differently. Instead of having two wires, coax has a
central core conductor of solid or stranded wire
(usually copper) enclosed in an insulating sheath,
which is, in turn, encased in an outer conduct or of
metal foil, braid, or a combination of the two. The
outer metallic wrapping serves both as a shield
against noise and as the second conductor, which
completes the circuit. This outer conductor is also
enclosed in an insulating sheath, and the whole
cable is protected by a plastic cover.
9

COAXIAL CABLE
 Coaxial cables are categorized by their radio
government (RG) ratings. Each RG number
denotes a unique set of physical specifications,
including the wire gauge of the inner conductor, the
thickness and type of the inner insulator, the
construction of the shield, and the size and type of
the outer casing.
10

FIBER-OPTIC CABLE
 A fiber-optic cable is made of glass or plastic and
transmits signals in the form of light.
 Optical fibers use reflection to guide light through a
channel. A glass or plastic core is surrounded by a
cladding of less dense glass or plastic. The
difference in density of the two materials must be
such that a beam of light moving through the core is
reflected off the cladding instead of being refracted
into it.
12

FIBER-OPTIC CABLE
 Current technology supports two modes (multimode and
single mode) for propagating light along optical
channels, each requiring fiber with different physical
characteristics. Multimode can be implemented in two
forms: step-index or graded-index.
 Multimode Multimode is so named because multiple
beams from a light source move through the core in
different paths. How these beams move within the cable
depends on the structure of the core.
 Single-mode uses step-index fiber and a highly focused
source of light that limits beams to a small range of
angles, all close to the horizontal. The single mode fiber
itself is manufactured with a much smaller diameter than
that of multimode fiber, and with substantialiy lower
density (index of refraction).
14

ADVANTAGES OF OPTICAL FIBER
 Higher bandwidth. Fiber-optic cable can support
dramatically higher bandwidths (and hence data
rates) than either twisted-pair or coaxial cable.
Currently, data rates and bandwidth utilization over
fiber-optic cable are limited not by the medium but
by the signal generation and reception technology
available.
 Less signal attenuation. Fiber-optic transmission
distance is significantly greater than that of other
guided media. A signal can run for 50 km without
requiring regeneration. We need repeaters every 5
km for coaxial or twisted-pair cable.
15

ADVANTAGES OF OPTICAL FIBER
 Immunity to electromagnetic interference.
Electromagnetic noise cannot affect fiber-optic
cables.
 Resistance to corrosive materials. Glass is more
resistant to corrosive materials than copper.
 Light weight. Fiber-optic cables are much lighter
than copper cables.
 Greater immunity to tapping. Fiber-optic cables are
more immune to tapping than copper cables.
Copper cables create antenna effects that can
easily be tapped.
16

DISADVANTAGES OF OPTICAL FIBER
 Installation and maintenance. Fiber-optic cable is a
relatively new technology. Its installation and
maintenance require expertise that is not yet
available everywhere.
 Unidirectional light propagation. Propagation of light
is unidirectional. If we need bidirectional
communication, two fibers are needed.
 Cost. The cable and the interfaces are relatively
more expensive than those of other guided media.
If the demand for bandwidth is not high, often the
use of optical fiber cannot be justified.
17

UNGUIDED MEDIA: WIRELESS
 Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
communication. Signals are normally broadcast
through free space and thus are available to
anyone who has a device capable of receiving
them.
 Part of the electromagnetic spectrum, ranging from
3 kHz to 900 THz, used for wireless
communication.
 Unguided signals can travel from the source to
destination in several ways: ground propagation,
sky propagation, and line-of-sight propagation. 18

SWITCHING
 A network is a set of connected devices. Whenever we
have multiple devices, we have the problem of how to
connect them to make one-to-one communication
possible.
 One solution is to make a point-to-point connection
between each pair of devices (a mesh topology) or
between a central device and every other device (a star
topology). These methods, however, are impractical and
wasteful when applied to very large networks.
 The number and length of the links require too much
infrastructure to be cost-efficient, and the majority of
those links would be idle most of the time. Other
topologies employing multipoint connections, such as a
bus, are ruled out because the distances between
devices and the total number of devices increase
beyond the capacities of the media and equipment. 20

SWITCHING
 A better solution is switching. A switched network
consists of a series of interlinked nodes, called
switches. Switches are devices capable of creating
temporary connections between two or more
devices linked to the switch. In a switched network,
some of these nodes are connected to the end
systems (computers or telephones, for example).
Others are used only for routing.
21

METHODS OF SWITCHING
 Traditionally, three methods of switching have been
important: circuit switching, packet switching, and
message switching.
 The first two are commonly used today. The third
has been phased out in general communications
but still has networking applications.
 We can divide today's networks into three broad
categories: circuit-switched networks, packet-
switched networks, and message-switched. Packet-
switched networks can further be divided into two
subcategories-virtual-circuit networks and datagram
networks. 22

