Data Communication and Network- Network Layer onward all layers

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Network Layer of OSI Model
Mr. C. P. Divate
Department of Computer Engineering

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Switching techniques in Computer Network
 In large networks, there can be multiple paths from sender to receiver.
 The switching technique will decide the best route for data transmission.
 Switching technique is used to connect the systems for making one-to-one
communication.

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Classification Of Switching Techniques

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1) Circuit Switching
 Circuit switching is a switching technique that establishes a dedicated path between sender and
receiver.
 In the Circuit Switching Technique, once the connection is established then the dedicated path
will remain to exist until the connection is terminated.
 Circuit switching in a network operates in a similar way as the telephone works.
 A complete end-to-end path must exist before the communication takes place.

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 In case of circuit switching technique, when any user wants to send the data, voice, video, a
request signal is sent to the receiver then the receiver sends back the acknowledgment to ensure
the availability of the dedicated path. After receiving the acknowledgment, dedicated path
transfers the data.
 Circuit switching is used in public telephone network. It is used for voice transmission.
 Fixed data can be transferred at a time in circuit switching technology.

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Communication through circuit switching has 3 phases:
1) Circuit establishment
2) Data transfer
3) Circuit Disconnect

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Circuit Switching can use either of the two technologies:
1) Space Division Switches
2) Time Division Switches

Time Division Switches: Time slot Interchange (TSI)

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Time Division Multiplexing(TDM) : works in two modes
i) Synchronous TDM
ii) Asynchronous TDM

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2) Space Division Switches:
Space Division Switches can be categorized in two ways:
1) Crossbar Switch 2) Multistage Switch

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2) Space Division Switches: i) Crossbar Switch
 Space Division Switching is a circuit switching technology
in which a single transmission path is accomplished in a
switch by using a physically separate set of crosspoints.
 Space Division Switching can be achieved by using
crossbar switch.
 A crossbar switch is a metallic crosspoint or semiconductor
gate that can be enabled or disabled by a control unit.
 The Crossbar switch is made by using the semiconductor.
 Space Division Switching has high speed, high capacity,
and non-blocking switches.
 The Crossbar switch is a switch that has n input lines and n
output lines. The crossbar switch has n2 intersection points
known as crosspoints.

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2) Space Division Switches: ii) Multistage Switch
 Multistage Switch is made by splitting the crossbar switch
into the smaller units and then interconnecting them.
 It reduces the number of crosspoints.
 If one path fails, then there will be an availability of
another path.

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2) Space Division Switches: ii) Multistage Switch

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Advantages Of Circuit Switching
 In the case of Circuit Switching technique, the communication channel is dedicated.
 It has fixed bandwidth.

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Disadvantages Of Circuit Switching
 Once the dedicated path is established, the only delay occurs in the speed of data transmission.
 It takes a long time to establish a connection approx 10 seconds during which no data can be
transmitted.
 It is more expensive than other switching techniques as a dedicated path is required for each
connection.
 It is inefficient to use because once the path is established and no data is transferred, then the capacity
of the path is wasted.
 In this case, the connection is dedicated therefore no other data can be transferred even if the channel
is free.

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2) Message Switching
 Message Switching is a switching technique in which a message is transferred as a complete unit and routed
through intermediate nodes at which it is stored and forwarded.
 In Message Switching technique, there is no establishment of a dedicated path between the sender and
receiver.
 The destination address is appended to the message. Message Switching provides a dynamic routing as the
message is routed through the intermediate nodes based on the information available in the message.
 Each and every node stores the entire message and then forward it to the next node. This type of network is
known as store and forward network.

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2) Message Switching
Advantages Of Message Switching
 Data channels are shared among the communicating devices that improve the efficiency of using available
bandwidth.
 Traffic congestion can be reduced because the message is temporarily stored in the nodes.
 Message priority can be used to manage the network.
 The size of the message which is sent over the network can be varied. Therefore, it supports the data of
unlimited size.
Disadvantages Of Message Switching
 The message switches must be equipped with sufficient storage to enable them to store the
messages until the message is forwarded.
 The Long delay can occur due to the storing and forwarding facility provided by the message
switching technique.

