CHAPTER 1: INTRODUCTION TO COMPUTER NETWORKS
A computer network is a collection of computers and other devices (nodes) that use a
common network protocol to share resources with each other over a network medium.
“interconnected collection of autonomous computers connected by a single technology”
[Tanenbaum]
To share information or receive a service via a network, group members must be able to
communicate with each other.
The following is a figure which shows a communication model.
Communication Model
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Source
Generates data to be transmitted
• Transmitter
Converts data into transmittable signals
• Transmission System
Carries data
• Receiver
Converts received signal into data
• Destination
Takes incoming data
The figure also shows an example of a public telephone network.
The networks can be classified as shown below
• Wired, Wireless and Fiber Optic Networks
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LANs, MANs and WANs
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Circuit Switched, Packet Switched and Virtual Circuit Switched Networks
• Access, Edge and Core Networks
The computer network can be classified based on architecture and access as shown below
• Architecture
Common LAN architectures: Ethernet IEEE 802.3, Token Ring, and FDDI.
• Access Possibilities
shared-media networks
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switching networks
• Transmission Technology
Broadcast links
Point-to-point links
The architecture based classification will be dealt later. The shared media networks: The
stations connected to the same media and can share all the resources like printers and
scanners and also software resources and share the same communication channel. Where
as incase of switching networks a switching element is used and will route the
information to the relevant output. The information comes from many sources and
forwarded only to the correct output.
Broad cast links have a single communication channel shared by all the machines on the
network. A short message called a packet is sent by any machine and received by all the
others in the network. The address of the receiver is present in the message all the
machine simply ignores. Actually there is a special address called broadcast address
where all the machines receive the packet. This type of transmission is called the
Broadcasting. Some broadcast systems allow the message to be sent to only a subset of
the machine or a group by using a bit in the address field to indicate that the message is
intended for the group. This method is called the multicasting. In contrast the point to
point link, the source and the destination have several links. The message may have to
visit an intermediate station before reaching the destination. The point to point link
between one sender and the receiver is also called as unicasting.
Wired network: All the machines are connected using a wire, that could be a copper wire
or fibre optic. They are many different topologies used to connect the machines. The
figure below shows how the machines are connected using the wire this is an example of
a bus topology.
All the machines are connected using a wire and can share all the resources.
Wireless network:
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To the wired network
router
base
statio
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mobile
hosts
The above figure shows a wireless network. It consists of mobile machines such as
laptops and there is a base station it is called as access point. The machines can access
other network using the access point. The access point is wired to the router which is a
switching element and is inturn connected to the wired network. IEEE 802.11 describes
the wireless technology.
Fiber optic network:
The machines can be connected using the fiber optic cable. This is mainly used in
connecting the systems in the backbone. Different servers and ISP provider equipment
are the examples of the systems in the backbone. The fiber optic cable uses light as the
signal to transmit information in the cable. It offers good bandwidth and less interference
but it is expensive to use this cable.
LANs
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company/univ local area network (LAN) connects end system to edge router
Ethernet:
shared or dedicated link connects end system and router10
Mbs,
100Mbps,Gigabit Ethernet
deployment: institutions, home LANs happening now
Occupies a small geographical area. Use only one type media and different
topologies. Printers scanners and machines can be connected.
LANs give lot of flexibility, speed ,reliability, adaptability, security private
ownership.
Connection to other LANs and WANs
MANs
It is larger than the LAN and occupies a city or a group of nearby corporate offices. It
uses the same technology as LAN. The example is the cable TV network. It uses the
coaxial cable. The service provider connects the home TVs this forms a large network.
The service is provided by the cable TV operator. Fiber optic cable is also used. It can
support both voice and data transmission.
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WANs
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Spans a large geographic area, e.g., a country or a continent
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A WAN consists of several transmission lines and routers Internet is an example
of a WAN
All the machines are connected using the subnets.
Compared to LAN the speed is very less
Used to connect different LANs
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Circuit switched network
The sender and the receiver has a dedicated link between them. For example consider the
telephone network when a sender places a call a dedicated link is established between the
sender and receiver as long as the call exists. Then the link is terminated when the call
ends.