CIRCUIT-SWITCHED NETWORKS
 A circuit-switched network consists of a set of
switches connected by physical links.
 A connection between two stations is a dedicated
path made of one or more links. However, each
connection uses only one dedicated channel on
each link. Each link is normally divided into n
channels by using FDM or TDM
 Figure shows a trivial circuit-switched network with
four switches and four links. Each link is divided
into n (n is 3 in the figure) channels by using FDM
or TDM.
23

 We have explicitly shown the multiplexing symbols to
emphasize the division of the link into channels even
though multiplexing can be implicitly included in the
switch fabric.
 The end systems, such as computers or telephones, are
directly connected to a switch. We have shown only two
end systems for simplicity. When end system A needs to
communicate with end system M, system A needs to
request a connection to M that must be accepted by all
switches as well as by M itself. This is called the setup
phase.
 A circuit (channel) is reserved on each link, and the
combination of circuits or channels defines the
dedicated path. After the dedicated path made of
connected circuits (channels) is established, data
transfer can take place. After all data have been
transferred, the circuits are teardown.
25

 Circuit switching takes place at the physical layer.
 Before starting communication, the stations must make a
reservation for the resources to be used during the
communication. These resources, such as channels
(bandwidth in FDM and time slots in TDM), switch buffers,
switch processing time, and switch input/output ports, must
remain dedicated during the entire duration of data transfer
until the teardown phase.
 Data transferred between the two stations are not packetized
(physical layer transfer of the signal). The data are a
continuous flow sent by the source station and received by the
destination station, although there may be periods of silence.
 There is no addressing involved during data transfer. The
switches route the data based on their occupied band (FDM)
or time slot (TDM). Of course, there is end-to end addressing
used during the setup phase.
26

DATAGRAM NETWORKS
 In data communications, we need to send messages from one
end system to another. If the message is going to pass
through a packet-switched network, it needs to be divided into
packets of fixed or variable size. The size of the packet is
determined by the network and the governing protocol.
 In packet switching, there is no resource allocation for a
packet. This means that there is no reserved bandwidth on
the links, and there is no scheduled processing time for each
packet.
 Resources are allocated on demand. The allocation is
done on a first come, first-served basis. When a switch
receives a packet, no matter what is the source or destination,
the packet must wait if there are other packets being
processed. As with other systems in our daily life, this lack of
reservation may create delay. For example, if we do not have
a reservation at a restaurant, we might have to wait.
27

DATAGRAM NETWORKS
 In a datagram network, each packet is treated
independently of all others. Even if a packet is
part of a multipacket transmission, the network
treats it as though it existed alone. Packets in this
approach are referred to as datagrams.
 Datagram switching is normally done at the network
layer.
 Figure shows how the datagram approach is used
to deliver four packets from station A to station X.
The switches in a datagram network are
traditionally referred to as routers. That is why
we use a different symbol for the switches in the
figure. 28

DATAGRAM NETWORKS
 In this example, all four packets (or datagrams) belong
to the same message, but may travel different paths to
reach their destination. This is so because the links may
be involved in carrying packets from other sources and
do not have the necessary bandwidth available to carry
all the packets from A to X.
 This approach can cause the datagrams of a
transmission to arrive at their destination out of order
with different delays between the packets. Packets may
also be lost or dropped because of a lack of resources.
 In most protocols, it is the responsibility of an upper-
layer protocol to reorder the datagrams or ask for lost
datagrams before passing them on to the application.
30

DATAGRAM NETWORKS
 The datagram networks are sometimes referred to as
connectionless networks. The term connectionless here
means that the switch (packet switch) does not keep
information about the connection state. There are no
setup or teardown phases. Each packet is treated the
same by a switch regardless of its source or destination.
 If there are no setup or teardown phases, how are the
packets routed to their destinations in a datagram
network? In this type of network, each switch (or packet
switch) has a routing table which is based on the
destination address. The routing tables are dynamic and
are updated periodically. The destination addresses and
the corresponding forwarding output ports are recorded
in the tables.
31

DATAGRAM NETWORKS
 Every packet in a datagram network carries a header that
contains, among other information, the destination address of
the packet. When the switch receives the packet, this
destination address is examined; the routing table is consulted
to find the corresponding port through which the packet should
be forwarded. This destination address, unlike the address in
a virtual-circuit-switched network, remains the same during
the entire journey of the packet.
 Switching in the Internet is done by using the datagram
approach to packet switching at the network layer.
32
Destination Address Output Port
1200 1
…………… ………
1250 5

VIRTUAL-CIRCUIT NETWORKS
 A virtual-circuit network is a cross between a
circuit-switched network and a datagram
network.
 As in a circuit-switched network, there are setup
and teardown phases in addition to the data
transfer phase.
 Resources can be allocated during the setup
phase, as in a circuit-switched network, or on
demand, as in a datagram network.
33

 As in a datagram network, data are packetized and
each packet carries an address in the header.
However, the address in the header has local
jurisdiction (it defines what should be the next
switch and the channel on which the packet is
being carried), not end-to-end jurisdiction. The
reader may ask how the intermediate switches
know where to send the packet if there is no final
destination address carried by a packet.
 As in a circuit-switched network, all packets follow
the same path established during the connection.
34

