OSI LAYERS FUNCTIONS, TOPOLOGIES,LAN,WAN AND METROPOLITAN NETWORK

DATA COMMUNICATIONS
DATA COMMUNICATIONS
The term telecommunication means
communication at a distance. The word data refers to
information presented in whatever form is agreed
upon by the parties creating and using the data. Data
communications are the exchange of data between
two devices via some form of transmission medium
such as a wire cable.
1.1

Fundamental
Characteristics
Delivery:
 The system must deliver data to the correct destination.
 Data must be received only by the intended device or user.
Accuracy:
 The system must deliver the data accurately.
 Data that have been altered in transmission and left uncorrected are unusable.
Timeliness:
 The system must deliver data in a time manner. Data delivered late is useless.
 In the case of video and audio, timely delivery means delivering data as they are
produced, in the same order that they are produced, and without significant delay.
Jitter:
 Jitter refers to the variation in the packet arrival time.
 It is the uneven delay in the delivery of audio or video packets that results in
uneven quality.
1.2

Components
Message:
 The message is the information (data) to be communicated.
 Popular forms of information include text, numbers, pictures, audio, and video.
Sender:
 The sender is the device that sends the data message.
 It can be a computer, workstation, telephone handset, video camera, and so on.
Receiver:
 The receiver is the device that receives the message.
 It can be a computer, workstation, telephone handset, television, and so on.
Transmission medium:
 The transmission medium is the physical path by which a message travels from
sender to receiver such as twisted-pair wire, coaxial cable, fiber optic cable, and
radio waves.
Protocol:
 protocol is a set of rules that govern data communications between the
communicating devices.
 Without a protocol, two devices may be connected but not communicating.
 The key elements of a protocol are syntax, semantics, and timing.
1.3

Components of a data communication system
1.4

Data Representation
Text :
 is a sequence of bits.
 Different sets of bit patterns have been designed to represent text.
 Each set is called a code, and the prevalent coding system is called Unicode, which uses 32 bits to
represent a symbol or character used in any language.
Numbers:
 are also represented by bit patterns.
 Instead of code, the number is directly converted to a binary number to simplify mathematical
operations.
Images:
 are also represented by bit patterns. An image is composed of a matrix of pixels (picture elements).
 Each pixel is assigned a bit pattern.
 The size and the value of the pattern depend on whether the image is binary, gray or color.
Audio:
 refers to the recording or broadcasting of sound or music.
 Audio is by nature different from text, numbers, or images.
 It is continuous, not discrete.
Video:
 refers to the recording or broadcasting of a picture or movie.
 Video can either be produced as a continuous entity, or it can be a combination of images in motion.
1.5

DATA FLOW (simplex, half-duplex, and full-duplex)
1.6

Simplex
 In simplex mode, the communication is unidirectional.
 Only one of the two devices on a link can transmit; the other can only receive.
 For example, keyboard can only introduce input; the monitor can only accept
output.
Half-Duplex
 In half-duplex mode, each station can both transmit and receive, but not at the
same time.
 When one device is ending, the other can only receive, and vice versa.
 Walkie-talkies and CB (citizens band) radios are both half-duplex systems.
 Thus the entire capacity of the channel can be utilized for each direction
Full-Duplex
 In full-duplex mode (also called duplex), both stations can transmit and receive
simultaneously.
 An example is the telephone network.
 Therefore, either the link must contain two separate physical transmission paths
(sending and receiving) or the capacity of the channel is divided between signals
traveling in both directions
1.7

NETWORKS
NETWORKS
A
A network
network is a set of devices (often referred to as
is a set of devices (often referred to as
nodes
nodes) connected by communication
) connected by communication links
links. A node can
. A node can
be a computer, printer, or any other device capable of
be a computer, printer, or any other device capable of
sending and/or receiving data generated by other nodes
sending and/or receiving data generated by other nodes
on the network. A link can be a cable, air, optical fiber,
on the network. A link can be a cable, air, optical fiber,
or any medium which can transport a signal carrying
or any medium which can transport a signal carrying
information.
information.
 Network Criteria
 Physical Structures
 Categories of Networks
Topics discussed in this section:
1.8

Network Criteria
 Performance

Depends on Network Elements

Measured in terms of Delay and Throughput
 Reliability

Failure rate of network components

Measured in terms of availability/robustness
 Security
 Data protection against corruption/loss of data due to:
 Errors
 Malicious users
1.9