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3) Packet Switching
 The packet switching is a switching technique in which the message is sent in one go, but it is divided into
smaller pieces, and they are sent individually.
 The message splits into smaller pieces known as packets and packets are given a unique number to identify
their order at the receiving end.
 Every packet contains some information in its headers such as source address, destination address and
sequence number.

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3) Packet Switching
 Packets will travel across the network, taking the shortest path as possible.
 All the packets are reassembled at the receiving end in correct order.
 If any packet is missing or corrupted, then the message will be sent to resend the message.
 If the correct order of the packets is reached, then the acknowledgment message will be sent.

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3) Packet Switching
There are two approaches to Packet Switching:
1) Datagram Packet switching 2) Virtual Circuit Switching

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3) Packet Switching 1) Datagram Packet switching
It is a packet switching technology in which packet is known as a datagram, is considered as an independent entity.
Each packet contains the information about the destination and switch uses this information to forward the packet to the
correct destination.
The packets are reassembled at the receiving end in correct order.
In Datagram Packet Switching technique, the path is not fixed.
Intermediate nodes take the routing decisions to forward the packets.
Datagram Packet Switching is also known as connectionless switching.

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3) Packet Switching 2) Virtual Circuit Switching
In the above diagram, A and B are the sender and receiver respectively. 1 and 2 are the nodes.
Call request and call accept packets are used to establish a connection between the sender and receiver.
When a route is established, data will be transferred.
After transmission of data, an acknowledgment signal is sent by the receiver that the message has been received.
If the user wants to terminate the connection, a clear signal is sent for the termination.

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3)Packet Switching
Advantages Of Packet Switching
 Cost-effective: In packet switching technique, switching devices do not require massive secondary
storage to store the packets, so cost is minimized to some extent. Therefore, we can say that the packet
switching technique is a cost-effective technique.
 Reliable: If any node is busy, then the packets can be rerouted. This ensures that the Packet Switching
technique provides reliable communication.
 Efficient: Packet Switching is an efficient technique. It does not require any established path prior to the
transmission, and many users can use the same communication channel simultaneously, hence makes use
of available bandwidth very efficiently.

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3)Packet Switching
Disdvantages Of Packet Switching
 Packet Switching technique cannot be implemented in those applications that require low delay
and high-quality services.
 The protocols used in a packet switching technique are very complex and requires high
implementation cost.
 If the network is overloaded or corrupted, then it requires retransmission of lost packets. It can
also lead to the loss of critical information if errors are nor recovered.

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Multiplexing:
 The transmission medium is used to send the signal from sender to receiver. The medium can
transmit only one signal at a time.
 If there are multiple signals to share one medium, then the medium must be divided in such a
way that each signal is given some portion of the available bandwidth. For example: If there are
10 signals and bandwidth of medium is100 units, then the 10 unit is shared by each signal.
 When multiple signals share the common medium, there is a possibility of collision.
Multiplexing concept is used to avoid such collision.
 Transmission services are very expensive.

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Multiplexing:
 The 'n' input lines are transmitted through a multiplexer and multiplexer combines the signals to
form a composite signal.
 The composite signal is passed through a Demultiplexer and demultiplexer separates a signal to
component signals and transfers them to their respective destinations.
Advantages of Multiplexing:
 More than one signal can be sent over a single medium.
 The bandwidth of a medium can be utilized effectively.

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Multiplexing Techniques:
 Multiplexing techniques can be classified as:

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Frequency-division Multiplexing (FDM):
 It is an analog technique.
 Frequency Division Multiplexing is a technique in which the available bandwidth of a single
transmission medium is subdivided into several channels.
 In the above diagram, a single transmission medium is subdivided into several frequency
channels, and each frequency channel is given to different devices. Device 1 has a frequency
channel of range from 1 to 5.

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 The input signals are translated into frequency bands by using modulation techniques, and they are combined by a
multiplexer to form a composite signal.
 The main aim of the FDM is to subdivide the available bandwidth into different frequency channels and allocate them to
different devices.
 Using the modulation technique, the input signals are transmitted into frequency bands and then combined to form a
composite signal.
 The carriers which are used for modulating the signals are known as sub-carriers. They are represented as f1,f2..fn.

