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Computer Networks

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Communication Network And Services

Q
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: A network consists of two or more computers that are linked in order to share resources such as printers and

CD-ROMs, exchange files, or allow Electronic communications
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Computer network can be classified on the basis of following
features :

By Scale : Computer networks may be classified according to the scale :
Local Area Network (LAN)

Metropolitan Area Network (MAN)
Wide Area Network (WAN)

By Connect ion Method : Computer networks can also be classified according to the hardware technology that is used to
connect the individual devices in the network such as Optical fibre, Ethernet, Wireless LAN
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This classification also called computer architecture
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Computer networks may be classified according to the network topology upon which the
network is based, such as :
Bus Network

Star Network

Ring Network

Mesh Network

Star-Bus Network

Tree or Hierarchical Topology Network

Q
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: To share data and network resources among the computers in a network is known as networking
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Just imagine that if you work in a bank or in a corporate office and all the computers in your of fice are
without networking
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Q
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: Computer network are of following type :

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Local Area Network (LAN)

Wide Area Network (WAN)

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Metropolitan Area Network (MAN)

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Local Area Net work : A local-area network is a computer network covering a small geographic area, like a home, office, or
groups of buildings e
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a school
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g
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The defining characteristics of LANs, in contrast to wide-area
networks (WAN s), includes their much higher data-transfer rates, smaller geographic range, and lack of need for leased

telecommunication lines
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Current Ethernet or other

IEEE 802
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IEEE has projects investigating the standardization of 100
Gbit/s, and possibly 40 Gbit/s
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LANs may have connections with other LANs
via leased lines, leased services
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e
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Any network whose communications links cross metropolitan, regional, or national boundaries
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Contrast with local area networks (LANs) or

metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area

respectively
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WANs are used to connect LANs and other

types of networks together, so that users and computers in one location can communicate with users and computers in other
locations
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Others, built by Internet service providers,

provide connections from an organization's LAN to the Internet
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At each end of the
leased line, a router connects to the LAN on one side and a hub within the WAN on the other
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Network protocols including TCP/IP deliver transport and addressing functions
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Transmission rate usually range from 1200 bits/second to 6 Mbit/s, although some connections such as

ATM and Leased lines can reach speeds greater than 156 Mbit/s
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Metropolitan an Area Network : Metropolitan area network s, or MAN s, are large computer networks usually spanning a city
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A Metropolitan Area Network is a

network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the
boundaries of the immediate town, city, or metropolitan area
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According to IEEE, A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to
entire cities
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A MAN might be owned

and operated by a single organization, but it usually will be used by many individuals and organizations
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They will often provide means for internet working of local networks
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Q
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:Network topology is the study of the arrangement or mapping of the devices of a network, especially the physical and
logical interconnections between nodes
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Physical Topology
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Types of the Topologies :
Bus

Star

Ring

Mesh

partially connected mesh (or simply 'mesh') or fully connected mesh or Tree
Hybrid

Bus : The type of network topology in which all of the nodes of the network are connected to a common transmission medium

which has exactly two endpoints ;this is the 'bus', which is also commonly referred to as the backbone, or trunk, all data that is
transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received
by all nodes in the network virtually simultaneously
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Requires less cable length than a star topology
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Terminators are required at both ends of the backbone cable
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Not meant to be used as a stand- alone solution in a large building
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All data that is transmitted between nodes in the network is transmitted to this central Star Topology node,
which is usually some type of device that then retransmits the data to some or all of the other nodes in the network, although
the central node may also be a simple common connection point without any active device to repeat the signals
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No disruptions to the network then connecting or removing devices
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Disadvantages :

Requires more cable length than a linear topology
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More expensive than linear bus topologies because of the cost of the concentrators
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to enable the flow of data to continue using a segment of the second ring to bypass the fault in the primary ring
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Mesh : The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that
communicating groups of any two endpoints, up to and including all the endpoints, is approximated by Reed's Law
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Disadvantages:

Large number of I/O ports are required
Cabling required is high

Difficult to reconfiguration and installation
Tree or Hierarchical : The type of network topology in which a central 'root' node, the top level of the hierarchy, is connected
to one or more other nodes that are one level lower in the hierarchy i
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, the second level, with a point-to-point link between

each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected
to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy, i
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, the
third level, connected to it, also with a point-to- point link, the top level central 'root' node being the only node that has no

other node above it in the hierarchy
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Advantages :

Point-to- point wiring for individual segments
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Disadvantages :

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Overall length of each segment is limited by the type of cabling used
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More difficult to configure and wire than other topologies

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Q
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: Apart from determining valid paths between sources and destinations within an interconnection network, a switching
technique is needed that specifies how messages are to be fragmented before passing them to the network and how the

resources along the path are t o be allocated
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Following are the different switching techniques :

Circuit Switching : In circuit switching when a connection is established, the origin-node identifies the first intermediate node
(node A) in the path to the end-node and sends it a communication request signal
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Afterwards, the end-node sends a
communication acknowledge signal to the origin- node through all the intermediate nodes that have been used in the

communication request
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To release the communication the origin-node sends a communication end signal
to the end-node
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After receiving and storing this message, the first intermediate node (node A)
identifies the second one (node B) and, when the transmission line is not busy, the f ormer sends the whole message (store-

and-forward philosophy)
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As can be seen in figure no communication release or
establishment is needed
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This process is
repeated as many times as needed to reach
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The

virtual circuit established on this way will be kept for the whole communication
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Then, the first intermediate node (node A) begins to
send packets to the following node in the virtual circuit without waiting to store all message packets received from the

origin-node
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In the communication release, when

the origin- node sends to the end-node a communication end packet, the latter answers with an acknowledge packet
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The virtual circuit information is kept for future connections
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Layered Architecture

Open System Interconnection , an ISO standard for worldwide communications that defines a networking framework f or

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implementing protocols in seven layers
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OSI reference model is a logical framework for standards for the network

communication
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Today
many network communication protocols are based on the standards of OSI model
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The seven layers can be grouped into three groups 1) Network

2) Transport

3) Application
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Transport : This layer is in charge of getting data received from the network and transforming them in a format nearer to the
data format understandable by the program
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When your computer is receiving data,
this layer gets the received packets and put them back together
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The application layer makes the interf ace between the program that is sending or is receiving data and the
protocol stack
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This layer provides network
services to the end-users like Mail, ftp, telnet, DNS
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Remote file access
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Inter- process communication
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Directory services
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Network virtual terminals
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Layer 6 -> Presentation Layer : Presentation layer is also called translation layer
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It can be viewed as the translator for the network
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Presentation layer provides :
Character code translation: for example, ASCII to EBCDIC
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Data compression: reduces the number of bits that need to be transmitted on the network
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For example, password encryption
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Session layer establish

and manages the session between the two users at different ends in a network
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The examples of session layers and the interactive logins and file

transfer sessions
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It also reports and logs and upper layer errors
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It provides:
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Session establishment, maintenance and termination: allows two application processes on different machines to establish,

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use and terminate a connection, called a session
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Protocols : The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC
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Transport
layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data
was transferred
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It relieves the higher layer protocols from any concern with the transfer of data between them and their peers
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For a

reliable network layer with virtual circuit capability, a minimal transport layer is required
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The transport
layer provides :

Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not
already small enough), and passes the smaller units down to the network layer
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Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments
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Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which
messages belong to which sessions
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Layer 3 - Network : This layer is in charge of packet addressing, converting logical addresses into physical addresses,

making it possible to data packets to arrive at their destination
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The packets
will use to arrive at their destination, based on factors like traffic and priorities
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It also translates the logical address into the physical address
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g
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It is also responsible for defining the route, managing the network problems and addressing The network
layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions,
priority of service, and other factors
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Subnet Traffic Control : Routers (network layer intermediate systems) can instruct a sending station to "throttle back" its
frame transmission when the router's buffer fills up
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Logical-Physical Address Mapping : translates logical addresses, or names, into physical addresses
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In the network layer and the layers below, peer protocols exist between a node and its
immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station
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Protocols : These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF
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Data Link layer defines the
format of data on the network
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The data link layer handles the
physical and logical connections to the packet's destination, using a network interface
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The frame created by this layer is sent to the physical layer, where the frame will be converted into an electrical signal to do
this, the data link layer provides :