 A virtual-circuit network is normally
implemented in the data link layer, while a
circuit-switched network is implemented in the
physical layer and a datagram network in the
network layer. But this may change in the future.
 The network has switches that allow traffic from
sources to destinations. A source or destination can
be a computer, packet switch, bridge, or any other
device that connects other networks.
 The network has switches that allow traffic from
sources to destinations. A source or destination can
be a computer, packet switch, bridge, or any other
device that connects other networks. 35

VIRTUAL-CIRCUIT NETWORKS :
ADDRESSING
 In a virtual-circuit network, two types of addressing are
involved: global and local (virtual-circuit identifier).
 Global Addressing: A source or a destination needs to have
a global address-an address that can be unique in the scope
of the network or internationally if the network is part of an
international network.
 Virtual-Circuit Identifier: The identifier that is actually used
for data transfer is called the virtual-circuit identifier (VCl). A
VCI, unlike a global address, is a small number that has only
switch scope; it is used by a frame between two switches.
When a frame arrives at a switch, it has a VCI; when it leaves,
it has a different VCl. Figure shows how the VCI in a data
frame changes from one switch to another. Note that a VCI
does not need to be a large number since each switch can
use its own unique set of VCls.
37

DATA LINK LAYER
 The data link layer transforms the physical layer, a
raw transmission facility, to a link responsible for
node-to-node (hop-to-hop) communication.
 Specific responsibilities of the data link layer
include framing, addressing, flow control, error
control, and media access control.
39

WORK OF DATA LINK LAYER
 The data link layer divides the stream of bits
received from the network layer into manageable
data units called frames.
 The data link layer adds a header to the frame to
define the addresses of the sender and receiver of
the frame.
 If the rate at which the data are absorbed by the
receiver is less than the rate at which data are
produced in the sender, the data link layer imposes
a flow control mechanism to avoid overwhelming
the receiver.
40

WORK OF DATA LINK LAYER
 The data link layer also adds reliability to the
physical layer by adding mechanisms to detect and
retransmit damaged, duplicate, or lost frames.
 When two or more devices are connected to the
same link, data link layer protocols are necessary to
determine which device has control over the link at
any given time.
41

ERROR DETECTION AND CORRECTION
 Types of Errors:
 Single-Bit Error: In a single-bit error, only 1 bit in the
data unit has changed.
 Burst Error: The term burst error means that 2 or
more bits in the data unit have changed from 1 to 0
or from 0 to 1.
42

REDUNDANCY
 The central concept in detecting or correcting
errors is redundancy. To be able to detect or
correct errors, we need to send some extra bits
with our data. These redundant bits are added by
the sender and removed by the receiver. Their
presence allows the receiver to detect or correct
corrupted bits.
43

DETECTION VERSUS CORRECTION
 The correction of errors is more difficult than
the detection. In error detection, we are looking
only to see if any error has occurred. The
answer is a simple yes or no. We are not even
interested in the number of errors.
 In error correction, we need to know the exact
number of bits that are corrupted and more
importantly, their location in the message. The
number of the errors and the size of the message
are important factors.
44

FORWARD ERROR CORRECTION VERSUS
RETRANSMISSION
 There are two main methods of error correction.
 Forward error correction is the process in which the
receiver tries to guess the message by using
redundant bits. This is possible, as we see later, if
the number of errors is small.
 Correction by retransmission is a technique in
which the receiver detects the occurrence of an
error and asks the sender to resend the message.
Resending is repeated until a message arrives that
the receiver believes is error-free.
45

CODING
 Redundancy is achieved through various
coding schemes. The sender adds redundant bits
through a process that creates a relationship
between the redundant bits and the actual data bits.
 The receiver checks the relationships between the
two sets of bits to detect or correct the errors.
 The ratio of redundant bits to the data bits and the
robustness of the process are important factors in
any coding scheme.
 coding schemes can be divided into two broad
categories: block coding and convolution
coding.
 Coding Techniques: Block Coding, Hamming Code,
Cyclic Code, Checksum
46

FRAMING
 Framing in the data link layer separates a message from
one source to a destination, or from other messages to
other destinations, by adding a sender address and a
destination address. The destination address defines
where the packet is to go; the sender address helps the
recipient acknowledge the receipt.
 Although the whole message could be packed in one
frame, that is not normally done. One reason is that a
frame can be very large, making flow and error control
very inefficient. When a message is carried in one very
large frame, even a single-bit error would require the
retransmission of the whole message. When a message
is divided into smaller frames, a single-bit error affects
only that small frame.
47

CHARACTER-ORIENTED PROTOCOLS
 In a character-oriented protocol, data to be carried
are 8-bit characters from a coding system such as
ASCII.
 The header, which normally carries the source and
destination addresses and other control
information, and the trailer, which carries error
detection or error correction redundant bits, are
also multiples of 8 bits.
 To separate one frame from the next, an 8-bit (1-
byte) flag is added at the beginning and the end of
a frame. The flag, composed of protocol dependent
special characters, signals the start or end of a
frame. 48

