A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 1 Sub Code:EC2352 Sub Name: Computer Networks Dept: ECE Sem/Year: VI/III UNIT-I PART-A 1. Differentiate guided and unguided transmission medium.(AUC MAY’13) Guided transmission medium Unguided transmission medium Guided indicate, medium is contained have any within the physical boundary Unguided medium does not have any physical boundary. Transmission takes place through wire It’s a wireless transmission medium. 2. State the role of digital subscriber line.(AUC MAY’13) Digital subscriber line (DSL) technology is one of the most promising for supporting high- speed digital communication over the existing local loops. However, like a cable modem, a DSL circuit is much faster than a regular phone connection, even though the wires it uses are copper like a typical phone line. in asymmetric DSL (ADSL) connection allows download speeds of up to about 1.5 megabits (not megabytes) per second, and upload speeds of 128 kilobits per second. 3. What is TCP/IP?(AUC NOV ’12) TCP/IP is two separate protocols, TCP and IP, that are used together. The Internet Protocol standard dictates how packets of information are sent out over networks. IP has a packet-addressing method that lets any computer on the Internet forward a packet to another computer that is a step (or more) closer to the packet's recipient. The Transmission Control Protocol ensures the reliability of data transmission across Internet connected networks. TCP checks packets for errors and submits requests for re-transmissions if errors are found; it also will return the multiple packets of a message into a proper, original sequence when the message reaches its destination 4. Compare datagram networks with virtual circuit subnets. (AUC NOV ’12)
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A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 1
Sub Code:EC2352 Sub Name: Computer Networks
Dept: ECE Sem/Year: VI/III
UNIT-I
PART-A
1. Differentiate guided and unguided transmission medium.(AUC MAY’13)
Guided transmission medium Unguided transmission medium
Guided indicate, medium is contained have
any within the physical boundary
Unguided medium does not have
any physical boundary.
Transmission takes place through wire It’s a wireless transmission medium.
2. State the role of digital subscriber line.(AUC MAY’13)
Digital subscriber line (DSL) technology is one of the most promising for supporting high-
speed digital communication over the existing local loops.
However, like a cable modem, a DSL circuit is much faster than a regular phone
connection, even though the wires it uses are copper like a typical phone line.
in asymmetric DSL (ADSL) connection allows download speeds of up to about 1.5
megabits (not megabytes) per second, and upload speeds of 128 kilobits per second.
3. What is TCP/IP?(AUC NOV ’12)
TCP/IP is two separate protocols, TCP and IP, that are used together.
The Internet Protocol standard dictates how packets of information are sent out over
networks. IP has a packet-addressing method that lets any computer on the Internet
forward a packet to another computer that is a step (or more) closer to the packet's
recipient.
The Transmission Control Protocol ensures the reliability of data transmission across
Internet connected networks. TCP checks packets for errors and submits requests for
re-transmissions if errors are found; it also will return the multiple packets of a message
into a proper, original sequence when the message reaches its destination
4. Compare datagram networks with virtual circuit subnets. (AUC NOV ’12)
A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 2
5. What is meant by data communication?(AUC NOV’11)
Data communication is the exchange of data (in the form of 1s and 0s) between two devices via
some form of transmission medium (such as a wire cable).
6. For n devices in a network, what is the no of cable links required for mesh and ring
topology. (AUC NOV’11)
Mesh topology – n (n-1)/2
Ring topology – n
7. What are the three criteria necessary for an effective and efficient network?
The most important criteria are performance, reliability and security. Performance of the network
depends on number of users, type of transmission medium, the capabilities of the connected
h/w and the efficiency of the s/w.
Reliability is measured by frequency of failure, the time it takes a link to recover from
the failure and the network’s robustness in a catastrophe.
Security issues include protecting data from unauthorized access and viruses.
8. What is the difference between a passive and an active hub?
An active hub contains a repeater that regenerates the received bit patterns before sending
them out. A passive hub provides a simple physical connection between the attached devices.