Physical Structures
 Type of Connection
 Point to Point - single transmitter and receiver
 Multipoint - multiple recipients of single transmission
 Physical Topology
 Connection of devices
 Type of transmission - unicast, mulitcast, broadcast
1.10

Types of connections: point-to-point and multipoint
1.11

MESH TOPOLOGY
 Each device has a dedicated point-
to-point link to every other device.
 The term dedicated means that the
link carries traffic only between the
two devices it connects.
 A mesh network with n nodes has
n(n-1) links [ n(n-1) / 2 in case of
duplex ].
 The use of dedicated links
eliminates traffic problems that
occur when link is shared.
 A mesh topology is robust.
 If one link becomes unusable, it
does not incapacitate the entire
system. Privacy or security is
maintained.
1.13

Disadvantages :
Installation and reconnection are difficult.
The sheer bulk of the wiring can be greater than the available
space.
The hardware required to connect each link (I/O ports & cable)
is expensive.
 Due to its major disadvantages, mesh topology is
usually implemented in a limited fashion.
An example of a mesh topology
 It is the connection of telephone regional offices in which
each regional office needs to be connected to every other
regional office.
MESH TOPOLOGY
1.14

STAR TOPOLOGY
 Each device has a dedicated
point-to-point link only to a
central controller, usually called a
hub.
 The devices are not directly
linked to one another.
 Unlike mesh, a star topology does
not allow direct traffic between
devices.
 The controller acts as an
exchange.
 If one device wants to send data
to another, it sends the data to the
controller, which then relays the
data to the other connected
device.
 It is used in LANs.
1.15

STAR TOPOLOGY
Advantages :
 A star topology is less expensive than a mesh topology.
 In a star, each device needs only one link and one I/O port to connect it to any number of
others.
 This factor also makes it easy to install and reconfigure.
 Far less cabling needs to be housed, and additions, moves, and deletions involve only one
connection: between that device and the hub.
 Star topology is also robust i.e., when one link fails, only that link is affected.
 All other links remain active.
 As long as the hub is working, it can be used to monitor link problems and bypass
defective links.
Disadvantage :
 star topology is the dependency of the whole topology on one single point, the hub.
 If the hub goes down, the whole system is dead.
1.16

BUS TOPOLOGY
 Unlike mesh or ring, it is multipoint.
 One long cable acts as a backbone to link all the devices in a network.
 Nodes are connected to the bus cable by drop lines and taps.
 A drop line is a connection running between the device and the main cable.
 A tap is a connector that either splices into the main cable to create a contact with
the metallic core.
 As signal travels along the backbone, some of its energy is transformed into heat.
 Therefore, it becomes weaker and weaker as it travels farther and farther.
 For this reason there is a limit on the number of taps a bus can support and distance
between the taps.
1.17

BUS TOPOLOGY
Advantages:
 Ease of installation.
 Backbone cable can be laid along the most efficient path, then connected to the
nodes by drop lines.
 In this way, a bus uses less cabling than mesh or star topologies.
Disadvantages :
 Difficult reconnection and fault isolation.
 Adding new devices require modification or replacement of the backbone.
 Signal reflection at the taps can cause degradation in quality.
 In addition, a fault or break in the bus cable stops all transmission.
1.18

RING TOPOLOGY
 Each device has a dedicated point-to-point connection with only the two
devices on either side of it.
 A signal is passed along the ring in one direction, from device to device,
until it reaches its destination.
 When a device receives a signal intended for another device, its repeater
regenerates the bits and passes them along
Advantages:
 Easy to install and reconfigure.
 Each device is linked to only its immediate neighbors.
 To add or delete a device requires changing only two connections.
 Fault isolation is simplified by making a device to issue an alarm if it does
not receive a signal within a specified period.
Disadvantages:
 A break in the ring (such as a disabled station) can disable the entire
network due to unidirectional traffic.
 Ring topology is less popular now.
1.20

Categories of Networks
 Local Area Networks (LANs)
 Short distances
 Designed to provide local interconnectivity
 Wide Area Networks (WANs)
 Long distances
 Provide connectivity over large areas
 Metropolitan Area Networks (MANs)
 Provide connectivity over areas such as a city, a campus
1.22