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Network
Transmitter
Receiver

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Advantages Of FDM:
 FDM is used for analog signals.
 FDM process is very simple and easy modulation.
 A Large number of signals can be sent through an FDM
simultaneously.
 It does not require any synchronization between sender and
receiver.
Disadvantages Of FDM:
 FDM technique is used only when low-speed channels
are required.
 It suffers the problem of crosstalk.
 A Large number of modulators are required.
 It requires a high bandwidth channel.
Applications Of FDM:
 FDM is commonly used in TV
networks.
 It is used in FM and AM
broadcasting. Each FM radio station
has different frequencies, and they
are multiplexed to form a composite
signal. The multiplexed signal is
transmitted in the air.

▶The network layer is responsible for the delivery of
individual packets from the source host to the destination
host
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Network Layer of OSI Model

The Network
Layer
Introduction
FUNCTIONS OF
NETWORK LAYER
Network Layer Design
Issues
Routing Algorithms
Congestion Control
Algorithms
References
OUTLINE
INTRODUCTION
NETWORK LAYER DESIGN ISSUES
ROUTING ALGORITHMS
CONGESTION CONTROL ALGORITHMS
FUNCTIONS OF NETWORK LAYER

The Network
Layer
INTRODUCTION I
 The network layer is concerned with getting packets from the source all
the way to the destination.
 Getting to the destination may require making many hops at intermediate
routers along the way.
 This function clearly contrasts with the function of the data link layer.
 The data link layer is concerned with only moving frames from one end of
a wire to the other.
 Thus, the network layer is the lowest layer that deals with end-to-end
transmission.
 To achieve its goals, the network layer must know about the topology of
the network (i.e., the set of all routers and links).
 It should also choose appropriate paths through the network, even if it is
large.
Introduction
FUNCTIONS OF
NETWORK LAYER
Issues
Routing Algorithms
Congestion Control
Algorithms
References

INTRODUCTION II
 It must also take care when choosing routes to avoid overloading some of the communication lines
and routes while leaving others idle.
 It must also handle problems when source and destination are in different networks.
 It is up to the network layer to deal with them.

INTRODUCTION III
 The network layer can also divide the huge packets into little chunks, if the packet is higher than the
most extensive data link layer.
 The network reassembles the fragments into packets at the receiving end. Routers and Gateways act
in the network layer.
 The communication of the network layer to the data link and transport layer is demonstrated in the
figure given below −

Functions of Network Layer:
 Internetworking: An internetworking is the main responsibility of the network layer. It provides a
logical connection between different devices.

 Packetizing: A Network Layer receives the segment from the upper layer and converts them into
packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

 Addressing: A Network layer adds the source and destination address to the header of the frame.
Addressing is used to identify the device on the internet.

 Addressing: Network Addressing is one of the major responsibilities of the network layer.
 Network addresses are always logical, i.e., software-based addresses.
 A host is also known as end system that has one link to the network. The boundary between the
host and link is known as an interface. Therefore, the host can have only one interface.
 A router is different from the host in that it has two or more links that connect to it. When a router
forwards the datagram, then it forwards the packet to one of the links. The boundary between the
router and link is known as an interface, and the router can have multiple interfaces, one for each
of its links. Each interface is capable of sending and receiving the IP packets, so IP requires each
interface to have an address.
 Each IP address is 32 bits long, and they are represented in the form of "dot-decimal notation"
where each byte is written in the decimal form, and they are separated by the period. An IP address
would look like 193.32.216.9 where 193 represents the decimal notation of first 8 bits of an
address, 32 represents the decimal notation of second 8 bits of an address.