9. Distinguish between peer-to-peer relationship and a primary-secondary relationship. Peer-to-peer relationship: All the devices share the link equally. Primary-secondary relationship: One device controls traffic and the others must transmit
through it.
10. Assume 6 devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device? Number of cables=n (n-1)/2=6(6-1)/2=15 Number of ports per device=n-1=6-1=5
11. Explain cross talk and what is needed to reduce it?
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Effect of one wire on another is called as cross talk. One wire will be the sending antenna and
the other wire will be the receiving antenna. We can use the shielded twisted pair cable or
coaxial cable for transmission, which contains metal foil to reduce cross talk.
11. Give the relationship between propagation speed and propagation time?
Propagation time = distance / propagation speed
The time required for a signal or a bit to travel from one point to another is called Propagation
time.
Propagation speed is the distance, a signal or a bit travel through a medium in one second.
12. What are the criteria used to evaluate transmission medium?
The criteria used to evaluate transmission medium are Throughput Propagation speed Propagation time Wavelength
13. What is refraction?
The phenomenon related to the bending of light when it passes from one medium to another.
14. Discuss the mode for propagating light along optical channels.
There are two modes for propagating light along optical channels, Multimode: Multiple beams from a light source move through the core in different paths.
Single mode: Fiber with extremely small diameter that limits beams to a few angles ,resulting in an almost horizontal beam.
15. Group the OSI layers by function.
The seven layers of the OSI model belonging to three subgroups. Physical, datalink and
network layers are the network support layers; they deal with the physical aspects of moving
data from one device to another. Session, presentation and application layers are the user
support layers; they allow interoperability among unrelated software systems. The transport
layer ensures end-to-end reliable data transmission.
16. What are the functions of a DTE? What are the functions of a DCE? Data terminal equipment is a device that is an information source or an information sink. It is
connected to a network through a DCE .Data circuit-terminating equipment is a device used as
an interface between a DTE and a network.
17. What does the electrical specification of EIA-232 describe? The electrical specification of EIA-232 defines that signals other than data must be sent using
OFF as less than -3 volts and ON as greater than +3 volts. The data must be transmitted using
NRZ-L encoding.
A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 4
PART-B
1. (i) Explain the TCP/IP reference model with a neat sketch(AUC MAY’13)
TCP/IP
Protocols
Protocols are the rules which govern accurate and dependable communication between two or
more parties. Network protocols ar; rules governing information exchange between two or more
computers on a Network. TCP/IP can handle communication on a very small scale to a very
large scale (Internet).
TCP/ICP
During 1960 the Advanced Research Projects Agency of Department of Defence (US)
sponsored the development of ARPAnet. During 1980 a new of protocols including TCP/IP was
developed. ARPA net was split into ARPA net for research and M1LNET for military
applications.
Types of services
Connection oriented services
Connectionless services
Connection oriented and connectionless services
In case of connection oriented service, there is no need to supply destination address. But in the
case of connectionless service, there is a need to supply the destination address.
Fig (a) shows a time line of the typical scenario that takes place for a connection oriented
transfer-first the server is started then sometimes later a client is started that connects to the
server.
Fig (b) above fig shows a time line of the typical scenario that takes place for a connectionless
protocol, the system calls are different. The client does not establish a connection with the
server Instead the client just sends a datagram to the server, using the send to system call
which requires the address of the destination as a parameter.
Similarly the server does not have to accept a connection from a client. Instead a server just
issues a recv from system call that waits until data arrive from some client. The recvfrom returns
the network address of client process.
TCP/IP suite
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The official name of TCP/IP protocol is TCP/IP internet protocol suite. TCP/IP stands for
Transmission control protocol/internet protocol. The TCP/IP protocol suite is a set of protocols
that allows communication across multiple diverse networks.
The TCP/IP protocols are defined in documents called Request for comments (RFC). RFCs are
maintained by the Network Information Center (NIC), the organization that handles the Address
registration for the/Internet RFCs define a number of Applications, the most widely used being
TELNET, FTP and so on.
TCP/IP is normally considered to be a 4 layer system. The layers are application layer, transport
layer, internet layer, and host to network layer.
