NETWORKS
A network is a set of devices (often
referred to as nodes) connected by communication links. A node can be a
computer, printer, or any other device capable of sending and/or receiving data
generated by other nodes on the network.
Distributed Processing
Most networks use distributed
processing, in which a task is divided among multiple computers. Instead of one
single large machine being responsible for all aspects of a process, separate
computers (usually a personal computer or workstation) handle a subset.
Network Criteria
A network must be able to meet a
certain number of criteria. The most important of these are performance,
reliability, and security.
Performance:
Performance can be measured in many
ways, including transit time and response time.Transit time is the amount of
time required for a message to travel from one device to another. Response time
is the elapsed time between an inquiry and a response. The performance of a
network depends on a number of factors, including the number of users, the type
of transmission medium, the capabilities of the connected hardware, and the
efficiency of the software. Performance is often evaluated by two networking
metrics: throughput and delay. We often need more throughput and less delay.
However, these two criteria are often contradictory. If we try to send more
data to the network, we may increase throughput but we increase the delay
because of traffic congestion in the network.
Reliability:
In addition to accuracy of delivery,
network reliability is measured by the frequency of failure, the time it takes
a link to recover from a failure, and the network's robustness in a
catastrophe.
Security:
Network security issues include
protecting data from unauthorized access, protecting data from damage and
development, and implementing policies and procedures for recovery from
breaches and data losses.
Type of Connection
A network is two or more devices
connected through links. A link is a communications pathway that transfers data
from one device to another. For visualization purposes, it is simplest to
imagine any link as a line drawn between two points. For communication to
occur, two devices must be connected in some way to the same link at the same
time. There are two possible types of connections: point-to-point and
multipoint. Point-to-Point
A
point-to-point connection provides a dedicated link between two devices. The
entire
capacity of the link is reserved for
transmission between those two devices. Most point-to-point
connections use an actual length of
wire or cable to connect the two ends, but other options, such
as microwave or satellite links, are
also possible. When you change television channels by
infrared remote control, you are
establishing a point-to-point connection between the remote
control and the television's control
system.
Multipoint
A multipoint (also called multidrop)
connection is one in which more than two specific devices share a single link.
In a multipoint environment, the capacity of the channel is shared, either
spatially or temporally. If several devices can use the link simultaneously, it
is a spatially shared connection. If users must take turns, it is
a timeshared connection.
Physical Topology
The term physical topology
refers to the way in which a network is laid out physically. One or more
devices connect to a link; two or more links form a topology. The topology of a
network is the geometric representation of the relationship of all the links
and linking devices (usually called nodes) to one another. There are four basic
topologies possible: mesh, star, bus, and ring
Mesh:
In a mesh topology, every device has a dedicated
point-to-point link to every other device. The term dedicated
means that the link carries traffic only between the two devices it connects.
To find the number of physical links in a fully connected mesh network with n
nodes, we first consider that each node must be connected to every other node.
Node 1 must be connected to n - I nodes, node 2 must be connected to n
– 1 nodes, and finally node n must be connected to n - 1 nodes.
We need n(n - 1) physical links. However, if each physical link allows
communication in both directions (duplex mode), we can divide the number of
links by 2. In other words, we can say that in a mesh topology, we need n(n
-1) /2 duplex-mode links.
To accommodate that many links,
every device on the network must have n – 1 input/output (VO) ports
to be connected to the other n - 1 stations.
1. The use of dedicated links
guarantees that each connection can carry its own data load, thus eliminating
the traffic problems that can occur when links must be shared by multiple
devices.
2. A mesh topology is robust. If one
link becomes unusable, it does not incapacitate the entire system. Third, there
is the advantage of privacy or security. When every message travels along a
dedicated line, only the intended recipient sees it. Physical boundaries
prevent other users from gaining access to messages. Finally, point-to-point
links make fault identification and fault isolation easy. Traffic can be routed
to avoid links with suspected problems. This facility enables the network
manager to discover the precise
location
of the fault and aids in finding its cause and solution.
1. Disadvantage of a mesh are related
to the amount of cabling because every device must be connected to every other
device, installation and reconnection are difficult.
2. Second, the sheer bulk of the wiring
can be greater than the available space (in walls, ceilings, or floors) can
accommodate. Finally, the hardware required to connect each link
(I/O
ports and cable) can be prohibitively expensive.
For these reasons a mesh topology is
usually implemented in a limited fashion, for example, as a backbone connecting
the main computers of a hybrid network that can include several other
topologies.
Star Topology:
In a star topology, each device has
a dedicated point-to-point link only to a central controller, usually called a
hub. The devices are not directly linked to one another. Unlike a mesh
topology, a star topology does not allow direct traffic between devices. The
controller acts as an exchange: If one device wants to send data to another, it
sends the data to the controller, which then relays the data to the other
connected device .