Link Establishment and Termination : Establishes and terminates the logical link between two nodes
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Frame Sequencing : Transmits/receives frames sequentially
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Detects and recovers from errors that occur in the
physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt
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Frame Error Checking : Checks received frames for integrity
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Layer 1 - Physical : The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception
of the unstructured raw bit stream over a physical medium
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Physical layer defines and cables, network cards and physical aspects
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This layer gets the frames sent by the Data Link layer and converts them into signals
compatible with the transmission media
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When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer,
which will put the frame back together and check for its integrity
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It determines:

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What signal state represents a binary 1
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How the receiving station delimits a frame
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Physical Medium Transmission : transmits bits as electrical or optical signals appropriate for the physical medium, and
determines:

What physical medium options can be used
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Protocols used at
physical layer are ISDN, IEEE 802 and IEEE 802
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Q
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: In LAN data link layer is divided in the 2 layers :
Logical Lick Control Sub-layer
Medium Access Layer

Logical Link Control Sub-layer : The uppermost sub-layer is Logical Link Control (LLC)
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The LLC
provides addressing and control of the data link
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Media Access Control Sub-layer: The sub-layer below it is Media Access Control (MAC)
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Other times it refers to a frame structure with

MAC addresses inside
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Both of these may
be compared to communication between people: The Media Access Control sub-layer also determines where one frame of

data ends and the next one starts
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TCP/IP

The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite is the engine for the Internet and networks

worldwide
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TCP/IP is
a set of protocols developed to allow cooperating computers to share resources across the network
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Certainly the ARPAnet is the best- known TCP/IP network
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TCP and IP are two of the protocols in this suite
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Term "Internet" applies to this entire set of networks
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This layered representation leads to the term protocol stack, which refers to the stack of layers
in the protocol suite
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By dividing the communication software into layers, the protocol stack allows for division
of labor, ease of implementation and code testing, and the ability to develop alternative layer implementations
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In this regard, a layer provides a service f or the layer

directly above it and makes use of services provided by the layer directly below it
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TCP/IP is
a family of protocols
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These include IP, TCP, and UDP
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g
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Initially TCP/IP was used mostly between minicomputers or mainframes
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Application Layer : The application layer is provided by the program that uses TCP/IP for communication
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Examples of applications include Telnet and the File Transfer Protocol (FTP)
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Multiple applications can be supported simultaneously
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Another transport layer protocol is the User Datagram Protocol It provides

connectionless, unreliable, best-eff ort service
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Usually, UDP is used by applications that need
a fast transport mechanism and can tolerate the loss of some data
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Internet Protocol (IP)

is the most important protocol in this layer
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IP
does not provide reliability, flow control, or error recovery
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IP provides a

routing function that attempts to deliver transmitted messages to their destination
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This is the basic unit of information transmitted across TCP/IP networks
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Network Interface Layer (link layer): The network interface layer, also called the link layer or the data-link layer , is the
interface to the actual network hardware
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In fact, TCP/IP does not specify any protocol here, but can use almost any network interface available, which
illustrates the flexibility of the IP layer
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2, X
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TCP/IP specifications do
not describe or standardize any network-layer protocols, they only standardize ways of accessing those protocols from the
internet work layer
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Write short note on A) TCP
B) IP

C) FTP

D) TELNET
Ans
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Data can be lost in the intermediate
network
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The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite
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It is so important
in the Internet protocol suite that sometimes the entire suite is referred to as "TCP/IP
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It is also responsible for
controlling the size and rate at which messages are exchanged between the server and the client
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At the same time that the IP takes care of handling the actual delivery of the data, the TCP takes
care of keeping track of the individual units of data packets that a message is divided into for efficient routing through the net
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Even though every packet has the same destination IP address, they can get routed differently through the network
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TCP is used extensively by many of the Internet's

most popular application protocols and resulting applications, including the World Wide Web, E-mail, File Transfer Protocol,
Secure Shell, and some streaming media applications
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IP is documented in RFC 791 and is the primary
network-layer protocol in the Internet protocol suite
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IP has two primary responsibilities: providing connectionless, best- effort delivery of datagram
through an internetwork; and providing fragmentation and reassembly of datagram to support data links with different