Packet switched network
No dedicated link present between the sender and receiver. When a data frame or packet
is sent it is sent to the subnet and to the intermediate system and reaches the destination.
The same message is broken into small packets and sent on the subnet all packets need
not take the same route. The switching elements decide the route.
Virtual circuit switched network
It is like circuit switched and a dedicated link present and a identifier is assigned to the
link and same channel used for different communication.
Internetwork
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internetwork – interconnection of networks – also called an “internet”
Subnetwork – a constituent of an internet
Intermediate system – a device used to connect two networks allowing hosts of
the networks to correspond with each other
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Bridge
Routers
Internet is an example of an internetwork.
network of networks”
“collection of networks interconnected by routers”
“a communication medium used by millions”Email, chat, Web “surfing”,
streaming media
millions of connected computing devices: hosts, end-systems
– PCs workstations, servers
– PDAs phones, toasters running network apps
communication links
– fiber, copper, radio, satellite
– Links have different bandwidth
routers: forward packets
Packet: a piece of message
Uses of computer network
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Business applications
Resource sharing: end systems (hosts):
• run application programs
• e.g. Web, email
• at “edge of network”
client/server model
• client host requests, receives service from always-on server
• e.g. Web browser/server; email client/server
Client/server model is applicable in an intranet.
E-mail: Now all the companies uses email as the means of communication
E-commerce: Now teleshopping and marketing is very popular and finding the
application in business
Mobile users are connected using network such as laptops palmtops etc
Like wise even home users have increased now and becoming popular.
Notes
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A Protocol can be defined as a set of rules governing the exchange of data
between two entities.
Used for communications between entities in a system
Two entities have to speak the same language to successfully communicate
Networks are complex and consist of many “pieces”:
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hosts
routers
switches
links of various media
applications
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• protocols
• reliability
• connection type
How to simplify the complex structures. A layered structured can be used to reduce the
complexity. Most of the network are organized as a stack of layers or levels each one
built over the other. The number of layers and the name of the layers and the function of
each layer differ from network to network. The purpose of each layer is to offer service to
layer above it. Layer n on one machine carries conversation with layer n on another
machine. The rules and conventions used collectively known as the layer n protocol.
For example consider a five layered network.
The entities comprising the corresponding layers on different machines are called peers.
The peers may be processes or hardware devices or human beings. Peers communicate
using protocol. No data is sent from layer n to layer n instead they send to the layer below
until the last layer is reached. Between the layers it is the virtual communication.
Between each pair of layers is the interface. It defines the primitive operation and
services what the lower layer makes available to the upper one. Network designers decide
about the function and the number of layers. It is very important to define a clear
interfaces. A set of protocol and layers is called the network architecture. A list of
protocol used by a certain system one protocol per layer is called a protocol stack.
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Consider the above figure
This shows how communication happens between two systems. A message M is
produced by the layer 5. It is given to layer 4 and it puts the header in front of the
message and passes to layer 3. The header includes the control information such as
sequence numbers to allow the layer 4 on the destination machine to deliver messages in
the right order. The layer 3 breaks up the message into smaller units called packets
adding layer 3 header to each packet. In this example M is split into 2 packets M1 and
M2. Layer 3 decides which of the outgoing line to use and sends on that line to layer 2.
Layer 2 adds a header and also a trailer and give the resulting unit to layer 1 for physical
transmission. At the receiving machine the message move upwards from layer to layer,
with header being stripped off as it progresses.
