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COMPUTER NETWORKS
Introduction
Computer network is a collection of autonomous computers interconnected by a single
technology. Two computers are said to be interconnected if they are able to exchange
information. The connection need not be via a copper wire; fiber optics, microwaves, infrared,
and communication satellites can also be used.
Why Networks?
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Distribute computation among nodes
Coordination between processes running on different nodes
Remote I/O Devices
Remote Data/File Access
Personal communications (e-mail, chat, A/V)
World Wide Web
Applications
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Business applications
Home Applications
Access to remote information.
Person-to-person communication.
Interactive entertainment.
Electronic commerce.
WHY DO WE NEED A STANDARD?
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Many types of connection media: telephone lines, optical fibers, cables, radios,
etc.
Many different types of machines and operating systems
Many different network applications
To reduce their design complexity, most networks are organized as a stack of layers or
levels, each one built upon the one below it. The number of layers, the name of each
layer, the contents of each layer, and the function of each layer differ from network to
network. The purpose of each layer is to offer certain services to the higher layers,
shielding those layers from the details of how the offered services are actually
implemented. In a sense, each layer is a kind of virtual machine, offering certain services
to the layer above it.
BASED ON CONNECTION
1. BUS Network: Bus network is a network architecture in which a set of clients are
connected via a shared communications line, called a bus. Bus networks are the
simplest way to connect multiple clients, but often have problems when two
clients want to communicate at the same time on the same bus.
Advantages
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Easy to implement and extend
Well suited for temporary networks (quick setup)
Typically the cheapest topology to implement
Failure of one station does not affect others
Disadvantages
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Difficult to administer/troubleshoot
Limited cable length and number of stations
A cable break can disable the entire network
Maintenance costs may be higher in the long run
Performance degrades as additional computers are added
Low security (all computers on the bus can see all data transmissions on the bus)
One virus in the network will affect all of them (but not as badly as a star or ring
network)
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2. STAR NETWORK: Star network is one of the most common computer network
topologies. In its simplest form, star network consists of one central or hub computer
which acts as a router to transmit messages.
Data on a star network passes through the hub, switch, or concentrator before continuing
to its destination. The hub, switch, or concentrator manages and controls all functions of
the
network.
It
also
acts
as
a
repeater
for
the
data
flow.
Advantages
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Easy to implement and extend, even in large networks
Well suited for temporary networks (quick setup)
The failure of a non-central node will not have major effects on the functionality
of the network.
Disadvantages
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Limited cable length and number of stations
Maintenance costs may be higher in the long run
Failure of the central node can disable the entire network.
One virus in the network will affect them all
3.RING NETWORK: Ring network is a topology of computer networks where each
user is connected to two other users, so as to create a ring. The most popular example is a
token ring network.
Advantages
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All stations have equal access
Each node on the ring acts as a repeater, allowing ring networks to span greater
distances than other physical topologies.
When using a coaxial cable to create a ring network the service becomes much
faster.
Disadvantages
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Often the most expensive topology
4.TREE OR EXPANDED STAR
A tree topology combines characteristics of linear bus and star topologies. It consists of
groups of star-configured workstations connected to a linear bus backbone cable. Tree
topologies allow for the expansion of an existing network, and enable schools to
configure a network to meet their needs.
4.Mesh Network: Mesh Network is a way to route data, voice and instructions between
nodes. It allows for continuous connections and reconfiguration around blocked paths by
"hopping" from node to node until a connection can be established.
Mesh networks are self-healing: the network can still operate even when a node breaks
down or a connection goes bad. As a result, a very reliable network is formed. This is
applicable to wireless networks, wired networks, and software interaction.
A mesh network is a networking technique which allows inexpensive peer network nodes
to supply back haul services to other nodes in the same network. A mesh network
effectively extends a network by sharing access to higher cost network infrastructure
Mesh is a network topology in which devices are connected with many redundant
interconnections between network nodes. In a true mesh topology every node has a
connection to every other node in the network.
There are two types of mesh topologies: full mesh and partial mesh.
Full mesh topology occurs when every node has a circuit connecting it to every other
node in a network. Full mesh is very expensive to implement but yields the greatest
amount of redundancy, so in the event that one of those nodes fails, network traffic can
be directed to any of the other nodes. Full mesh is usually reserved
for backbone networks.