AN ISOLATED LAN IN THE PAST AND TODAY
1.23

WANs: a switched WAN and a point-to-point WAN
1.24

A heterogeneous network made of four WANs and two LANs
1.25

THE INTERNET
THE INTERNET
The
The Internet
Internet has revolutionized many aspects of our
has revolutionized many aspects of our
daily lives. It has affected the way we do business as
daily lives. It has affected the way we do business as
well as the way we spend our leisure time. The Internet
well as the way we spend our leisure time. The Internet
is a communication system that has brought a wealth of
is a communication system that has brought a wealth of
information to our fingertips and organized it for our
information to our fingertips and organized it for our
use
use.
.
Organization of the Internet
Internet Service Providers (ISPs)
1.26

Parallel Transmission
The mechanism for parallel transmission is a conceptually
simple one: Use n wires to send n bits at one time.
That way each bit has its own wire, and all n bits of one group
can be transmitted with each clock tick from one device to
another
1.29

Serial Transmission
In serial transmission one bit follows another, so we need
only one communication channel rather than n to transmit data
between two communicating devices
The advantage of serial over parallel transmission is that
with only one communication channel, serial transmission
reduces the cost of transmission over parallel by roughly a
factor of n.
1.30

Asynchronous transmission
Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the same.
In asynchronous transmission, we send 1 start bit (0) at the
beginning and 1 or more stop bits (1s) at the end of each byte. There
may be a gap between bytes.
1.31

Synchronous Transmission
In synchronous transmission, we send bits one after another
without start or stop bits or gaps. It is the responsibility of the
receiver to group the bits.
1.32

Isochronous
 In real-time audio and video, in which uneven delays between
frames are not acceptable,
 synchronous transmission fails. For example, TV images are
broadcast at the rate
 of 30 images per second; they must be viewed at the same
rate. If each image is sent by
 using one or more frames, there should be no delays between
frames. For this type of
 application, synchronization between characters is not enough;
the entire stream of bits
 must be synchronized. The isochronous transmission
guarantees that the data arrive at a fixed rate.
1.33

PROTOCOLS
PROTOCOLS
A protocol is synonymous with rule. It consists of a set
A protocol is synonymous with rule. It consists of a set
of rules that govern data communications. It
of rules that govern data communications. It
determines what is communicated, how it is
determines what is communicated, how it is
communicated and when it is communicated. The key
communicated and when it is communicated. The key
elements of a protocol are syntax, semantics and timing
elements of a protocol are syntax, semantics and timing
 Syntax
 Semantics
 Timing
1.34

Elements of a Protocol
 Syntax
 Structure or format of the data
 Indicates how to read the bits - field delineation
 Semantics
 Interprets the meaning of the bits
 Knows which fields define what action
 Timing
 When data should be sent and what
 Speed at which data should be sent or speed at which it is
being received.
1.35

Common protocol used
Protocol Acronym Remarks
Point To Point PPP Used to manage network communication over a
modem
Transfer/Transmission Control
Protocol
TCP / IP Backbone protocol. The most widely used protocol.
Internetwork package exchange IPX Standard protocol for Novell NOS
NetBIOS extended user interface NetBEUI Microsoft protocol that doesn’t support routing to
other network. Running only Windows-based
clients.
File transfer Protocol FTP used to send and received file from a remote host
Simple mail Transfer protocol SMTP Used to send Email over a network
Hyper text transfer protocol HTTP Used for Internet to send document that encoded in
HTML
Apple Talk Apple Talk Protocol suite to network Macintosh computer and a
peer-to-peer network protocol
OSI Model OSI Layers A way of illustrating how information functions
travels through network of its 7 layers.
36

Organizations For Communication
Standards
Standards are developed by cooperation among standards
creation committees, forums, and government regulatory
agencies.
Standards Creation Committees
International Standards Organization (ISO)
International Telecommunications Union (ITU)
American National Standards Institute (ANSI)
Institute of Electrical and Electronics Engineers (IEEE)
Electronic Industries Association (EIA)
Internet Engineering Task Force (IETF)

Types of Standards
There are two major categories
 1. De facto
 2. De jure
De facto standards
 Means ―from the fact‖
 It just happened or accepted without any formal plan.
 IBM has developed its own standards for a PC
 It was accepted internationally as a De facto standard
De jure standards
 Means ―by law
 These standards are planned and developed by authorized standards
creating bodies
 IEEE Standards, OSI and ISO Standards are examples for these standards.