Host to Network layer is also called physical and data link layer of OSI Reference model. This
layer cannot define any protocol. It is responsible for accepting and transmitting IP datagrams.
In the Application layer in TCP/IP can be equated in the combination of session, presentation
and application layer of OSI reference model.
TCP/IP suite Relation of protocol in the TCP/IP suite
ICMP – Internet Control Message Protocol: The protocol that handle error and control
information between gateways and hosts. While ICMP messages are transmitted using IP
datagrams, these messages are generated by TCP/IP networking s/w itself not user processes.
Specially routers and hosts use ICMP to send reports of problem
IP-Internet Protocol: IP is a protocol that provides the packet delivery service for TCP, UDP and
ICMP. TCP/IP standard protocol defines IP datagram as the unit of information passed across
an internet.
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ARP-Address Resolution Protocol: The protocol that maps an internet address into a hardware
address. It finds physical address using logical address. .
RARPReverse Address Resolution Protocol: The protocol that maps the hardware address into
an internet address.
TCP – Transmission Control Protocol: A connection oriented protocol that provides a reliable,
full duplex, byte stream for a user process. Most Internet application programs use TCP. Since
TCP uses IP, the entire protocol suite is called TCP/IP protocol “family.
UDP – User Datagram Protocol: A connectionless protocol for the user processes. Unlike TCP,
which is reliable protocol, there is no guaranteed that UDP datagrams ever reach their intended
destination.
FTP – File Transfer Protocol: TCP/IP standard, high level protocol for transferring files from one
machine to another. It uses two connections to transfer file. Control and data connection. SMTP
– Simple Mail Transfer Protocol: TCP/IP standard protocol for transferring electronic messages
from one machine to another.
SNMP – Simple Network Management Protocol: A protocol used to manage device such as
hosts, routers and printers.
TELNET- Remote Login: TCP/IP protocol for remote terminal services. TELNET allows an
interactive user on the client system to start a login session on a remote system.
TFTP – Trivial file Transfer Protocol
It is a simple file transfer protocol. It is not as complex as FTP. It does not have much code and
consumes less memory It’ can be used on small machines.
(ii) Compare the performance of TCP/IP and ISO/OSI reference model.
Transmission Control Protocol is used by Internet applications like email, world wide web, FTP,
etc. TCP/IP was developed by the Department of Defense (DOD) to connect various devices to
a common network (Internet).
The main purpose behind developing the protocol was to build a robust and automatically
recovering phone line failure while on the battlefield. On the other hand, Open Systems
Interconnection was developed by the International Organization for Standardization (ISO). This
model was made up of two components, namely, seven-layer model and the subset of
protocols.
A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 7
TCP/IP is a four-layered structure, with each layer having their individual protocol. Let us have a
look at the four layers:
Link Layer
The layer includes the physical and logical connections from the host's link. It is also known as
Network Access layer and Network Interface layer. It explains how the data is transmitted from
the host, through the network. The physical connectors like the coaxial cables, twisted pair
wires, the optical fiber, interface cards, etc., are a part of this layer. This layer can be used to
connect different network types like ATM, Token ring, Ethernet, LAN, etc.
Internet Layer
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This layer is also known as the Network Layer. The main function of this layer is to route the
data to its destination. The data that is received by the link layer is made into data packets (IP
datagrams). The data packets contain the source and the destination IP address or logical
address. These packets are sent on any network and are delivered independently. This
indicates that the data is not received in the same order as it was sent. The protocols at this
layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), etc.
Transport Layer
This layer is responsible for providing datagram services to the Application layer. This layer
allows the host and the destination devices to communicate with each other for exchanging
messages, irrespective of the underlying network type. Error control, congestion control, flow
control, etc., are handled by the transport layer. The protocol that this layer uses is TCP
(Transmission Control Protocol) and UDP (User Datagram Protocol). TCP gives a reliable, end-
to-end, connection-oriented data transfer, while UDP provides unreliable, connectionless data
transfer between two computers.
Application Layer
It provides the user interface for communication. This is the layer where email, web browsers or
FTP run. The protocols in this layer are FTP, SMTP, HTTP, etc.