Partial mesh topology is less expensive to implement and yields less redundancy than full
mesh topology. With partial mesh, some nodes are organized in a full mesh scheme but
others are only connected to one or two in the network. Partial mesh topology is
commonly found in peripheral networks connected to a full meshed backbone.
Star-Bus Network: Star-Bus Network is a combination of a star network and a bus
network. A hub (or concentrator) is used to connect the nodes to the network. It is a
combination of the linear bus and star topologies and operates over one main
communication line.
TYPES OF NETWORK
Networks can be divided into three types based on geographical areas covered:
LANs, MANs, and WANs
LANs: Local Area Networks: LANs may use a transmission technology consisting of a
cable, to which all the machines are attached
 A LAN connects network devices over a relatively short distance. A networked
office building, school, or home usually contains a single LAN, though sometimes
one building will contain a few small LANs (perhaps one per room), and
occasionally a LAN will span a group of nearby buildings.
 In addition to operating in a limited space, LANs are also typically owned,
controlled, and managed by a single person or organization. They also tend to use
certain connectivity technologies, primarily Ethernet and Token Ring.
 Developed in 1970s.
 Medium: optical fibers, coaxial cables, twisted pair, wireless.
 Low latency (except in high traffic periods).
 High speed networks (10 to 100 Mb/sec).
 Speeds adequate for most distributed systems
MAN: Metropolitan Area Networks: The best-known example of a MAN is the
cable television network available in many cities.
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Generally covers towns and cities (50 kms)
Developed in 1980s.
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Medium: optical fibers, cables.
Data rates adequate for distributed computing applications.
A typical standard is DQDB (Distributed Queue Dual Bus).
Typical latencies < 1 msec.
Message routing is fast.
Man Based On Cable TV
DQDB: Distributed Queue Dual Bus Defined in IEEE 802.6
Data Over Cable Service Interface Distributed Queue Dual Bus (DQDB) is a Data-link
layer communication protocol for Metropolitan Area Networks (MANs), specified in the
IEEE 802.6 standard, designed for use in MANs. DQDB is designed for data as well as
voice and video transmission based on cell switching technology (similar to ATM).
DQDB, which permits multiple systems to interconnect using two unidirectional logical
buses, is an open standard that is designed for compatibility with carrier transmission
standards such as SMDS, which is based on the DQDB standards.
For a MAN to be effective it requires a system that can function across long, city-wide
distances of several miles, have a low susceptibility to error, adapt to the number of
nodes attached and have variable bandwidth distribution. Using DQDB, networks can be
thirty miles long and function in the range of 34 Mbps to 155 Mbps. The data rate
fluctuates due to many hosts sharing a dual bus as well as the location of a single host in
relation to the frame generator, but there are schemes to compensate for this problem
making DQDB function reliably and fairly for all hosts.
The DQDB is composed of a two bus lines with stations attached to both and a frame
generator at the end of each bus. The buses run in parallel in such a fashion as to allow
the frames generated to travel across the stations in opposite directions.
Below is a picture of the basic DQDB architecture:
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Upstream(A) means upstream on bus A
Downstream(A) means downstream on bus A
Upstream(B) means upstream on bus B
Downstream(B) means downstream on bus B
Head(A) means the uppermost node on bus A
Head(B) means the uppermost node on bus B
WAN: Wide Area Networks: It contains a collection of machines intended for running
user (i.e., application) programs. The machines are connected by a communication
subnet. In most wide area networks, the subnet consists of two distinct components:
transmission lines and switching elements. Transmission lines move bits between
machines. They can be made of copper wire, optical fiber, or even radio links. Switching
elements are specialized computers that connect three or more transmission lines. When
data arrive on an incoming line, the switching element must choose an outgoing line on
which to forward them. These switching computers have been called by various names in
the past; the name router is now most commonly used.
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Developed in 1960s.
Generally covers large distances (states, countries, continents).
Medium: communication circuits connected by routers.
Routers forwards packets from one to another following a route from the sender
to the receiver. Store-and-Forward
Hosts are typically connected (or close to) the routers.