 Character-oriented framing was popular when only
text was exchanged by the data link layers.
 The flag could be selected to be any character not
used for text communication.
 Character-oriented protocols present problem in
data communications. The universal coding
systems in use today, such as Unicode, have 16-bit
and 32-bit characters that conflict with 8-bit
characters.
 Byte stuffing (or character stuffing) used for
video, Audio, Picture transmission.
49

 Byte stuffing is the process of adding 1 extra byte
whenever there is a flag or escape character in the text.
50

BIT-ORIENTED PROTOCOLS
 In a bit-oriented protocol, the data section of a
frame is a sequence of bits to be interpreted by the
upper layer as text, graphic, audio, video, and so
on.
 However, in addition to headers (and possible
trailers), we still need a delimiter to separate one
frame from the other.
 Most protocols use a special 8-bit pattern flag
01111110 as the delimiter to define the beginning
and the end of the frame, as shown in Figure.
51

FLOW AND 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 in the data link layer is based on
automatic repeat request, which is the
retransmission of data.
 Data link layer can combine framing, flow control,
and error control to achieve the delivery of data
from one node to another by “The protocols”.
53

PROTOCOLS
 The protocols are normally implemented in software
by using one of the common programming
languages.
 Protocols can be divided into those that can be
used for noiseless(error-free) channels and those
that can be used for noisy (error-creating) channels.
 The protocols in the first category cannot be used in
real life, but they serve as a basis for understanding
the protocols of noisy channels.
54

SIMPLEST PROTOCOL
 No flow or error control.
 It is very simple. The sender sends a sequence of
frames without even thinking about the receiver. To
send three frames, three events occur at the sender
site and three events at the receiver site.
 Note that the data frames are shown by tilted
boxes; the height of the box defines the
transmission time difference between the first bit
and the last bit in the frame.
56

STOP-AND-WAIT PROTOCOL
 If data frames arrive at the receiver site faster than
they can be processed, the frames must be stored
until their use. Normally, the receiver does not have
enough storage space, especially if it is receiving
data from many sources. This may result in either
the discarding of frames or denial of service. To
prevent the receiver from becoming overwhelmed
with frames, we somehow need to tell the sender to
slow down. There must be feedback from the
receiver to the sender.
 The protocol we discuss now is called the Stop-
and-Wait Protocol because then sender sends one
frame, stops until it receives confirmation from the
receiver (okay to go ahead), and then sends the
next frame.
 We add flow control to our previous protocol.
58

NOISY CHANNELS : STOP-AND-WAIT AUTOMATIC
REPEAT REQUEST
 Stop-and-Wait Automatic Repeat Request (Stop-and
Wait ARQ), adds a simple error control mechanism
to the Stop-and-Wait Protocol.
 To detect and correct corrupted frames, we need to add
redundancy bits to our data frame.
 When the frame arrives at the receiver site, it is checked
and if it is corrupted, it is silently discarded. The
detection of errors in this protocol is manifested by the
silence of the receiver.
 Lost frames are more difficult to handle than corrupted
ones. In our previous protocols, there was no way to
identify a frame. The received frame could be the
correct one, or a duplicate, or a frame out of order. The
solution is to number the frames. When the receiver
receives a data frame that is out of order, this means
that frames were either lost or duplicated.
60

STOP-AND-WAIT AUTOMATIC REPEAT
REQUEST
 The completed and lost frames need to be resent in
this protocol. If the receiver does not respond when
there is an error, how can the sender know which
frame to resend? To remedy this problem, 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. Since
the protocol uses the stop-and-wait mechanism,
there is only one specific frame that needs an ACK
even though several copies of the same frame can
be in the network. 61

STOP-AND-WAIT AUTOMATIC REPEAT
REQUEST
 Since an ACK frame can also be corrupted and lost,
it too needs redundancy bits and a sequence
number. The ACK frame for this protocol has a
sequence number field. In this protocol, the sender
simply discards a corrupted ACK frame or ignores
an out-of-order one.
 Sequence Numbers: In Stop-and-Wait ARQ, we use
sequence numbers to number the frames. The
sequence numbers are based on modulo-2
arithmetic.
 Acknowledgment Numbers: In Stop-and-Wait ARQ,
the acknowledgment number always announces in
modulo-2 arithmetic the sequence number of the
next frame expected. 62

STOP-AND-WAIT AUTOMATIC REPEAT REQUEST
63

PIPELINING
 In networking and in other areas, a task is often
begun before the previous task has ended.
 This is known as pipelining.
 There is no pipelining in Stop-and-Wait ARQ
because we need to wait for a frame to reach the
destination and be acknowledged before the next
frame can be sent.
64

GO-BACK-N AUTOMATIC REPEAT REQUEST
 To improve the efficiency of transmission (filling the
pipe), multiple frames must be in transition while
waiting for acknowledgment.
 In other words, we need to let more than one frame
be outstanding to keep the channel busy while the
sender is waiting for acknowledgment.
 In “Go-Back-N Automatic Repeat Request” In this
protocol we can send several frames before
receiving acknowledgments; we keep a copy of
these frames until the acknowledgments arrive.
 Sequence Numbers: In the Go-Back-N Protocol,
the sequence numbers are modulo ,where m
is the size of the sequence number field in bits.
66
2m