Design issues for layers
• Addressing
• Error Control
• Flow Control
• Multiplexing
• Routing
Addressing Level
Level in architecture at which entity is named
• Unique address for each end system (computer) and each intermediate system
(router)
• Network level address
IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
• Process within the system
Port number (TCP/IP)
Service access point or SAP
Addressing Scope
• Global nonambiguity
Global address identifies unique system
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There is only one system with address X
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Global applicability
It is possible at any system (any address) to identify any
other system (address) by the global address of the other
system Address X identifies that system from anywhere on
the network
• e.g. MAC address on IEEE 802 networks
Connection Identifiers
• Connection oriented data transfer (virtual circuits)
• Allocates a connection name during the transfer phase
• the advantages are:
• Reduced overhead as connection identifiers are shorter than global
addresses
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Routing may be fixed and identified by connection name
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Entities may want multiple connections – multiplexing
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State information
Error Control
Guard against loss or damage of data and control information
Error control is implemented as two separate functions:
Error detection
Sender inserts error detecting bits
Receiver checks these bits
If OK, acknowledge
If error, discard packet
Retransmission
If no acknowledge in given time, re-transmit
Performed at various layers of protocol
Flow Control
Done by receiving entity
Function to limit amount or rate of data sent by a transmitting entity
Simplest form: stop-and-wait procedure
More efficient protocols: Credit systems Sliding window
Needed at application as well as network layers
Multiplexing
-Supporting multiple connections on one machine
-Mapping of multiple connections at one level to a single connection at another
-Carrying a number of connections on one fiber optic cable
-Aggregating or bonding ISDN lines to gain bandwidth
Routing
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Determine path or route that packets will follow
Use routing protocol based on a routing algorithm
“Good” path should be least cost path
Cost : depends on the following factors.
Average queuing delay
Propagation delay
Bandwidth, mean queue length, etc.
End systems and routers maintain routing tables
Dynamic or static
OSI Model
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Not a network architecture, because it does not specify the exact services and
protocols to be used in each layer, it just formally defines and codifies the concept
of layered network architecture
• Each layer describe what happens at each stage in the processing of data for
transmission
• Layers help to reduce complexity
• Each layer relies on the next lower layer to perform more primitive functions
• Each layer provides services to the next higher layer
• Changes in one layer should not require changes in other layers
The functions of different layers
Physical
• responsible for transmitting raw bits over a communication path
• concerned with issues such as
-mechanical interfaces, e.g. design of a network connector
-electrical interfaces, e.g. voltage level of bits
-procedural interfaces, e.g. whether transmission may
proceed simultaneously in both directions
Data Link
• Responsible for the transfer of data between the ends of a physical link
• Provides for error detection, "framing", and flow control
• Resolves problems due to damaged, lost, or duplicate frames
• Formatted messages are referred to as frames rather than packets
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Network
• Responsible for the source to destination routing
• Addresses and resolves all inherent problems related to the transmission of data
between heterogeneous networks
• Formatted messages are referred to as packets
• In broadcast networks the network layer is often thin or nonexistent, because of
easy to solve routing problems
• Sometimes no need for a network layer if using point-to-point link
Transport
• Provides for error-free delivery of data
• Accepts data from the session layer and splits data into smaller packets if
necessary
• passes these packets to the network layer, and ensures that packets arrive in
sequence, with no losses or duplications, at their destination
Session
• Provides for coordination between communicating processes between nodes.
• Manages dialog control (e.g. Can allow traffic to go in both direction at the same
time, or in only one direction at time.)
• Responsible for synchronizing the flow of data, and reestablishing a connection
in the event a failure occurs.
Presentation
• Provides for data formats, and code conversions
• Concerned with syntax and semantics of data being transmitted
• Encodes messages in a form that is suitable for electronic transmission
• Data compression and encryption is done at this layer
Application
• Consists of protocols that define specific user-oriented applications such as email, file transfer, and virtual terminal
Note:
Differences between a computer network (CN) and a distributed system(DS)
• CN collection of computers connected by single technology
• DS collection independent computers appears as one coherent system
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• Middleware responsible for the DS
• WWW is the example of DS
• DS software system built on top of network
The two services a network offers
Connection oriented
• A connection is established between ES’s (end System) that is used for duration
of call
Call setup
Data transfer
Call termination
E.g: Virtual circuits at this layer
• IS’s ( intermediate system) connect two or more networks
IS appear as ES to each network
Logical connection set up between ESs
-Concatenation of logical connections across networks Individual network virtual circuits
joined by IS
• Advantages
– Fixed path
– Order of message preserved
– No loss of data
• Reliable
– But the process of acknowledgement adds overhead and delay
– Example: telephone, ftp
Connectionless
• Each packet sent independently
• Routing decisions made at every IS
• Corresponds to datagram service in packet switched network
• Network layer protocol common to all ES’s and routers
– Known generically as the internet protocol
• Internet Protocol
– One such internet protocol developed for ARPANET
– Example: Telegraph systems, email, remote login
– Advantages
– Flexibility
– Robust
– No unnecessary overhead
– Unreliable
– Not guaranteed delivery
– Not guaranteed order of delivery
– Packets can take different routes
– Reliability is responsibility of next layer up (e.g. TCP)
The following table shows an example of 6 different services
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Service primitives
A service is specified by a set of primitives available to a user process to access the
service. These primitives tell the service to perform some action or report on an action
taken by a peer entity. The set of primitives available depends on the nature of the service
being provided. The primitives for connection oriented are different from the
connectionless service.