Functions of Network Layer: Addressing
 Type of IPAddress:
1) Classful IP Addressing
2) Classless IP Addressing
1) Classful Addressing:
 An IP address is 32-bit long.
 An IP address is divided into Five sub-classes:
a) Class A
b) Class B
c) Class C
d) Class D
e) Class E

1) Classful Addressing:
 An IP address is divided into two parts:
 Network ID: It represents the number of
networks.
 Host ID: It represents the number of hosts.
 In the diagram, we observe that each class
have a specific range of IP addresses.
 The class of IP address is used to
determine the number of bits used in a
class and number of networks and hosts
available in the class

1) Class A:
 In Class A, an IP address is assigned to those networks that contain a large number of hosts.
 The network ID is 8 bits long.
 The host ID is 24 bits long.
 In Class A, the first bit in higher order bits of the first octet is always set to 0 and the remaining
7 bits determine the network ID. The 24 bits determine the host ID in any network.
 The total number of networks in Class A = 27 = 127 network address
 The total number of hosts in Class A = 224 - 2 = 16,777,214 host address
 IP addresses belonging to class A ranges from 1.x.x.x – 127.x.x.x

2) Class B:
 In Class B, an IP address is assigned to those networks that range from small-sized to large-
sized networks.
 The Network ID is 16 bits long.
 The Host ID is 16 bits long..
 In Class B, the higher order bits of the first octet is always set to 10, and the remaining14 bits
determine the network ID. The other 16 bits determine the Host ID.
 The total number of networks in Class B = 214 = 16383 network address
 The total number of hosts in Class B = 216 - 2 = 65534 host address
 IP addresses belonging to class B ranges from 128.0.x.x – 191.255.x.x.

3) Class C:
 In Class C, an IP address is assigned to only small-sized networks.
 The Network ID is 24 bits long.
 The host ID is 8 bits long.
 In Class C, the higher order bits of the first octet is always set to 110, and the remaining 21 bits
determine the network ID. The 8 bits of the host ID determine the host in a network.
 The total number of networks = 221 = 2097152 network address
 The total number of hosts = 28 - 2 = 254 host address
 IP addresses belonging to class C ranges from 192.0.0.x – 223.255.255.x

4) Class D:
 In Class D, an IP address is reserved for multicast addresses.
 It does not possess subnetting.
 The higher order bits of the first octet is always set to 1110, and the remaining bits determines
the host ID in any network.
 IP addresses belonging to class D ranges from 224.0.0.0 – 239.255.255.255.

5) Class E:
 In Class E, an IP address is used for the future use or for the research and development purposes.
 It does not possess any subnetting.
 The higher order bits of the first octet is always set to 1111, and the remaining bits determines the
host ID in any network.
 IP addresses of class E ranges from 240.0.0.0 – 255.255.255.254.

Classful Network Architecture

Public and Private IPAddresses
The are two types of Internet Protocol (IP) addresses: Public and Private.
A router will typically have two types of network interfaces:
 An Internal Interface
 An external Interface
Each of these interfaces will have an IP address.

Public and Private IPAddresses
 A Public IP address (External) is assigned to every device that connects to the Internet and
each IP address is unique. Therefore, there cannot exist two device with the same public IP
address.
 The public IP address is assigned to the device by the Internet Service Provider as soon as
the device is connected to the Internet.
 A public IP address can be static, dynamic or shared.

Public IP as a Static IP address
 Public static - some times called Dedicated - means the IP address never changes and is tied
to a single user, device, server or website.

Public IP as a Dynamic IP address
 Public dynamic - means the IP address can change from time-to-time (for example, when
you lose connection and re-connect or the ISP might change the address periodically).

Public IP as a Shared IPs
 Public shared - in some cases, an ISP can assign a public IP address to a group of users, and
then employ NAT to isolate their traffic.

Private IP address
 (Internal) is only used by devices communicating to each other on the same network.
 Devices with private IP addresses cannot connect to the Internet directly.
 Likewise, computers or other devices outside the local network cannot connect directly to a
device with a private IP.
 An IP address is considered private if the IP number falls within one of the IP address ranges
reserved for private networks such as a Local Area Network (LAN).
 The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks
of the IP address space for private networks (local networks):
o 10.0.0.0 - 10.255.255.255 (Total Addresses: 16,777,216)
o 172.16.0.0 - 172.31.255.255 (Total Addresses: 1,048,576)
o 192.168.0.0 - 192.168.255.255 (Total Addresses: 65,536)

What is Subnetting?
 Subnetting is the practice of dividing a network into two or smaller networks.
 It increases routing efficiency, which helps to enhance the security of the network and reduces
the size of the broadcast domain.
 This method divides a network into smaller subnets.
 It also helps you to reduce the size of the routing tables, which is stored in routers.
Classless addressing / Subnetting