Typical latencies: 100ms - 500ms.
Problems with delays if using satellites.
Typical speed: 20 - 2000 Kbits/s.
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Not (yet) suitable for distributed computing.
New standards are changing the landscape.
In this model, each host is frequently connected to a LAN on which a router is present,
although in some cases a host can be connected directly to a router. The collection of
communication lines and routers (but not the hosts) form the subnet.
WAN
4. ROUTING
Routing is the act of moving information across an internetwork from a source to a
destination. Routing involves two basic activities: determining optimal routing paths and
transporting information groups (typically called packets) through an internetwork. In the
context of the routing process, the latter of these is referred to as packet switching. The
routing algorithm is that part of the network layer software responsible for deciding
which output line an incoming packet should be transmitted on. Two major classes:
• Static Routing
• Dynamic Routing
A static routing table is created, maintained, and updated by a network administrator,
manually. A static route to every network must be configured on every router for full
connectivity.
A dynamic routing table is created, maintained, and updated by a routing protocol
running on the router. Examples of routing protocols include RIP (Routing Information
Protocol), EIGRP (Enhanced Interior Gateway Routing Protocol), and OSPF (Open
Shortest Path First).
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Non-adaptive algorithms do not base their routing decisions on measurements or
estimates of the current traffic and topology.
The choice of the route to use is computed in advance, off-line, and downloaded
to the routers when the network is booted (Static Routing).
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Adaptive algorithms attempt to change their routing decisions to reflect changes
in topology and the current traffic. (Dynamic Routing).
5. IP address
Every machine on the Internet has a unique identifying number, called an IP Address. A
typical IP address looks like this:
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216.27.61.137
To make it easier for us humans to remember, IP addresses are normally expressed in
decimal format as a "dotted decimal number" like the one above. But computers
communicate in binary form. Look at the same IP address in binary:
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11011000.00011011.00111101.10001001
 The four numbers in an IP address are called octets, because they each have eight
positions when viewed in binary form. If you add all the positions together, you
get 32, which is why IP addresses are considered 32-bit numbers. Since each of
the eight positions can have two different states (1 or 0) the total number of
possible combinations per octet is 28 or 256. So each octet can contain any value
between 0 and 255. Combine the four octets and you get 232 or a possible
4,294,967,296 unique values!
 Out of the almost 4.3 billion possible combinations, certain values are restricted
from use as typical IP addresses. For example, the IP address 0.0.0.0 is reserved
for the default network and the address 255.255.255.255 is used for broadcasts.
 The octets serve a purpose other than simply separating the numbers. They are
used to create classes of IP addresses that can be assigned to a particular business;
government or other entity based on size and need. The octets are split into two
sections: Net and Host. The Net section always contains the first octet. It is used
to identify the network that a computer belongs to. Host (sometimes referred to as
Node) identifies the actual computer on the network. The Host section always
contains the last octet. There are five IP classes plus certain special addresses:
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Default Network - The IP address of 0.0.0.0 is used for the default network.
There are different classes of IP addresses such as
o
o
o
o
o
Class A
Class B
Class C
Class D
Class E
Class A:
This class is for very large networks, such as major international
companies might have. The first octet is used as Net identifier. The other three
octets are used to identify each Host. In Class A networks, the high order bit value
in the first octet is always zero.
Eg:
Net
115.
Host
24.53.107
Loopback: The IP address 127.0.0.1 is used as the loopback address. This means
that it is used by the host computer to send a message back to itself. It is
commonly used for troubleshooting and network testing.
Class B:
Class B is used for medium sized networks. Class B addresses include the
second octet as part of the Net identifier. The other two octets are used to identify
each host. Class B networks have a first bit value of 1 and a second bit value of 0
in the first octet.
Eg:
Net
145.24
Host
53.107
Class C:
Class C is used for small to mid-size businesses. Class C addresses include
the second and third octets as part of the Net identifier. The last octet is used to
identify each host. Class C networks have a first bit value of 1, a second bit value
of 1 and a third bit value of 0 in the first octet.
Eg:
Net
195.24.53
Host
107
Class D:
Class D is used for multicast. Class D networks have first bit value of 1,
second bit value of 1, third bit value of 1 and fourth bit value of 0 in the first
octet. The other 28 bits are used to identify the group of computers the multicast
message is intended for.