 Frames from a sending station are numbered
sequentially. However, because we need to include
the sequence number of each frame in the header,
we need to set a limit. If the header of the frame
allows m bits for the sequence number, the
sequence numbers range from 0 to 2m - 1. For
example, if m is 4, the only sequence numbers are
0 through 15 inclusive. However, we can repeat the
sequence. So the sequence numbers are :
0,1,2,3,4,5,6, 7,8,9, 10, 11, 12, 13, 14, 15,0,
1,2,3,4,5,6,7,8,9,10, 11, ...
67

 Sender maintains a list of sequence numbers that it
is allowed to send (sender window). The size of the
sender’s window is at most 2^k – 1. The sender is
provided with a buffer equal to the window size.
 The receiver acknowledges a frame by sending an
ACK frame that includes the sequence number of
the next frame expected. This also explicitly
announces that it is prepared to receive the next N
frames, beginning with the number specified. This
scheme can be used to acknowledge multiple
frames. It could receive frames 2, 3, 4 but withhold
ACK until frame 4 has arrived. By returning an ACK
with sequence number 5, it acknowledges frames
2, 3, 4 in one go.
68

 Sliding window algorithm is a method of flow
control for network data transfers. TCP, the
Internet's stream transfer protocol, uses a
sliding window algorithm.
 A sliding window algorithm places a buffer
between the application program and the
network data flow.
 Sender sliding Window:
69

 It looks for a specific frame (frame 4 as shown in
the figure) to arrive in a specific order. If it receives
any other frame (out of order), it is discarded and it
needs to be resent. However, the receiver window
also slides by one as the specific frame is received
and accepted as shown in the figure. The receiver
acknowledges a frame by sending an ACK frame
that includes the sequence number of the next
frame expected.
70

71

72

73

o The receiver always maintains a window of size 1
as shown in Fig.
Hence, Sliding Window Flow Control :
o Allows transmission of multiple frames
o Assigns each frame a k-bit sequence number
o Range of sequence number is [0…2k-1], i.e.,
frames are counted modulo 2k.
74

SELECTIVE-REPEAT ARQ
 The selective-repetitive ARQ scheme retransmits
only those for which NAKs are received or for which
timer has expired, this is shown in the Fig. This is
the most efficient among the ARQ schemes, but the
sender must be more complex so that it can send
out-of-order frames. The receiver also must have
storage space to store the post-NAK frames and
processing power to reinsert frames in proper
sequence.
75

PIGGYBACKING
 The three protocols we discussed in this section are
all unidirectional: data frames flow in only one
direction although control information such as ACK
and NAK frames can travel in the other direction. In
real life, data frames are normally flowing in both
directions: from node A to node B and from node B
to node A.
 So, instead of sending separate acknowledgement
packets, a portion (few bits) of the data frames can
be used for acknowledgement. This phenomenon is
known as piggybacking.
 The piggybacking helps in better channel utilization.
Further, multi-frame acknowledgement can be
done. 77

HIGH-LEVEL DATA LINK CONTROL
 HDLC is a bit-oriented protocol.
 It specifies a packitization standard for serial links.
 HDLC supports several modes of operation,
including a simple sliding-window mode for reliable
delivery. Since Internet provides retransmission at
higher levels (i.e., TCP), most Internet applications
use HDLC's unreliable delivery mode, Unnumbered
Information.
78

HDLC STATIONS AND CONFIGURATIONS
 HDLC specifies the following three types of stations for
data link control:
 Primary Station
 Secondary Station
 Combined Station
 Primary Station : Within a network using HDLC as its
data link protocol, if a configuration is used in which
there is a primary station, it is used as the controlling
station on the link. It has the responsibility of controlling
all other stations on the link (usually secondary
stations). A primary issues commands and secondary
issues responses. Despite this important aspect of being
on the link, the primary station is also responsible for the
organization of data flow on the link. It also takes care of
error recovery at the data link level (layer 2 of the OSI
model). 79

 Secondary Station: If the data link protocol being
used is HDLC, and a primary station is present, a
secondary station must also be present on the data
link. The secondary station is under the control of
the primary station. It has no ability, or direct
responsibility for controlling the link. It is only
activated when requested by the primary station. It
only responds to the primary station. The
secondary station's frames are called responses. It
can only send response frames when requested by
the primary station. A primary station maintains a
separate logical link with each secondary station.
80

 Combined Station: A combined station is a
combination of a primary and secondary station. On
the link, all combined stations are able to send and
receive commands and responses without any
permission from any other stations on the link. Each
combined station is in full control of itself, and does
not rely on any other stations on the link. No other
stations can control any combined station. May
issue both commands and responses.
 http://nptel.ac.in/courses/106105080/13
81