The five different service primitives for implementing a simple connection oriented
service
Listen: The server executes LISTEN to indicate that it is prepared to accept the incoming
connection. The server process is blocked until a request for connection appears
Connect: the client process executes a CONNECT call to establish the connection with
the server. Specify the address too.
When the server receives this packet it unblocks the server and sends back the
acknowledgement and this releases the client. At this point the client and server both are
running. The connection established.
Receive: the server executes RECEIVE to prepare the first request. This call blocks the
server.
Send: the client executes SEND to transmit its request followed by the execution of
receive to get the reply. If the client has additional requests it makes now
Disconnect: The client use DISCONNECT to end the connection. The server also issues
a acknowledgement to terminate the connection it send the disconnect.
The following figure shows the relationship between the service and the protocol
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A service is the set of primitives or operations where as protocol are the rules.
Example networks
Internet
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internetwork – interconnection of networks – also called an “internet”
Subnetwork – a constituent of an internet
Intermediate system – a device used to connect two networks allowing hosts of
the networks to correspond with each other
Bridge
Routers
Internet is an example of an internetwork.
internet : collection of networks interconnected by router and/or bridges
The Internet
• The global collection of thousands of individual machines and networks
Intranet
• Corporate internet operating within the organization
• Uses Internet (TCP/IP and http) technology to deliver documents and
resources
End System (ES)
• Device attached to one of the networks of an internet
• Supports end-user applications or services
• ES sometimes called DTE
Intermediate System (IS)
• Device used to connect two networks
• Permits communication between end systems attached to different
networks
• Examples: Routers and Bridges
Bridge
– IS used to connect two LANs using similar LAN protocols
– Address filter passing on packets to the required network only
– OSI layer 2 (Data Link)
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Router
– Connects two (possibly dissimilar) networks
– Uses internet protocol present in each router and end system
– OSI Layer 3 (Network)
X.25
– First public data network
– Connection number used for data transfer of packets
– data packets contain 3 byte header and upto 128 bytes of data
– X.25 replaced by Frame Relay
Frame Relay
 Frame Relay is a way of sending information over a WAN by dividing data into
packets
 It operates at the Physical and Data Link layers of the OSI reference model
 It relies on upper-layer protocols such as TCP for error correction
 Frame Relay is a switched data link-layer protocol that handles multiple virtual
circuits using (HDLC) encapsulation
 Frame Relay interface can be either a carrier-provided public network or a
network of privately owned equipment, serving a single enterprise
Frame Relay –benefits
 Reduced internetworking costs
 Statistically multiplexed traffic from multiple sources over private backbone
networks can reduce the number of circuits and corresponding cost of bandwidth
 Lower Equipment Costs
 Lower cost than dedicated leased lines
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Increased performance & reduced network complexity
Reduces the amount of processing (as compared to X.25)
Efficiently utilizing high speed digital transmission lines, frame relay can improve
performance and response times of applications.
 Increased interoperability via international standards
 Frame relay can be implemented over existing technology
 Access devices often require only software changes or simple hardware
modifications to support the interface standard
 Existing packet switching equipment and T1/E1 multiplexers often can be
upgraded to support frame relay over existing backbone networks.
Frame Relay overwiew
 Packet Switched
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Uses Virtual Circuits (Connection Oriented Service)
Logical connection created between two (DTE) devices
across a Frame Relay packet-switched network (PSN)
Ethernet
• “dominant” LAN technology:
• cheap $20 for 100Mbs!