Classless addressing / Subnetting
Why Use Subnetting?
 Subnetting process helps to allocate IP addresses that prevent large numbers of IP network
addresses from remaining unused.
 It helps you to maximize IP addressing efficiency.
 Extend the life of IPV4.
 Public IPV4 Addresses are rare.
 Dividing a single networkinto multiple networks.
 Converting Host bits into networkbits. i.e. converting 0’s into 1’s.
 Sub netting can be done in three ways.
 Requirement of Network
 Requirement of Host
 / Notation

 The key concept of sub netting is borrowing bits from the
host portion of the network to create a sub network.
 Rules govern this borrowing, ensuring that some bits are
left for a Host ID.
 The rules require that two bits remain available to use for
the Host ID.

If 2 Bits available for Creating Subnets
Address class Network bits Host bits
Bits available for
subnet Hosts
CLASS A 8 24 24-2 = 22
CLASS B 16 16 16-2 = 14
CLASS C 24 8 8-2 = 6

128 64 32 16 8 4 2 1
1 0 0 0 0 0 0 0 128
1 1 0 0 0 0 0 0 192
1 1 1 0 0 0 0 0 224
1 1 1 1 0 0 0 0 240
1 1 1 1 1 0 0 0 248
1 1 1 1 1 1 0 0 252
1 1 1 1 1 1 1 0 254
1 1 1 1 1 1 1 1 255
Binary place
value
Binary
Digit Decimal
Equivalent
Sub Net mask value

CIDR – Classless Inter-Domain Routing.
CIDR addressing method allows the use of a prefix to designate the number of network bits in
the mask
For Example: 200.15.1.53/25 (CIDR Notation)
In this example the CIDR value is /25 means the first 25 bits in the mask are network bits (1s)

Subnet mask : It is used to determine the network portion and
Host portion of an IP address.
Address Class Subnet mask Octet Format
CLASS A 255.0.0.0 N.H.H.H
CLASS B 255.255.0.0 N.N.H.H
CLASS C 255.255.255.0 N.N.N.H
N = Network H = Host

Network = 2n [n = number of ON bits (1’s)]
Host = 2h [h = number of OFF bits (0’s)]
Valid Host = Host – 2 *
 * First IP(Host) of Network is use for network address
 * Last IP(Host) of network is use for Broadcast address
192.168.1.0
255.255.255.0
11111111.11111111.11111111. 00000000
IP Address
Subnet Mask
Binary format
Network Bits Host Bits

 192.168.1.0 divided into in 64 Hosts.
Subnet mask
Binary form
CIDR
Network
Host
Valid Host
Block size
New Binary form
New Subnet mask
New CIDR
: 255.255.255.0
: 11111111.11111111.11111111.00000000
: /24
: 2n => 22 = 4
: 2h => 26 = 64
: Host – 2 => 64 – 2 = 62
: 256 – 192 = 64
: 11111111.11111111.11111111.11000000
: 255.255.255.192
: /26
n = no. of
ON bits.
h = no. of
OFF bits

No
.
Network/Network
ID
Broadcast ID First Host Last Host
1 192.168.1.0 192.168.1.63 192.168.1.1 192.168.1.62
2 192.168.1.64 192.168.1.127 192.168.1.65 192.168.1.126
3 192.168.1.128 192.168.1.191 192.168.1.129 192.168.1.190
4. 192168.1.192 192.168.1.255 192.168.1.193 192.168.1.254
 192.168.1.0 divided into in 64 Hosts.

 172.18.0.0 divided into in 8 Networks.
Subnet mask
Binary form
CIDR
Network
Host
V
alid Host
Block Size
: 255.255.0.0
: 11111111.11111111.00000000.00000000
: /16
: 2n => 23 = 8
: 2h => 213 = 8192
: Host – 2 => 8192 – 2 = 8190
: 256 – 224 = 32
New Binary form
New Subnet mask
New CIDR
: 11111111.11111111.11100000.00000000
: 255.255.224.0
: /19
n = no. of
ON bits.
h = no. of
OFF bits