OSI model
The Open Systems Interconnected (OSI) model divides the network into seven layers and
explains the routing of the data from source to destination. It is a theoretical model which
explains the working of the networks. It was developed by the International Organization for
Standardization (ISO) for their own network suite. Here are the details of OSI's seven layers:
Physical Layer
As the name suggests, this is the layer where the physical connection between two computers
takes place. The data is transmitted via this physical medium to the destination's physical layer.
The popular protocols at this layer are Fast Ethernet, ATM, RS232, etc.
Data Link Layer
The main function of this layer is to convert the data packets received from the upper layer into
frames, and route the same to the physical layer. Error detection and correction is done at this
layer, thus making it a reliable layer in the model. It establishes a logical link between the nodes
and transmit frames sequentially.
Network Layer
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The main function of this layer is to translate the network address into physical MAC address.
The data has to be routed to its intended destination on the network.
This layer is also responsible to determine the efficient route for transmitting the data to its
destination. While doing so, it has to manage problems like network congestion, switching
problems, etc. The protocols used here are IP, ICMP, IGMP, IPX, etc.
Transport Layer
This layer provides end-to-end delivery of data between two nodes. It divides data into different
packets before transmitting it.
On receipt of these packets, the data is reassembled and forwarded to the next layer. If the data
is lost in transmission or has errors, then this layer recovers the lost data and transmits the
same.
Session Layer
This layer is responsible to establish and terminate connections between two communicating
machines.
This connection is known as a session, hence the name. It establishes full-duplex, half-duplex
and simplex connection for communication. The sessions are also used to keep a track of the
connections to the web server.
Presentation Layer
The data conversion takes place at this layer. The data that it receives from the application layer
is converted into a suitable format that is recognized by the computer.
For example, the conversion of a file from .wav to .mp3 takes place at this layer.
Application Layer
This layer provides a user interface by interacting with the running application. E-mail, FTP, web
browsers, etc., are the network applications that run on this layer.
The entire communication industry stands on the backbone of TCP/IP and OSI reference model.
It is absolutely vital to learn the above differences, if anyone wants to be an expert in the field of
communication.
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2. Explain in detail about network dependent and network independent layers of OSI
reference model. (AUC MAY’13,NOV’12,nov’11)
ISO / OSI MODEL:
ISO refers International Standards Organization was established in 1947, it is a multinational
body dedicated to worldwide agreement on international standards.
OSI refers to Open System Interconnection that covers all aspects of network communication. It
is a standard of ISO.
Here open system is a model that allows any two different systems to communicate regardless
of their underlying architecture. Mainly, it is not a protocol it is just a model.
OSI MODEL
The open system interconnection model is a layered framework. It has seven separate but
interrelated layers. Each layer having unique responsibilities.
ARCHITECTURE
The architecture of OSI model is a layered architecture. The seven layers are,
1. Physical layer
2. Datalink layer
ORGANIZATION OF LAYERS
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The seven layers are arranged by three sub groups.
1. Network Support Layers
2. User Support Layers
3. Intermediate Layer
Network Support Layers:
Physical, Datalink and Network layers come under the group. They deal with
the physical aspects of the data such as electrical specifications, physical
connections, physical addressing, and transport timing and reliability.
User Support Layers:
Session, Presentation and Application layers comes under the
group. They deal with the interoperability between the software systems. Intermediate
Layer
The transport layer is the intermediate layer between the network support
and the user support layers.
FUNCTIONS OF THE LAYERS PHYSICAL LAYER
The physical layer coordinates the functions required to transmit a bit
stream over a physical medium. It deals with the mechanical and electrical
specifications of the interface and the transmission medium.
The functions are,
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1. Physical Characteristics Of Interfaces and Media:
It defines the electrical and mechanical characteristics of the
interface and the media.
It defines the types of transmission medium
2. Representation of Bits
To transmit the stream of bits they must be encoded into signal.
It defines the type of encoding weather electrical or optical.
3. Data Rate
It defines the transmission rate i.e. the number of bits sent per
second.