Eg:
Net
224.
Host
24.53.107
Class E:
Class E is used for experimental purposes only. Class E networks have
first bit value of 1, second bit value of 1, third bit value of 1 and fourth bit value
of 1 in the first octet. The other 28 bits are used to identify the group of computers
the multicast message is intended for.
Eg:
Net
240.
Host
24.53.107
Broadcast:
Messages that are intended for all computers on the network are sent as
broadcasts. These messages always use the IP address 255.255.255.255.
6. DOMAIN NAME SERVICE (DNS)
The Domain Name System (DNS) is the method by which Internet addresses in
mnemonic form such as sunc.scit.wlv.ac.uk. are converted into the equivalent numeric IP
address such as 134.220.4.1. To the user and application process this translation is a
service provided either by the local host or from a remote host via the Internet. The DNS
server (or resolver) may communicate with other Internet DNS servers if it cannot
translate the address itself.
The system accesses the DNS through a resolver. The resolver gets the hostname and
returns the IP address or gets an IP address and looks up a hostname. The resolver returns
the IP address before asking the TCP to open a connection or sending a datagram using
UDP
DNS Name Structure
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DNS names are constructed hierarchically. The highest level of the hierarchy
being the last component or label of the DNS address. Labels can be up to 63
characters long and are case insensitive. A maximum length of 255 characters is
allowed. Labels must start with a letter and can only consist of letters, digits and
hyphens. The root of the DNS tree is a special node with a null label. The seven
3-character domain names.
meaning
code
com Commercial. Now international.
edu Educational.
gov Government.
int
International Organization.
mil Military.
net
Network related.
org Miscellaneous Organization.
Fig: 1 DNS
7. PROTOCOL
A protocol is a set of guidelines or rules. A communications protocol is a formal
description of digital message formats and the rules for exchanging those messages in or
between
computing systems
and
in telecommunications.
Protocols
may
include signaling, authentication and error detection and correction capabilities.
To reduce their design complexity, most networks are organized as a stack of
layers or levels, each one built upon the one below it. The number of layers, the name of
each layer, the contents of each layer, and the function of each layer differ from network
to network. The purpose of each layer is to offer certain services to the higher layers,
shielding those layers from the details of how the offered services are actually
implemented. The fundamental idea is that a particular piece of software (or hardware)
provides a service to its users but keeps the details of its internal state and algorithms
hidden from them.
Layer n on one machine carries on a conversation with layer n on another machine. The
rules and conventions used in this conversation are collectively known as the layer n
protocol. Basically, a protocol is an agreement between the communicating parties on
how communication is to proceed. A five-layer network is illustrated in Fig. The entities
comprising the corresponding layers on different machines are called peers. The peers
may be processes, hardware devices, or even human beings. In other words, it is the peers
that communicate by using the protocol.
8. THE INTERNET
The Internet is not a network at all, but a vast collection of different networks that use
certain common protocols and provide certain common services. It is an unusual system
in that it was not planned by anyone and is not controlled by anyone. The Internet was
largely populated by academic, government, and industrial researchers. One new
application, the WWW (World Wide Web) changed all that and brought millions of new,
nonacademic users to the net.
Architecture of the Internet
The modem is a card within the PC that converts the digital signals the computer
produces to analog signals that can pass unhindered over the telephone system. These
signals are transferred to the ISP’s POP (Point of Presence), where they are removed
from the telephone system and injected into the ISP’s regional network. From this point
on, the system is fully digital and packet switched. The ISP’s regional network consists of
interconnected routers in the various cities the ISP serves. If the packet is destined for a
host served directly by the ISP, the packet is delivered to the host. Otherwise, it is handed
over to the ISP’s backbone operator. International backbone networks, with thousands of
routers connected by high-bandwidth fiber optics. Large corporations and hosting
services that run server farms (machines that can serve thousands of Web pages per
second) often connect directly to the backbone. If a packet given to the backbone is
destined for an ISP or company served by the backbone, it is sent to the closest router and
handed off there. Packets can be forwarded from any backbone to any other backbone.