S No Network/Network ID Broadcast ID First Host Last Host
1 172.18.0.0 172.18.31.255 172.18.0.1 172.18.31.254
2 172.18.32.0 172.18.63.255 172.18.32.1 172.18.63.254
3 172.18.64.0 172.18.91.255 172.18.64.1 172.18.91.254
4 172.18.92.0 172.18.127.255 172.18.92.1 172.18.127.254
5 172.18.128.0 172.18.159.255 172.18.128.1 172.18.159.254
6 172.18.160.0 172.18.191.255 172.18.160.1 172.18.191.254
7 172.18.192.0 172.18.223.255 172.18.192.1 172.18.223.254
8 172.18.224.0 172.18.255.255 172.18.224.1 172.18.255.254
 172.18.0.0 divided into in 8 Networks.

 Find the Broadcast ID of this network 110.60.21.15/11
Subnet mask
Binary form
CIDR
Network
Host
Valid Host
Block Size
: 255.0.0.0
: 11111111.00000000.00000000.00000000
: /8
: 2n => 23 = 8
: 2h => 221 = 2097152
: Host – 2 => 2097152 – 2 = 2097150
: 256 – 224 = 32
New Binary form
New Subnet mask
New CIDR
: 11111111.11100000.00000000.00000000
: 255.224.0.0
: /11
n = no. of
ON bits.
h = no. of
OFF bits

2 110.32.0.0 110.63.255.255 110.32.0.1 110.63.255.254
S No Network/Network ID Broadcast ID First Host Last Host
1 110.0.0.0 110.31.255.255 110.0.0.1 110.31.255.254
2 110.32.0.0 110.63.255.255 110.32.0.1 110.63.255.254
3 110.64.0.0 110.91.255.255 110.64.0.1 110.91.255.254
4 110.92.0.0 110.127.255.255 110.92.0.1 110.127.255.254
5 110.128.0.0 110.159.255.255 110.128.0.1 110.159.255.254
6 110.160.0.0 110.191.255.255 110.160.0.1 110.191.255.254
7 110.192.0.0 110.223.255.255 110.192.0.1 110.223.255.254
8 110.224.0.0 110.255.255.255 110.224.0.1 110.255.255.254
 Find the Broadcast ID of this address 110.60.21.15/11

Routing
• The process of moving a packet of data from source to
destination.
• Routing is usually performed by a dedicated device
called a Router.
• It is the key feature of Internet because it enables
messages to pass from one computer to another and
eventually reach the target machine

Classification of Routing Algorithms
Adaptive Algorithms
• These are the algorithms which change their routing decisions
whenever network topology or traffic load changes.
• The changes in routing decisions are reflected in the topology as
well as traffic of the network.
• Also known as Dynamic routing, these make use of dynamic
information such as current topology, load, delay, etc. to select
routes.
• Optimization parameters are distance, number of hops and
estimated transit time.

Classification of Adaptive Algorithms
• (a) Isolated – In this method each, node makes its routing decisions using the
information it has without seeking information from other nodes. The sending
nodes doesn’t have information about status of particular link. Disadvantage is
that packet may be sent through a congested network which may result in
delay. Examples: Hot potato routing, backward learning.
• (b) Centralized – In this method, a centralized node has entire information about
the network and makes all the routing decisions. Advantage of this is only one
node is required to keep the information of entire network and disadvantage is
that if central node goes down the entire network is done.
• (c) Distributed – In this method, the node receives information from its neighbors
and then takes the decision about routing the packets. Disadvantage is that the
packet may be delayed if there is change in between interval in which it receives
information and sends packet.

Classification of Adaptive Algorithms
• (a)Isolated –

Non-Adaptive Algorithms
• These are the algorithms which do not change their routing decisions once they
have been selected. This is also known as static routing as route to be taken is
computed in advance and downloaded to routers when router is booted.
• (a) Flooding – This adapts the technique in which every incoming packet is sent
on every outgoing line except from which it arrived. One problem with this is that
packets may go in loop and as a result of which a node may receive duplicate
packets. These problems can be overcome with the help of sequence numbers,
hop count and spanning tree.
• (b) Random walk – In this method, packets are sent host by host or node by node
to one of its neighbors randomly. This is highly robust method which is usually
implemented by sending packets onto the link which is least queued.