HDLC OPERATIONAL MODES
 A mode in HDLC is the relationship between two
devices involved in an exchange; the mode
describes who controls the link. Exchanges over
unbalanced configurations are always conducted in
normal response mode. Exchanges over symmetric
or balanced configurations can be set to specific
mode using a frame design to deliver the
command. HDLC offers three different modes of
operation. These three modes of operations are:
 Normal Response Mode (NRM)
 Asynchronous Response Mode (ARM)
 Asynchronous Balanced Mode (ABM)
82

 Normal Response Mode : This is the mode in
which the primary station initiates transfers to the
secondary station. The secondary station can only
transmit a response when, and only when, it is
instructed to do so by the primary station.
 Asynchronous Response Mode : In this mode,
the primary station doesn't initiate transfers to the
secondary station. In fact, the secondary station
does not have to wait to receive explicit permission
from the primary station to transfer any frames. Due
to the fact that this mode is asynchronous, the
secondary station must wait until it detects and idle
channel before it can transfer any frames. This is
when the ARM link is operating at half-duplex.
83

 Synchronous Balanced Mode :This mode is used in
case of combined stations. There is no need for
permission on the part of any station in this mode. This
is because combined stations do not require any sort of
instructions to perform any task on the link.
 Normal Response Mode is used most frequently in
multi-point lines, where the primary station controls
the link. Asynchronous Response Mode is better for
point-to-point links, as it reduces overhead.
Asynchronous Balanced Mode is not used widely today.
The "asynchronous" in both ARM and ABM does not
refer to the format of the data on the link. It refers to the
fact that any given station can transfer frames
without explicit permission or instruction from any
other station. 84

HDLC FRAME STRUCTURE
 The Flag field: Every frame on the link must begin and
end with a flag sequence field (F). Stations attached to
the data link must continually listen for a flag sequence.
The flag sequence is an octet looking like 01111110.
Flags are continuously transmitted on the link between
frames to keep the link active. Two other bit sequences
are used in HDLC as signals for the stations on the link.
 HDLC is a code-transparent protocol. It does not rely on
a specific code for interpretation of line control. This
means that if a bit at position N in an octet has a specific
meaning, regardless of the other bits in the same octet.
If an octet has a bit sequence of 01111110, but is not a
flag field, HLDC uses a technique called bit-stuffing to
differentiate this bit sequence from a flag field.
86

 The Address field: The address field (A) identifies
the primary or secondary stations involvement in
the frame transmission or reception. Each station
on the link has a unique address.
 The Control field: HDLC uses the control field (C)
to determine how to control the communications
process. This field contains the commands,
responses and sequences numbers used to
maintain the data flow accountability of the link,
defines the functions of the frame and initiates the
logic to control the movement of traffic between
sending and receiving stations.
87

 The Poll/Final Bit (P/F):The 5th bit position in the
control field is called the poll/final bit, or P/F bit. It
can only be recognized when it is set to 1. If it is set
to 0, it is ignored. The poll/final bit is used to
provide dialogue between the primary station and
secondary station.
 The Information field or Data field: This field is
not always present in a HDLC frame. It is only
present when the Information Transfer Format is
being used in the control field. The information field
contains the actually data the sender is transmitting
to the receiver in an I-Frame and network
management information in U-Frame.
88

 The Frame check Sequence field: This field
contains a 16-bit, or 32-bit cyclic redundancy check
bits.
89

HDLC COMMANDS AND RESPONSES
 Information transfer format command and
response (I-Frame) : The function of the
information command and response is to transfer
sequentially numbered frames, each containing an
information field, across the data link.
 Supervisory format command and responses
(S-Frame) :Supervisory (S) commands and
responses are used to perform numbered
supervisory functions such as acknowledgment,
polling, temporary suspension of information
transfer, or error recovery. Frames with the S format
control field cannot contain an information field.
90

HDLC COMMANDS AND RESPONSES
 Unnumbered Format Commands and responses
(U-Frame) :The unnumbered format commands
and responses are used to extend the number of
data link control functions. The unnumbered format
frames have 5 modifier bits, which allow for up to
32 additional commands and 32 additional
response functions.
91

MULTIPLE ACCESS
 In the protocols we described, we assumed that
there is an available dedicated link (or channel)
between the sender and the receiver. This
assumption may or may not be true.
92

MULTIPLE ACCESS
 Data link layer can be considered as two
sublayers. The upper sublayer is responsible for
data link control, and the lower sublayer is
responsible for resolving access to the shared
media. If the channel is dedicated, we do not need
the lower sublayer.
 The upper sublayer that is responsible for flow and
error control is called the logical link control (LLC)
layer.
 The lower sublayer that is mostly responsible for
multiple access resolution is called the media
access control (MAC) layer.
93

RANDOM ACCESS
 In random access or contention methods, no station
is superior to another station and none is assigned
the control over another. No station permits, or
does not permit, another station to send.
 At each instance, a station that has data to send
uses a procedure defined by the protocol to make a
decision on whether or not to send. This decision
depends on the state of the medium (idle or busy).
95