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maximum-transmission unit (MTU) sizes
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IP Header Length ( IHL)Indicates the datagram header length in 32-bit
words
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Total Length Specifies the length, in bytes, of the entire IP packet, including the data and header
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This field is used to help piece together datagram
fragments
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The low-order bit
specifies whether the packet can be fragmented
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The third or high-order bit is not used
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Time-to-Live Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded
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Protocol Indicates which upper-layer protocol receives incoming packets after IP processing is complete
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Source Address Specifies the sending node
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Options Allows IP to support various options, such as security
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IP Addressing : As with any other network-layer protocol, the IP addressing scheme is integral to the process of routing IP

datagram through an internetwork
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These IP addresses
can be subdivided and used to create addresses for sub-networks
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An Internet Service Provider (ISP) can obtain blocks of network addresses from the

InterNIC and can itself assign address space as necessary
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IP Address Format : The 32-bit IP address is grouped eight bits at a time, separated by

dots, and represented in decimal format (known as dotted decimal notation )
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The minimum value for an octet is 0, and the maximum value for an octet is 255
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C) File Transfer Protocol ( FTP) : Transferring data from one host to another is one of the most frequently used operations
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Additionally, FTP provides security and authentication measures to prevent unauthorized access to
data
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Security is

handled by requiring the user to specify a user name and password for the other computer
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This is not quite the same thing as

more recent "network file system" or "netbios" protocols
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You use it to copy the file to your own system
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FTP uses TCP as
transport protocol to provide reliable end-to-end connections and implements two types of connections in managing data
transfers
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It is on this port that an FTP

server listens for and accepts new connections
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This is also the connection across which the FTP
server will send messages to the client in response to these control commands
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It is across this connection that FTP transfers the data
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Therefore, it is possible for an entire FTP session to open and close without a data connection ever having been
opened
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FTP can transfer data only from the client to the server, or from
the server to the client, but not both
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Data connections initiated by the server are active, while those initiated by the client are passive
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The server
FTP application typically only consists of a PI and DTP
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It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet
standards
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We can

start a remote session by specifying a computer to connect to
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The Telnet program runs on the computer and connects your PC to a server on the network
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This enables we to control the server and communicate with other servers on the network
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Telnet is a common way to remotely control
Web servers
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TELNET clients have been available
on most Unix systems for many years and are available virtually for all platforms
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TELNET is a client-server protocol, based on a reliable connection- oriented transport
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TELNET is generally used with the following applications :

(1) Enterprise networks to access host applications, e
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on IBM Mainframes
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g
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(3) MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community
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Comparision between OSI Model and TCP/IP Model

Â
Peer-To-Peer Connection

The two major types of network architecture systems are :
1) Peer-to-Peer

2) Client-Server
1) Peer-to-Peer : Peer-to-peer network operating systems allow users to share resources and files located on their
computers and to access shared resources found on other computers
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In a peer-to-peer network, all computers are considered equal; they all have the same
abilities to use the resources available on the network
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AppleShare and Windows for Work groups are examples of programs that

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can function as peer-to-peer network operating systems
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Setup - An operating system such as Windows XP already in place may only need to be reconfigured for peer-to-peer
operations
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Security - Does not provide the security available on a client/server network
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Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or

network traffic (network servers)
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Clients rely on servers for
resources, such as files, devices, and even processing power
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The servers become the heart of the system,

providing access to resources and providing security
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The network operating system provides the mechanism to integrate all the components of the network and
allow multiple users to simultaneously share the same resources irrespective of physical location
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Scalability - Any or all elements can be replaced individually as needs increase
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Interoperability - All components: client/network/server, work together
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Disadvantages of Client/Server Network:

Expense - Requires initial investment in dedicated server
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Dependence - When server goes down, operations will cease across the network

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