Review Questions
1. The fundamental characteristics of a network are _________,
____________,__________ and __________
2. Without ______________, devices can be connected but not be communicated
3. Telephone network is an example for __________________data transfer modes
4. Performance of network is measured in terms of _________________ and
_______________
5. A Mesh network with “n” nodes requires_______________ duplex links
6. A Star topology , each device requires ______________ link(s) and__________
I/O ports to communicate to any other device in that network
7. In Star topology,_____________ act as an exchange
8. ____________________acts as a backbone to link all the devices in a network
using bus topology
9. Elements of a protocol are_________________, ________________and
________________
10. Name some Commonly used protocols
1.39

SWITCHING
 A one-to-one communication between devices in a large network is
infeasible using topologies such as star, mesh, etc.
 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 and others for routing.
 Of these message-switching has been phased out in general communications
but is still used in some applications like electronic mail.
 In message switching, each switch stores the whole message and forwards it
to the next switch.

Taxonomy of switched networks
1.41

CIRCUIT-SWITCHED NETWORKS
1.42

 A circuit-switched network is made of a set of switches connected by
physical links, in which each link is divided into n channels.
 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.
 The end systems such as computers or telephones are directly connected to
a switch.
 3 important phases to connect and transfer the information:
 Set up phase , Data transfer phase and Tear down phase.
1.43

 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 (bandwidthin 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-toend addressing used during the setup phase, as we
will see shortly.
1.44

 Before the two parties (or multiple parties in a conference call)
can communicate, a dedicated circuit (combination of channels in
links) needs to be established.
 The end systems are normally connected through dedicated lines
to the switches, so connection setup means creating dedicated
channels between the switches.
 The resources, such as channels (bandwidth in FDM and time
slots in TDM), switch buffers, switch processing time, and switch
I/O ports, must be reserved until the teardown phase.
 For example as shown in figure, when system A needs to connect
to system M, it sends a setup request that includes the address of
system M, to switch I.
 Switch I finds a channel between itself and switch IV that can be
dedicated for this purpose.
Setup Phase

 Switch I then sends the request to switch IV, which finds a
dedicated channel between itself and switch III.
 Switch III informs system M of system A's intention at this
time.
 In the next step to making a connection, an acknowledgment
from system M needs to be sent in the opposite direction to
system A.
 Only after system A receives this acknowledgment the
connection is established.
 An end-to-end addressing is required for creating a connection
between the two end systems. It could be addresses of the
computers in a TDM network, or telephone numbers in an
FDM network

Data Transfer Phase
 After the establishment of the dedicated circuit (channels), the
two parties can transfer data.
 There is no addressing involved during data transfer. The
switches route the data based on their occupied band (FDM) or
time slot (TDM).
 Data transferred between the two stations is a continuous flow
of signal, may be with periods of silence.
Teardown Phase
 When one of the parties needs to disconnect, a signal is sent to
each switch to release the resources.

Efficiency
 The circuit-switched networks are not as efficient as the other two
types of networks because resources are allocated during the entire
duration of the connection.
 These resources are unavailable to other connections.
 In a telephone network, people normally terminate the
communication when they have finished their conversation.
 However, in computer networks, a computer can be connected to
another computer even if there is no activity for a long time

Delay
 Although a circuit-switched network normally has low efficiency,
the delay in this type of network is minimal.
 During data transfer the data are not delayed at each switch since
the resources are allocated for the duration of the connection.
 The total delay is due to the time needed to create the connection,
transfer data, and disconnect the circuit.
 The delay caused by the setup is the sum of four parts: the
propagation time of the source computer request, the request signal
transfer time, the propagation time of the acknowledgment from the
destination computer, and the signal transfer time of the
acknowledgment.
 The delay due to data transfer is the sum of two parts: the
propagation time and data transfer time. The third one is the time
needed to tear down the circuit.
 If the receiver requests disconnection, then it causes the maximum
delay.

PACKET-SWITCHED NETWORK
In a packet-switched network, there is no resource
reservation; resources are allocated on demand.

DATAGRAM NETWORKS
 In a packet-switched network, the message is
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.
 Therefore, there is no reserved bandwidth on the
links, and no scheduled processing time.
 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. The
lack of reservation creates delay.
 In a datagram network, each packet is treated
independently of all others regardless of its source
or destination. Datagram switching is normally
done at the network layer.
DATAGRAM NETWORKS
 In the above figure, all four packets (or
datagram's) belong to the same message, but
may travel different paths to reach their
destination.
 This approach can cause the datagram's 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 datagram's or
ask for lost datagram's before passing them on to
the application.
 The datagram networks are referred to as
connectionless networks.
 The term connectionless means that the 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.