RANDOM ACCESS
 In a random access method, each station has the right
to the medium without being controlled by any other
station. However, if more than one station tries to send,
there is an access conflict-collision-and the frames will
be either destroyed or modified. To avoid access conflict
or to resolve it when it happens, each station follows a
procedure.
 Two features give this method its name.
 First, there is no scheduled time for a station to transmit.
Transmission is random among the stations. That is why
these methods are called random access.
 Second, no rules specify which station should send
next. Stations compete with one another to access the
medium. That is why these methods are also called
contention methods.
96

RANDOM ACCESS
 The random access methods have evolved from a very
interesting protocol known as ALOHA, which used a
very simple procedure called multiple access (MA).
 The method was improved with the addition of a
procedure that forces the station to sense the medium
before transmitting. This was called carrier sense
multiple access.
 Carrier sense multiple access later evolved into two
parallel methods:
 carrier sense multiple access with collision detection
(CSMA/CD) . CSMA/CD tells the station what to do
when a collision is detected.
 carrier sense multiple access with collision avoidance
(CSMA/CA). CSMA/CA tries to avoid the collision.
97

ALOHA
 ALOHA, the earliest random access method, was
developed at the University of Hawaii in early 1970.
 The original ALOHA protocol is called pure ALOHA.
 The idea is that each station sends a frame whenever it
has a frame to send. However, since there is only one
channel to share, there is the possibility of collision
between frames from different stations.
 Figure shows four stations (unrealistic assumption) that
contend with one another for access to the shared
channel. The figure shows that each station sends two
frames; there are a total of eight frames on the shared
medium. Some of these frames collide because multiple
frames are in contention for the shared channel.
98

ALOHA
 There are four stations that contend with one
another for access to the shared channel. The
figure shows that each station sends two frames;
there are a total of eight frames on the shared
medium. Some of these frames collide because
multiple frames are in contention for the shared
channel.
 Figure shows that only two frames survive: frame
1.1 from station 1 and frame 3.2 from station 3. We
need to mention that even if one bit of a frame
coexists on the channel with one bit from another
frame, there is a collision and both will be
destroyed.
100

ALOHA
 It is obvious that we need to resend the frames that
have been destroyed during transmission. The pure
ALOHA protocol relies on acknowledgments from
the receiver.
 When a station sends a frame, it expects the
receiver to send an acknowledgment. If the
acknowledgment does not arrive after a time-out
period, the station assumes that the frame (or the
acknowledgment) has been destroyed and resends
the frame.
 vulnerable time, the length of time in which there is
a possibility of collision.
101

ALOHA
 A collision involves two or more stations. If all these
stations try to resend their frames after the time-out,
the frames will collide again.
 Pure ALOHA dictates that when the time-out period
passes, each station waits a random amount of
time before resending its frame. The randomness
will help avoid more collisions. We call this time the
back-off time TB.
 Pure ALOHA has a second method to prevent
congesting the channel with retransmitted frames.
After a maximum number of retransmission
attempts Kmax a station must give up and try later.
102

SLOTTED ALOHA
 Slotted ALOHA was invented to improve the efficiency of
pure ALOHA.
 In slotted ALOHA we divide the time into slots of Tfr s
and force the station to send only at the beginning of the
time slot.
 Because a station is allowed to send only at the
beginning of the synchronized time slot, if a station
misses this moment, it must wait until the beginning of
the next time slot.
 This means that the station which started at the
beginning of this slot has already finished sending its
frame. Of course, there is still the possibility of collision if
two stations try to send at the beginning of the same
time slot.
 However, the vulnerable time is now reduced to one-half
103

CARRIER SENSE MULTIPLE ACCESS (CSMA)
 To minimize the chance of collision and, therefore,
increase the performance, the CSMA method was
developed. The chance of collision can be reduced
if a station senses the medium before trying to use
it. Carrier sense multiple access (CSMA) requires
that each station first listen to the medium (or check
the state of the medium) before sending.
 In other words, CSMA is based on the principle
"sense before transmit" or "listen before talk."
105

 The possibility of collision still exists because of
propagation delay; when a station sends a frame, it
still takes time (although very short) for the first bit
to reach every station and for every station to sense
it.
 In other words, a station may sense the medium
and find it idle, only because the first bit sent by
another station has not yet been received. At time
t0 station B senses the medium and finds it idle, so
it sends a frame.
 At time t1 (t1 > t0) station D senses the medium
and finds it idle because, at this time, the first bits
from station B have not reached station C.
 Station C also sends a frame. The two signals
collide and both frames are destroyed. 106

107

 The vulnerable time for CSMA is the propagation
time Tp .
 This is the time needed for a signal to propagate
from one end of the medium to the other. When a
station sends a frame, and any other station tries to
send a frame during this time, a collision will result.
But if the first bit of the frame reaches the end of
the medium, every station will already have heard
the bit and will refrain from sending.
108

109

IEEE STDS.
 In 1985, the Computer Society of the IEEE started
a project, called Project 802, to set standards to
enable intercommunication among equipment from
a variety of manufacturers.
 The relationship of the 802 Standard to the
traditional OSI model is shown in Figure. The IEEE
has subdivided the data link layer into two
sublayers:
 logical link control (LLC) and media access control
(MAC). IEEE has also created several physical
layer standards for different LAN protocols.
110