4. Synchronization of Bits
The sender and receiver must be synchronized at bit level.
5. Line Configuration
It defines the type of connection between the devices.
Two types of connection are,
1. point to point
2. multipoint
6. Physical Topology
It defines how devices are connected to make a network.
Five topologies are,
1. mesh
2. star
3. tree
4. bus
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5. ring
7. Transmission Mode
It defines the direction of transmission between devices.
Three types of transmission are,
1. simplex
2. half duplex
3. full duplex
DATALINK LAYER
Datalink layer responsible for node-to-node delivery.
The responsibilities of Datalink layer are,
1. Framing
It divides the stream of bits received from network layer into manageable data
units called frames.
2. Physical Addressing
It adds a header that defines the physical address of the sender
and the receiver.
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If the sender and the receiver are in different networks, then the
receiver address is the address of the device which connects the
two networks.
3. Flow Control
It imposes a flow control mechanism used to ensure the data
rate at the sender and the receiver should be same.
4. Error Control
To improve the reliability the Datalink layer adds a trailer
which contains the error control mechanism like CRC,
Checksum etc.
5. Access Control
When two or more devices connected at the same link, then the
Datalink layer used to determine which device has control
over the link at any given time.
NETWORK LAYER
When the sender is in one network and the receiver is in some
other network then the network layer has the responsibility for the source to
destination delivery.
The responsibilities are,
1. Logical Addressing
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If a packet passes the network boundary that is when the sender and
receiver are places in different network then the network layer adds a
header that defines the logical address of the devices.
2. Routing
When more than one networks connected and to form an
internetwork, the connecting devices route the packet to its final
destination.
Network layer provides this mechanism.
TRANSPORT LAYER
The network layer is responsible for the end to end delivery of the
entire message. It ensures that the whole message arrives in order and intact. It
ensures the error control and flow control at source to destination level.
The responsibilities are,
1. Service point Addressing
A single computer can often run several programs at the same time.
The transport layer gets the entire message to the correct process on
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that computer.
It adds a header that defines the port address which used to identify the
exact process on the receiver.
2. Segmentation and Reassembly
A message is divided into manageable units called as segments.
Each segment is reassembled after received that information at the
receiver end.
To make this efficient each segment contains a sequence number.
3. Connection Control
The transport layer creates a connection between the two end ports.
It involves three steps. They are,
1. connection establishment
2. data transmission
3. connection discard
4. Flow Control
Flow control is performed at end to end level
5. Error Control
Error control is performed at end to end level.
SESSION LAYER
It acts as a dialog controller. It establishes, maintains and synchronizes the
interaction between the communication devices.
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The responsibilities are,
1. Dialog Control
The session layer allows two systems to enter into a dialog.
It allows the communication between the devices.
2. Synchronization
It adds a synchronization points into a stream of bits.
PRESENTATION LAYER
The presentation layer is responsible for the semantics and the syntax of the
information exchanged.
The responsibilities are,
1. Translation
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Different systems use different encoding systems.
The presentation layer is responsible for interoperability between
different systems.
The presentation layer t the sender side translates the information from the
sender dependent format to a common format. Likewise, at the receiver
side presentation layer translate the information from common format to
receiver dependent format.
2. Encryption
To ensure security encryption/decryption is used
Encryption means transforms the original information to another form
Decryption means retrieve the original information from the
encrypted data
3. Compression
It used to reduce the number of bits to be transmitted.
APPLICATION LAYER
The application layer enables the user to access the network. It provides
interfaces between the users to the network.
The responsibilities are,
1. Network Virtual Terminal
It is a software version of a physical terminal and allows a user to log on to a
remote host.
2. File Transfer, Access, and Management
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It allows a user to access files in a remote computer, retrieve files, and
manage or control files in a remote computer.
3. Mail Services
It provides the basis for e-mail forwarding and storage.