A star topology is less expensive
than a mesh topology. In a star, each device needs only one link and one I/O
port to connect it to any number of others. This factor also makes it easy to
install and reconfigure. Far less cabling needs to be housed, and additions,
moves, and deletions involve only one connection: between that device and the
hub.
Other advantages include robustness.
If one link fails, only that link is affected. All other links remain active.
This factor also lends itself to easy fault identification and fault isolation.
As long as the hub is working, it can be used to monitor link problems and
bypass defective links.
One big disadvantage of a star
topology is the dependency of the whole topology on one single point, the hub.
If the hub goes down, the whole system is dead. Although a star requires far
less cable than a mesh, each node must be linked to a central hub. For this
reason, often more cabling is required in a star than in some other topologies
(such as ring or bus).
Bus Topology:
The preceding examples all describe
point-to-point connections. A bus topology, on the other hand, is
multipoint. One long cable acts as a backbone to link all the devices in
a network
Nodes are connected to the bus cable
by drop lines and taps. A drop line is a connection running between the device
and the main cable. A tap is a connector that either splices into the main
cable or punctures the sheathing of a cable to create a contact with the
metallic core. As a signal travels along the backbone, some of its energy is
transformed into heat. Therefore, it becomes weaker and weaker as it travels
farther and farther. For this reason there is a limit on the number of taps a
bus can support and on the distance between those taps.
Advantages of a bus topology include
ease of installation. Backbone cable can be laid along the most efficient path,
then connected to the nodes by drop lines of various lengths. In this way, a
bus uses less cabling than mesh or star topologies. In a star, for example,
four network devices in the same room require four lengths of cable reaching
all the way to the hub. In a bus, this redundancy is eliminated. Only the
backbone cable stretches through the entire facility. Each drop line has to reach
only as far as the nearest point on the backbone.
Disadvantages include difficult
reconnection and fault isolation. A bus is usually designed to be optimally
efficient at installation. It can therefore be difficult to add new devices.
Signal reflection at the taps can cause degradation in quality. This
degradation can be controlled by limiting the number and spacing of devices
connected to a given length of cable. Adding new devices may therefore require
modification or replacement of the backbone.
In addition, a fault or break in the
bus cable stops all transmission, even between devices on the same side of the
problem. The damaged area reflects signals back in the direction of origin,
creating noise in both directions.
Bus topology was the one of the
first topologies used in the design of early local area networks.
Ethernet LANs can use a bus
topology, but they are less popular.
Ring Topology In a ring topology,
each device has a dedicated point-to-point connection with only the two devices
on either side of it. A signal is passed along the ring in one direction, from
device to device, until it reaches its destination. Each device in the ring
incorporates a repeater. When a device receives a signal intended for another
device, its repeater regenerates the bits and passes them along
A ring is relatively easy to install
and reconfigure. Each device is linked to only its immediate neighbors (either
physically or logically). To add or delete a device requires changing only two
connections. The only constraints are media and traffic considerations (maximum
ring length and number of devices). In addition, fault isolation is simplified.
Generally in a ring, a signal is circulating at all times. If one device does
not receive a signal within a specified period, it can issue an alarm. The
alarm alerts the network operator to the problem and its location.
However, unidirectional traffic can
be a disadvantage. In a simple ring, a break in the ring (such as a disabled
station) can disable the entire network. This weakness can be solved by using a
dual ring or a switch capable of closing off the break. Ring topology was
prevalent when IBM introduced its local-area network Token Ring. Today, the
need for higher-speed LANs has made this topology less popular. Hybrid Topology
A network can be hybrid. For example, we can have a main star topology with
each branch connecting several stations in a bus topology as shown in Figure
Local Area Networks:
Local area networks, generally
called LANs, are privately-owned networks within a single building or campus of
up to a few kilometres in size. They are widely used to connect personal
computers and workstations in company offices and factories to share resources
(e.g., printers) and exchange information. LANs are distinguished from other
kinds of networks by three characteristics:
(1) Their size,
(2) Their transmission technology, and
(3) Their topology.
LANs are restricted in size, which
means that the worst-case transmission time is bounded and known in advance.
Knowing this bound makes it possible to use certain kinds of designs that would
not otherwise be possible. It also simplifies network management. LANs may use
a transmission technology consisting of a cable to which all the machines are
attached, like the telephone company party lines once used in rural areas.