IEEE STDS FOR LAN:IEEE 802.3
111

IEEE STDS.
 The original Ethernet was created in 1976 at
Xerox's Palo Alto Research Center (PARC).
 Since then, it has gone through four generations:
Standard Ethernet (10 Mbps), Fast Ethernet (100
Mbps), Gigabit Ethernet (l Gbps), and Ten-Gigabit
Ethernet (l0 Gbps).
112

STANDARD ETHERNET :MAC SUBLAYER
 In Standard Ethernet, the MAC sublayer governs
the operation of the access method. It also frames
data received from the upper layer and passes
them to the physical layer.
 Frame Format: Ethernet does not provide any
mechanism for acknowledging received frames,
making it what is known as an unreliable medium.
Acknowledgments must be implemented at the
higher layers.
113

FRAME FORMAT
 Preamble: It alerts the receiving system to the coming
frame and enables it to synchronize its input timing. The
pattern provides only an alert and a timing pulse.
 Start frame delimiter (SFD): The SFD warns the
station or stations that this is the last chance for
synchronization. The last 2 bits is 11 and alerts the
receiver that the next field is the destination address.
 Destination address (DA)
 Source address (SA)
 Length or type: The original Ethernet used this field as
the type field to define the upper-layer protocol using the
MAC frame. The IEEE standard used it as the length
field to define the number of bytes in the data field. Both
uses are common today.
114

STANDARD ETHERNET
 Data. This field carries data encapsulated from the
upper-layer protocols
 CRC: The last field contains error detection
information, in this case a CRC-32.
 Frame Length: Minimum: 64 bytes (512 bits)
Maximum: 1518 bytes (12,144 bits)
 Addressing: Each station on an Ethernet network
(such as a PC, workstation, or printer) has its own
network interface card (NIC). The NIC fits inside the
station and provides the station with a 6-byte
physical address.
 Example of an Ethernet address in hexadecimal
notation :06:01 :02:01:2C:4B 115

STANDARD ETHERNET :PHYSICAL LAYER
 All standard implementations use digital signaling
(baseband) at 10 Mbps. At the sender, data are
converted to a digital signal using the Manchester
scheme.
 10Base5, thick Ethernet, or Thicknet: The
nickname derives from the size of the cable, which
is roughly the size of a garden hose and too stiff to
bend with your hands. 10Base5 was the first
Ethernet specification to use a bus topology with an
external transceiver connected via a tap to a thick
coaxial cable.
116

 10Base2, Thin Ethernet: It uses a bus topology,
but the cable is much thinner and more flexible. The
cable can be bent to pass very close to the stations.
In this case, the transceiver is normally part of the
network interface card (NIC), which is installed
inside the station.
 10Base-T: Twisted-Pair Ethernet : The third
implementation is called l0Base-T or twisted-pair
Ethernet. 10Base-T uses a physical star topology.
The stations are connected to a hub via two pairs of
twisted cable.
 Two pairs of twisted cable create two paths (one for
sending and one for receiving) between the station
and the hub. Any collision here happens in the hub.
118

 10Base-F: Fiber Ethernet: Although there are
several types of optical fiber l0-Mbps Ethernet, the
most common is called 10Base-F. 10Base-F uses a
star topology to connect stations to a hub. The
stations are connected to the hub using two fiber-
optic cables.
119

WIRELESS LANS: IEEE 802.11
 The standard defines two kinds of services: the
basic service set (BSS) and the extended service
set (ESS).
 Basic Service Set: A basic service set is made of
stationary or mobile wireless stations and an
optional central base station, known as the access
point (AP).
 The BSS without an AP is a stand-alone network
and cannot send data to other BSSs. It is called
an ad hoc architecture. In this architecture,
stations can form a network without the need of
an AP; they can locate one another and agree to
be part of a BSS. A BSS with an AP is
sometimes referred to as an infrastructure
network. 120

121

 Extended Service Set: An extended service set
(ESS) is made up of two or more BSSs with APs. In
this case, the BSSs are connected through a
distribution system, which is usually a wired LAN.
The distribution system connects the APs in the
BSSs.
 IEEE 802.11 does not restrict the distribution
system; it can be any IEEE LAN such as an
Ethernet.
 The extended service set uses two types of
stations: mobile and stationary. The mobile stations
are normal stations inside a BSS. The stationary
stations are AP stations that are part of a wired
LAN.
122

123

MAC SUBLAYER: IEEE 802.11
 IEEE 802.11 defines two MAC sublayers: the
distributed coordination function (DCF) and point
coordination function (PCF).
 DCF uses CSMA/CA as the access method.
 The point coordination function (PCF) is an optional
access method that can be implemented in an
infrastructure network (not in an ad hoc network). It
is implemented on top of the DCF and is used
mostly for time-sensitive transmission.
 PCF has a centralized, contention-free polling
access method. The AP performs polling for
stations that are capable of being polled. The
stations are polled one after another, sending any
data they have to the AP.
124

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