DATAGRAM NETWORKS
• A switch in a datagram network uses a routing table
that is based on the destination address.
• The destination address in the header of a packet in a
datagram network remains the same during the entire
journey of the packet.
• 1

DATAGRAM NETWORKS
Routing Table
 In datagram network, packets are routed to their destinations by means of a
routing table.
 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 as shown in the figure.
 Every packet in a datagram network carries a header that contains, among
other information, the destination address of the packet that remains the
same in its journey.
 When the switch receives the packet, destination address is examined and
the routing table is consulted to find the corresponding port through which
the packet should be forwarded.

DATAGRAM NETWORKS

DATAGRAM NETWORKS
Efficiency
 The efficiency of a datagram network is better than that of a circuit-
switched network; resources are allocated only when there are packets
to be transferred.
 The resources can be reallocated if it's idle, for other packets
Delay
 Each packet may experience a wait at a switch before it is forwarded.
In addition, since not all packets in a message necessarily travel
through the same switches, the delay is not uniform for the packets of
a message.
 In the above figure, the packet travels through two switches.
 There are three transmission times (3T), three propagation delays (3t),
and two waiting times (W1 + W2).
 The total delay is 3T + 3t + W1 + W2

VIRTUAL-CIRCUIT NETWORKS
A virtual-circuit network is a hybrid of a circuit-switched and datagram network. Some
of its characteristics are:
1. As in a circuit-switched network, there are setup and teardown phases in
addition to the data transfer phase.
2. Resources can be allocated during the setup phase, as in a circuit-switched
network, or on demand, as in a datagram network.
3. As in a datagram network, data are packetized and each packet carries an address
in the header. However, the address in the header defines what should be the next
switch and the channel on which the packet is being carried, not end-to-end.
4. As in circuit-switched network, all packets follow the same path established
during the connection.
5. A virtual-circuit network is normally implemented in the data link layer.

VIRTUAL-CIRCUIT 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

 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.
 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 the next switch should be 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. The answer will
be clear when we discuss virtual-circuit identifiers in the next section. 4.
As in a circuit-switched network, all packets follow the same path
established during the connection.
 As in a circuit-switched network, all packets follow the same path
established during the connection.
 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.

Virtual Circuit Network
 Three Phases of virtual circuit network:
 Setup
 Data Transfer
 Teardown
 Data Transfer in a VCN is shown in below diagram.

Structure of a Switch
• Circuit switches are of two types: Space and Time Division
Space Division
• A crossbar switch connects n inputs to m outputs in a grid, using
electronic microswitches (transistors) at each crosspoint.
• The major limitation of this design is the number of crosspoints
required.
• To connect n inputs to m outputs using a crossbar switch requires n
× m crosspoints. For example, to connect 1000 inputs to 1000
outputs requires a switch with 1,000,000 crosspoints

Space Division
•The solution to the limitations of the crossbar switch is the multistage switch,
which combines crossbar switches in several (normally three) stages.
•To design a three-stage switch, we follow these steps: 1. We divide the N input
lines into groups, each of n lines. For each group, we use one crossbar of size n ×
k, where k is the number of crossbars in the middle stage. In other words, the first
stage has N/n crossbars of n × k crosspoints. 2. We use k crossbars, each of size
(N/n) × (N/n) in the middle stage. 3. We use N/n crossbars, each of size k × n at
the third stage.

Time Division Switch: Time Slot Interchange
Time-division switching uses time-division multiplexing
(TDM) inside a switch. The most popular technology is called
the time-slot.
Imagine that each input line wants to send data to an output
line according to the following pattern: (1 → 3), (2 → 4), (3 →
1), and (4 → 2), in which the arrow means “to.”