Distance Vector Routing: https://www.youtube.com/watch?v=hkaYIg01dDk
Link State Routing: https://www.youtube.com/watch?v=kW6zV-040SY
Dijkastra’s Algorithm: https://www.youtube.com/watch?v=Lfb8qkXzHY0
Bellman Ford Algorithm: https://www.youtube.com/watch?v=SiI03wnREt4
Routing Algorithms
Flooding and Flow control algorithm: https://www.youtube.com/watch?v=hcXxnG1T3pE
Non-Adaptive Algorithms
Adaptive Algorithms

The Network
Layer
Introduction
Issues
Routing Algorithms
Congestion Control
Algorithms
References
STORE-AND-FORWARD PACKET SWITCHING I
Components of a computer network:
► Hosts (computers, handheld deveices etc.)
► Switches.
► Routers.
► Wireless access points.
D
C
B
A E F
Packet
Process P1
Host H1
Router ISP’s equipment
H2
LAN
P2
FIGURE: The environment of the network layer protocols.

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 The transport layer is a 4th layer from the top.
 The main role of the transport layer is to provide the communication services directly to the
application processes running on different hosts.
 The transport layer provides a logical communication between application processes running on
different hosts.
▶ Process-to-process delivery

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 The transport layer provides a logical communication between application processes running on
different hosts.
 Although the application processes on different hosts are not physically connected, application
processes use the logical communication provided by the transport layer to send the messages to
each other.
 The transport layer protocols are implemented in the end systems but not in the network routers.

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 A computer network provides more than one
protocol to the network applications.
 For example, TCP and UDP are two transport layer
protocols that provide a different set of services to
the network layer.
 Each of the applications in the application layer has
the ability to send a message by using TCP or UDP.
 The application communicates by using either of
these two protocols.
 Both TCP and UDP will then communicate with the
internet protocol in the internet layer.
 The applications can read and write to the transport
layer. Therefore, we can say that communication is a
two-way process.

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 The services provided by the transport layer are similar to those of the data link layer.
 The data link layer provides the services within a single network while the transport layer provides
the services across an internetwork made up of many networks.
 The data link layer controls the physical layer while the transport layer controls all the lower layers.
Services provided by the Transport Layer

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The services provided by the transport layer protocols can be divided into five categories:
 End-to-end delivery
 Addressing
 Reliable delivery
 Flow control
 Multiplexing.

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1) End-to-end delivery: The transport layer transmits the entire message to the destination. Therefore, it
ensures the end-to-end delivery of an entire message from a source to the destination.

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2) Reliable delivery: The transport layer provides reliability services by retransmitting the lost
and damaged packets.
The reliable delivery has four aspects:
 Error control
 Sequence control
 Loss control
 Duplication control

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2) Reliable delivery: i) Error control
 The data link layer checks for the error between each network.
 If an error is introduced inside one of the routers, then this error will not be caught by the data link layer.
 It only detects those errors that have been introduced between the beginning and end of the link.
 Therefore, the transport layer performs the checking for the errors end-to-end to ensure that the packet
has arrived correctly.

85
2) Reliable delivery: ii) Sequence control
 On the sending end, the transport layer is responsible for ensuring that the packets received from the
upper layers can be delivered to and used by the lower layers.
 On the receiving end, it ensures that the various segments of a transmission can be correctly reassembled.

86
2) Reliable delivery: iii) Loss control
 Loss Control is a third aspect of reliability.
 The transport layer ensures that all the fragments
of a transmission arrive at the destination, not
some of them.
 On the sending end, all the fragments of
transmission are given sequence numbers by a
transport layer.
 These sequence numbers allow the receiver?s
transport layer to identify the missing segment.
2) Reliable delivery: iv) Duplication Control
 Duplication Control is the fourth aspect of reliability.
 The transport layer guarantees that no duplicate data arrive at the destination.
 Sequence numbers are used to identify the lost packets; similarly, it allows the receiver to
identify and discard duplicate segments.