4. Directory Services
It provides distributed database sources and access for global information about
various objects and services
3. Explain the different types of switching networks and mention it’s advantages and
disadvantages. (AUC NOV ’12)
SWITCHING
Whenever we have multiple devices, we have the problem of how to connect them
to make one to one communication possible. Point to point connection between every pair
of devices are make the network very huge also cost inefficient.
A switched network consists of a series of interlinked nodes, called switches.
Switches are hardware and / or software devices capable of creating temporary
connections between two or more devices linked to the switch but not to each other.
There are three methods in switching are,
1. circuit switching
2. packet switching
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3. message switching
CIRCUIT SWITCHING
Circuit switching creates a direct physical connection between two devices such as
phones or computers. A circuit switch is a device with n input and m output that creates a
temporary connection between an input link and an output link. The number of inputs does
not have to match the number of outputs. An n by n folded switch can connect n lines in
full duplex mode.
Circuit switching today can use either of two technologies:
1. space division switches
2. time division switches
SPACE DIVISION SWITCHES
In space division switches, the paths in the circuit are separated from each other
spatially. It is very useful in analog networks.
Crossbar switches:
A crossbar switch connects n inputs to m outputs in a grid, using electronic micro
switches at each cross point. The major limitation is number of cross points required. To
connect n inputs to m outputs it needs n * m cross points.
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Multistage switches:
It combines crossbar switches in several stages. The design of multistage switches
are depends on the number of stages and the number of switches required in each stage.
Normally the middle stage has fewer switches than the first and last stages. They
provide several options for connecting each pair of linked devices is known as
multipath.
TIME DIVISION SWITCHES
Time division switches uses time division multiplexing to achieve switching. There
are two methods are,
1. time slot interchange
2. TDM buds
Time slot interchange:
A time slot interchange is used to order the slots based on the desired connections. It
consists of random access memory with several memory locations. The RAM fills up the
incoming data from time slots in the order received. Slots are then sent out in an order based
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on the decisions of a control unit.
TDM bus:
The input and output lines are connected to a high speed bus through the input
and output gates. Each input gate is closed during one of the four time slots. During the
same time slot, only one output gate is also closed. This pair of gates allows a burst of
data to be transmitted from one specific input line to one specific output line using the bus.
The control unit opens and closes the gates according to switching need.
PUBLIC SWITCHED TELEPHONE NETWORK:
An example of a circuit switched network is the public switched telephone
network. The switching centers are organized as five classes are,
Regional office
Sectional office
Primary office
Toll office
End office
DISADVANTAGES:
1. Circuit switching is less suited to data and other non voice communications.
2. less data rate
3. inflexible
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wwww..rreaauull..
4. No priorities allowed.
PACKET SWITCHING
In packet switched network, data are transmitted in discrete units of potentially
variable length blocks called packets. The maximum length of the packet is established by
the network. Longer transmission is broken up in to multiple packets.
There are two popular methods are,
1. datagram approach
2. virtual circuit approach
DATAGRAM APPROACH:
In the datagram approach to packet switching, each packet is treated
independently from all there. Even when one packet represents just a piece of a
multipacket transmission, the networks treats it as though it existed alone. Packets in this
technology are referred to datagram.
The datagram approach can be used to deliver four packets from station A to
station X. In this example, all four packets belong to same message but may go by
different paths to reach their destination.
This approach can cause the datagram of a transmission to arrive at their
destination out of order .It is responsibility of transport layer in most protocols to reorder the
data grams before passing them on to the destination port.
The link joining each pair of nodes can contain multiple channels. Each of these
channels is capable, in turn, of carrying data grams either from several different sources or
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from one source. Multiplexing can be done using TDM or FDM.
Devices A and B are sending data grams to devices X and Y. Some paths use one
channel while other uses more than one. the bottom link is carrying two packets from
different sources in the same direction. The link on the right, however, is carrying
data grams in two directions.
VIRTUAL CIRCIUT APPROACH:
In the virtual circuit approach to packet switching, the relationship
between all packets belonging to a message or session is preserved. A single route is
chosen between sender and receiver at the beginning of the session. When the data are
sent, all packets of the transmission travel one after another along that route.
Today, virtual circuit transmission is implemented in two formats: switched