 Packet switch has four components: input ports, output ports,
the routing processor, and the switching fabric.
 An input port performs the physical and data-link functions of
the packet switch. The bits are constructed from the received
signal. The packet is decapsulated from the frame. Errors are
detected and corrected. The packet is now ready to be routed
by the network layer.
 The output port performs the same functions as the input port,
but in the reverse order. First the outgoing packets are queued,
then the packet is encapsulated in a frame, and finally the
physical-layer functions are applied to the frame to create the
signal to be sent on the line.
Packet Switch Components

 Routing Processor The routing processor performs the functions of the
network layer. The destination address is used to find the address of the
next hop and, at the same time, the output port number from which the
packet is sent out. This activity is sometimes referred to as table lookup
because the routing processor searches the routing table. In the newer
packet switches, this function of the routing processor is being moved to
the input ports to facilitate and expedite the process.
 Switching Fabrics The most difficult task in a packet switch is to move
the packet from the input queue to the output queue. The speed with which
this is done affects the size of the input/output queue and the overall delay
in packet delivery. In the past, when a packet switch was actually a
dedicated computer, the memory of the computer or a bus was used as the
switching fabric. The input port stored the packet in memory; the output
port retrieved the packet from memory. Today, packet switches are
specialized mechanisms that use a variety of switching fabrics. We briefly
discuss some of these fabrics here.
Packet Switch Components

Review Questions
1. In a packet-switched network, the message is divided into ______________of fixed or variable size.
2. In datagram network, packets are routed to their destinations by means of a ________________.
3. A switch in a datagram network uses a routing table that is based on the _________________address
4. Datagram switching is normally done at the _____________ layer.
5. The datagram networks are referred to as _________________networks.
6. In a packer switched datagram network, the delay is not uniform for all the packets of a message (YES/NO)
7. A virtual-circuit network is normally implemented in the ___________layer
8. Circuit Switched network is normally implemented at the ______________ layer
Fig. 1 Fig. 2
Fig 1 and 2 belongs to which type of Packet
switched networks?

PROTOCOL LAYERING
 Scenarios
First Scenario (Single Layer)
Second Scenario (Three Layer)

 Each layer may add a Header and a Trailer to its Data (which consists
of the next higher layer's Header, Trailer and Data as it moves
through the layers). The Headers contain information that
specifically addresses layer-to-layer communication. For example,
the Transport Header (TH) contains information that only the
Transport layer sees. All other layers below the Transport layer pass
the Transport Header as part of their Data.
The OSI model

 The Physical Layer
 Establishes the physical characteristics of the network (e.g., the
type of cable, connectors, length of cable, etc.)
 Defines the electrical characteristics of the signals used to
transmit the data (e.g. signal voltage swing, duration of voltages,
etc.)

Transmits the binary data (bits) as electrical or optical signals
depending on the medium.
 The Data Link Layer

Defines how the signal will be placed on or taken off the NIC.
The data frames are broken down into individual bits that can be
translated into electric signals and sent over the network. On the
receiving side, the bits are reassembled into frames for
processing by upper levels.

Error detection and correction is also performed at the data link
layer. If an acknowledgement is expected and not received, the
frame will be resent. Corrupt data is also identified at the data
link layer.

Because the Data-Link Layer is very complex, it is sometimes
divided into sublayers (as defined by the IEEE 802 model). The
lower sublayer provides network access. The upper sublayer is
concerned with sending and receiving packets and error
checking.

 The Network Layer
 Primarily concerned with addressing and routing. Logical addresses
(e.g., an IP address) are translated into physical addresses (i.e., the
MAC address) for transmission at the network layer. On the receiving
side, the translation process is reversed.
 It is at the network layer where the route from the source to
destination computer is determined. Routes are determined based on
packet addresses and network conditions. Traffic control measures
are also implemented at the network layer.
 The Transport Layer
 On the sending side, messages are packaged for efficient transmission
and assigned a tracking number so they can be reassembled in proper
order. On the receiving side, the packets are reassembled, checked for
errors and acknowledged.
 Performs error handling in that it ensures all data is received in the
proper sequence and without errors. If there are errors, the data is
retransmitted.

 The Session Layer

Is responsible for establishing, maintaining, and terminating a connection
called a 'session'.

A session is an exchange of messages between computers (a dialog).
Managing the session involves synchronization of user tasks and dialog
control (e.g., who transmits and for how long). Synchronization involves
the use of checkpoints in the data stream. In the event of a failure, only the
data from the last checkpoint has to be resent.
 Logon, name recognition and security functions take place at the Session
Layer.
 The Presentation Layer

It is responsible for data translation (formatting), compression, and
encryption.

The Presentation Layer is primarily concerned with translation; interpreting
and converting the data from various formats. For example, EBCIDIC
characters might be converted into ASCII. It is also where data is
compressed for transmission and uncompressed on receipt. Encryption
techniques are implemented at the Presentation Layer.