Traditional LANs run at speeds of 10 Mbps to 100 Mbps, have low delay
(microseconds or nanoseconds), and make very few errors. Newer LANs operate at
up to 10 Gbps Various topologies are possible for broadcast LANs. Figure1 shows
two of them. In a bus (i.e., a linear cable) network, at any instant at most
one machine is the master and is allowed to transmit. All other machines are
required to refrain from sending. An arbitration mechanism is needed to resolve
conflicts when two or more machines want to transmit simultaneously. The
arbitration mechanism may be centralized or distributed. IEEE 802.3, popularly
called Ethernet, for example, is a bus-based broadcast network with
decentralized control, usually operating at 10 Mbps to 10 Gbps. Computers on an
Ethernet can transmit whenever they want to; if two or more packets collide,
each computer just waits for a
random time and tries again later.
Fig.1: Two broadcast networks . (a) Bus. (b) Ring.
A second type of broadcast system is
the ring. In a ring, each bit propagates around on its own, not waiting for the
rest of the packet to which it belongs. Typically, each bit circumnavigates the
entire ring in the time it takes to transmit a few bits, often before the
complete packet has even been transmitted. As with all other broadcast systems,
some rule is needed for arbitrating simultaneous accesses to the ring. Various
methods, such as having the machines take turns, are in use. IEEE 802.5 (the
IBM token ring), is a ring-based LAN operating at 4 and 16 Mbps. FDDI is
another example of a ring network.
Metropolitan Area Network (MAN):
Metropolitan Area Network:
A metropolitan area network, or MAN,
covers a city. The best-known example of a MAN is the cable television network
available in many cities. This system grew from earlier community antenna
systems used in areas with poor over-the-air television reception. In these
early systems, a large antenna was placed on top of a nearby hill and signal
was then piped to the subscribers' houses. At first, these were
locally-designed, ad hoc systems. Then companies began jumping into the business,
getting contracts from city governments to wire up an entire city. The next
step was television programming and even entire channels designed for cable
only. Often these channels were highly specialized, such as all news, all
sports, all cooking, all gardening, and so on. But from their inception until
the late 1990s, they were intended for television reception only. To a first
approximation, a MAN might look something like the system shown in Fig. In this
figure both television signals and Internet are fed into the centralized head
end for subsequent distribution to people's homes. Cable television is not the
only MAN. Recent developments in high-speed wireless Internet access resulted
in another MAN, which has been standardized as IEEE 802.16.
Fig.2:
Metropolitan area network based on cable TV.
A MAN is implemented by a standard
called DQDB (Distributed Queue Dual Bus) or IEEE 802.16. DQDB has two
unidirectional buses (or cables) to which all the computers are attached.
Wide Area Network (WAN).
Wide Area Network:
A wide area network, or WAN, spans a
large geographical area, often a country or continent. It contains a collection
of machines intended for running user (i.e., application) programs. These
machines are called as hosts. The hosts are connected by a communication
subnet, or just subnet for short. The hosts are owned by the customers (e.g.,
people's personal computers), whereas the communication subnet is typically
owned and operated by a telephone company or Internet service provider. The job
of the subnet is to carry messages from host to host, just as the telephone
system carries words from speaker to listener.
Separation of the pure communication aspects of the network
(the subnet) from the application aspects (the hosts), greatly simplifies the
complete network design. 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. In most WANs, the network contains numerous
transmission lines, each one connecting a pair of routers. If two routers that
do not share a transmission line wish to communicate, they must do this
indirectly, via other routers. When a packet is sent from one router to another
via one or more intermediate routers, the packet is received at each
intermediate router in its entirety, stored there until the required output
line is free, and then forwarded. A subnet organized according to this
principle is called a store-and-forward or packet-switched subnet. Nearly all
wide area networks (except those using satellites) have store-and-forward
subnets. When the packets are small and all the same size, they are often
called cells.
The principle of a packet-switched
WAN is so important. Generally, when a process on some host has a message to be
sent to a process on some other host, the sending host first cuts the message
into packets, each one bearing its number in the sequence. These packets are
then injected into the network one at a time in quick succession. The packets
are transported individually over the network and deposited at the receiving
host, where they are reassembled into the original message and delivered to the
receiving process. A stream of packets resulting from some initial message is
illustrated in Fig.
In this figure, all the packets
follow the route ACE, rather than ABDE or ACDE. In some networks all packets
from a given message must follow the same route; in others each packed is
routed separately. Of course, if ACE is the best route, all packets may be sent
along it, even if each packet is individually routed.
Fig.3.1:
A stream of packets from sender to receiver.
Not all WANs are packet switched. A
second possibility for a WAN is a satellite system. Each router has an antenna
through which it can send and receive. All routers can hear the output from the
satellite, and in some cases they can also hear the upward transmissions of
their fellow routers to the satellite as well. Sometimes the routers are
connected to a substantial point-to-point subnet, with only some of them having
a satellite antenna. Satellite networks are inherently broadcast and are most
useful when the broadcast property is important.
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