87
3) Flow Control
 Flow control is used to prevent the sender from overwhelming the receiver.
 If the receiver is overloaded with too much data, then the receiver discards the packets and asking for
the retransmission of packets.
 This increases network congestion and thus, reducing the system performance.
 The transport layer is responsible for flow control.
 It uses the sliding window protocol that makes the data transmission more efficient as well as it
controls the flow of data so that the receiver does not become overwhelmed.
 Sliding window protocol is byte oriented rather than frame oriented.

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4) Multiplexing: The transport layer uses the multiplexing to improve transmission efficiency.
i. Upward multiplexing:
 Upward multiplexing means multiple transport layer connections use the same network connection.
 To make more cost-effective, the transport layer sends several transmissions bound for the same
destination along the same path; this is achieved through upward multiplexing.

89
4) Multiplexing: The transport layer uses the multiplexing to improve transmission efficiency.
ii. Downward multiplexing:
 Downward multiplexing means one transport layer connection uses the multiple network connections.
 Downward multiplexing allows the transport layer to split a connection among several paths to
improve the throughput.
 This type of multiplexing is used when networks have a low or slow capacity.

90
5) Addressing:
 Data generated by an application on one machine must be transmitted to the correct application on
another machine.
 In this case, addressing is provided by the transport layer.
 The transport layer provides the user address which is specified as a station or port.
 The port variable represents a particular TS user of a specified station known as a Transport Service
access point (TSAP).
 Each station has only one transport entity.
 The transport layer protocols need to know which upper-layer protocols are communicating.

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5) Addressing:

▶The session layer is responsible for dialog control and
synchronization
92

Dialog control : The session layer allows two systems to enter into a dialog.
▶It allows the communication between two processes to take place in either half
duplex (one way at a time) or full-duplex (two ways at a time) mode.
Synchronization : The session layer allows a process to add checkpoints, or
synchronization points, to a stream of data.
▶For example, if a system is sending a file of 2000 pages, it is advisable to insert
checkpoints after every 100 pages to ensure that each 100-page unit is received
and acknowledged independently. In this case, if a crash happens during the
transmission of page 523, the only pages that need to be resent after system
recovery are pages 501 to 523. Pages previous to 501 need not be resent.
94

▶The presentation layer is responsible for translation,
compression, and encryption.
95

Translation : The processes (running programs) in two systems are usually
exchanging information in the form of character strings, numbers, and so on.
▶ The information must be changed to bit streams before being transmitted.
Because different computers use different encoding systems, the presentation
layer is responsible for interoperability between these different encoding
methods.
▶The presentation layer at the sender changes the information from its sender-
dependent format into a common format.
▶The presentation layer at the receiving machine changes the common format
into its receiver-dependent format.
97

Encryption : To carry sensitive information, a system must be able to
ensure privacy.
▶Encryption means that the sender transforms the original information
to another form and sends the resulting message out over the network.
▶Decryption reverses the original process to transform the message back
to its original form.
Compression : Data compression reduces the number of bits contained
in the information.
▶Data compression becomes particularly important in the transmission
of multimedia such as text, audio, and video.
98

▶The application layer is responsible for providing
services to the user
99

▶ Network virtual terminal : A network virtual terminal is a software
version of a physical terminal, and it allows a user to log on to a remote
host.
▶ To do so, the application creates a software emulation of a terminal at the
remote host.
▶ The user's computer talks to the software terminal which, in turn, talks to
the host, and vice versa.
▶The remote host believes it is communicating with one of its own terminals
and allows the user to log on.
▶File transfer, access, and management : This application allows a user to
access files in a remote host (to make changes or read data), to retrieve files
from a remote computer for use in the local computer, and to manage or
control files in a remote computer locally.
10
1

▶Mail services : This application provides the basis for e-
mail forwarding and storage.
▶Directory services : This application provides distributed
database sources and access for global information about
various objects and services.
10
2

▶At each layer, additional information is added to the data packet
▶An example would be information related to the IP protocol that is added
at Layer 3
10
3

Data
Header
10
4
Trailer
A general concept of packets serves as a prerequisite to
the understanding of the ISO-OSI model.

▶ Packet arrival information
▶Receiver’s address
▶Sender’s address
▶ Synchronization character
10
5

▶ Error correction code
◦ Character oriented
◦ VRC (Parity Checking)
▶ Packet oriented error correction codes
◦ LRC
◦ CRC
10
6

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