The redirector operates at the presentation layer by redirecting I/O
operations across the network.
 The Application Layer

Provides the operating system with direct access to network services.

It serves as the interface between the user and the network by providing
services that directly support user applications.

Review Questions
1. The ____________ model is 7-layer architecture where each layer is having some specific
functionality to perform.
2. The full form of OSI is OSI model is ______________
3. The media access control sublayer resides in ___________________________.
4. UDP in the INTERNET protocol suite is related to _________________________.
5. TCP/IP model does not have ______ , ________ layer but OSI model have this layer.
6. ___________ layer is used to link the network support layers and user support layers
7. Network support layers are ______________________________.
8. User support layers are ________________________________.
9. _________________ address is used to identify a process on a host by the transport layer.
10. ____________ layer provides the services to user.
11. The data rate is decided by the _____________ layer
12. Match the following
a) Network - i) Port Address
b) Data Link - ii) Logical
c) Transport - iii) Physical
1.77

IEEE Project 802
 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.
 Project 802 does not seek to replace any part of the OSI model or TCP/IP
protocol suite. Instead, it is a way of specifying functions of the physical
layer and the data-link layer of major LAN protocols.
 The relationship of the 802 Standard to the TCP/IP protocol suite 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.

 In IEEE Project 802, flow control, error control, and part of
the framing duties are collected into one sublayer called the
logical link control (LLC).
 Framing is handled in both the LLC sublayer and the MAC
sublayer. The LLC provides a single link-layer control
protocol for all IEEE LANs.
 This means LLC protocol can provide interconnectivity
between different LANs because it makes the MAC sublayer
transparent.
 Media access control that defines the specific access method
for each LAN.
 For example, it defines CSMA/CD as the media access
method for Ethernet LANs and defines the token-passing
method for Token Ring and Token Bus LANs
1.79

Frame Format
1.82
Preamble. This field contains 7 bytes (56 bits) of alternating 0s
and 1s that alert the receiving system to the coming frame and
enable it to synchronize its clock if it’s out of synchronization. The
pattern provides only an alert and a timing pulse. The 56-bit
pattern allows the stations to miss some bits at the beginning of the
frame. The preamble is actually added at the physical layer and is
not (formally) part of the frame

 Start frame delimiter (SFD). This field (1 byte: 10101011)
signals the beginning of the frame. The SFD warns the station
or stations that this is the last chance for synchronization. The
last 2 bits are (11)2 and alert the receiver that the next field is
the destination address. This field is actually a flag that defines
the beginning of the frame. We need to remember that an
Ethernet frame is a variable-length frame. It needs a flag to
define the beginning of the frame. The SFD field is also added
at the physical layer.
 Destination address (DA). This field is six bytes (48 bits)
and contains the linklayer address of the destination station
or stations to receive the packet. We will discuss addressing
shortly. When the receiver sees its own link-layer address, or a
multicast address for a group that the receiver is a member of,
or a broadcastaddress, it decapsulates the data from the frame
and passes the data to the upperlayer protocol defined by the
value of the type field.
1.83

 Source address (SA). This field is also six bytes and contains
the link-layer address of the sender of the packet. We will
discuss addressing shortly.
 Type. This field defines the upper-layer protocol whose
packet is encapsulated in the frame. This protocol can be IP,
ARP, OSPF, and so on. In other words, it serves the same
purpose as the protocol field in a datagram and the port number
in a segment or user datagram. It is used for multiplexing and
demultiplexing.
 Data. This field carries data encapsulated from the upper-
layer protocols. It is a minimum of 46 and a maximum of 1500
bytes.
 CRC. The last field contains error detection information, in
this case a CRC-32. The CRC is calculated over the addresses,
types, and data field. If the receiver calculates the CRC and finds
that it is not zero (corruption in transmission), it discards the
frame.

Ethernet Evolution
Standard
Fast
Gigabit
10 Gigabit

OSI LAYERS FUNCTIONS, TOPOLOGIES,LAN,WAN AND METROPOLITAN NETWORK

OSI LAYERS FUNCTIONS, TOPOLOGIES,LAN,WAN AND METROPOLITAN NETWORK

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OSI LAYERS FUNCTIONS, TOPOLOGIES,LAN,WAN AND METROPOLITAN NETWORK