• first wildey used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race: 10, 100, 1000 Mbps
Wireless LAN
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Advantages
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wireless LANs: untethered (often mobile) networking
IEEE 802.11 standard:
MAC protocol
unlicensed frequency spectrum: 900Mhz, 2.4Ghz
Basic Service Set (BSS)
contains:
wireless hosts
access point (AP): base station
BSS’s combined to form distribution system (DS)
Mobility
Flexibility
Hard to wire areas
Reduced cost of wireless systems
Improved performance of wireless systems
Adhoc networks
• Ad hoc network: IEEE 802.11 stations can dynamically form network without AP
• Applications:
– “laptop” meeting in conference room, car
– interconnection of “personal” devices
– battlefield
• IETF MANET (Mobile Ad hoc Networks) working group
LAN generations
• First
– Typified by CSMA/CD and token ring
– Provided terminal to host and client server
– Moderate data rates
• Second
– Typified by FDDI
– Needed for backbone LANs
– Support of high performance workstations
• Third
– Typified by ATM
– Provide the aggregate throughput and real time support for multimedia
applications
ATM
• ATM is a high-speed switching network architecture
• ATM can be used to carry data, voice, and video
– separately or simultaneously over same network path
• ATM has a robust quality of service (QoS)
– can provide seamless interconnectivity between LAN’s and WAN’s
– supports a wide range of data rates:
• 25 to 155 Mbps over copper
• 100 to 622 Mbps and higher over fiber
• common implementation is 155-Mbps ATM
• ATM is specified via a three-layer reference model:
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Physical layer (OSIs physical layer)
ATM layer (generally OSIs data link layer)
ATM adaptation layer (AAL) (generally OSIs higher-level layers (transport,
session, and application)
Physical layer (2 sublayers)
– Physical medium PM (lower sublayer)
• definition for the medium
• the bit-timing capabilities.
– Transmission convergence (TC) (upper sublayer)
• makes sure that valid cells are being created and transmitted
• involves breaking off individual cells from the data stream of the
higher layer (the ATM layer)
• checking the cell’s header
• Encoding the bit values
ATM layer
– service-independent layer
– creates cell headers and trailers
– defines virtual channels and paths and gives them unique identifiers
– cells are multiplexed or demultiplexed.
– ATM layer creates the cells and uses the physical layer to transmit them.
ATM adaptation layer (AAL) (2 sublayers)
– Segmentation and reassembly SAR (lower sublayer)
• packages variable size packets into fixed-size cells at the
transmitting end
• repackages the cells at the receiving end
• responsible for finding and dealing with cells that are out of order
or lost
– convergence sublayer CS (upper sublayer)
• provides the interface for the various services (e.g. data, voice, and
video).
• users connect to CS through service access points
• (SAPs).
ATM cells are always 53 bytes long
partitioned into
– 5 byte header  contains addressing information
– 48 byte payload  contains user data
ATM virtual connections consist of either
– permanent or switched virtual circuits
– that logically connect source and destination sites
– Virtual circuits are identified by specific virtual channel identifiers (VCIs).
A collection of virtual channels that all have the same endpoints is called a virtual
path connection (VPC)
VPCs are specified by virtual path identifiers (VPIs)
Virtual connections established
– VCI and VPI assignments are made dynamically by ATM end nodes and
switches at the time data are to be transmitted
– VCI is not of interest to e.g. public switches they would only use the VPI
ATM LAN
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Local area network emulation (LANE) interface
– can provide a service interface for the network layer that functions exactly
as the same as Ethernet/802.3 and token ring
– LANs with this interface – Emulated LANs (ELAN)
• involve special client/server processes that enables MAC-to-ATM
address resolution
• support connectionless nature of local area networks
Questions:
1. Compare computer networks and distributed systems. What are the
applications of computer networks?
2. A system has a n layer hierarchy. Applications generate messages of M
bytes.At each of the layer a n byte header is added. What fraction of the
network bandwidth is filled with the headers?
3. Bring out the design issues of computer networks. Differentiate between
services and protocols.
4. Explain the following with respect to network software; protocol hierarchy,
protocol layers
5. Compare the connection oriented and connectionless services
6. Differentiate between broadcasting and multicasting..
7. Why does ATM uses cells?
8. Explain client server model with an example.
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