Introduction
In its simplest form, data communication takes place between two devices that are
directly connected by some form of point-to-point transmission medium.
Data communications are the exchange of data between two devices via some form of
transmission medium such as a wire cable. For data communications to occur, the
communicating devices must be pail of a Communication system made up of a cornbination of
hardware (physical equipment) and software (programs).
The effectiveness of a data Communications system depends on four fundamental
characteristics: delivery, accuracy, timeliness, and jitter.
1. Delivery.
The system must deliver data to the correct destination. Data must be received by the
intended device or user and only by that device or user.
2. Accuracy.
The system must deliver the data accurately. Data that have been altered in transmission
and left uncorrected are unusable.
3. Timeliness.
The system must deliver data in a timely manner. Data delivered late are useless. In the
case of video and audio, timely delivery means delivering data as they are produced, in the
same order that they are produced, and without significant delay. This kind of delivery is
called real-time transmission.
4. Jitter.
Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets. For exarnple, let us assume that video packets arc sent
every 30 ms. If some of the packets arrive with 30-ms delay and others with 40-ms delay, an
uneven quality in the video is the result.
Components
Task
Data Communication and networking
Process
A data communications system has five components (see Fig 1.1).
1. Message.
The message is the information (data) to he communicated. Popular forms of
information include text, numbers, pictures, audio, and video.
2. Sender.
The sender is the device that sends the data message. It can be a computer, workstation,
telephone handset, video camera, and so on.
3. Receiver.
The receiver is the device that receives the message. It can be a computer, workstation,
telephone handset, television, and so on.
4. Transmission medium.
The transmission medium is the physical path by which a message travels from sender to
receiver. Sonic examples of transmission media include twisted-pair wire, coaxial cable,
fiber-optic cable, and radio waves.
5. Protocol.
A protocol is a set of sents an agreement between the devices may be connected but not
cannot be understood by a person rules that govern data communications. It
reprecommunicating devices without a protocol, two communicating, just as a person
speaking French who speaks only Japanese.
Data Representation
Information today comes in different forms such as text, numbers, images, audio, and
video.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex as
shown in Figure 1.2.
Simplex
In simplex mode, the communication js unidirectional, as on a one-way street, Only one
of the two devices on a link can transmit; the other can only receive (see Figure 1.2a).
Keyboards and traditional monitors are examples of simplex devices. The keyboard can
only introduce input; the monitor can only accept output. The simplex mode can use the
entire capacity of the channel to send data in one direction.
Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the same
time:
When one device is sending, the other can only receive, and vice versa (sec Figure 1.2b).
The half-duplex mode is like a one-lane road with traffic allowed in both threetions.
When cars are traveling in one direction, cars going the other way must wait. In a half-duplex
transmission, the entire capacity of a channel is taken over by whichever of the two devices is
transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex
systems.
The half-duplex mode is used in cases where there is no need for communication in
both directions at the same time: the entire capacity of the channel can be utilized tr each
direction.
Full-Duplex
In full-duplex mode (also called duplex), both stations can transmit and receive
simultaneously (see Figure 1.2c).
The full-duplex mode is like a two-way street with traffic flowing in both directions at
the same time. In full-duplex mode, signals going in one direction share the capacity of the link
with signals going in the other direction. This sharing can occur in two ways: Either the link
must contain two physically separate transmission paths, one for sending and the other for
receiving; or the capacity of the channe1 is divided between signals traveling in both directions.
One common example of full-duplex communication is the telephone network. When
two people are communicating by a telephone line, both can talk and listen at the same time.
The full-duplex mode is used when communication in both directions is required all the
time. The capacity of the channel, however, must be divided between the two directions.
Network
A computer network allows sharing of resources and information among devices
connected to the network. The Advanced Research Projects Agency (ARPA) funded the
design of the "Advanced Research Projects Agency Network" (ARPANET) for the
United States Department of Defense. It was the first operational computer network in the
world. Development of the network began in 1969, based on designs developed during the
1960s.
A computer network is a group of computers that are connected to each other for the
purpose of communication. Networks may be classified according to a wide variety of
characteristics.
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.
Physica1 Structures
Before discussing networks, we need to define some network attributes.
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-topoint
connections use an actual length of wire or cable to con- fleet the two ends, but other
options, such as microwave or satellite links, are also possible (see Figure 1 .3a). 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 (see Figure 1.3b).
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.
Protocols & standards and standards organizations
Protocol (computing)
In computing, a protocol is a set of rules which is used by computers to communicate
with each other across a network. A protocol is a convention or standard that controls or
enables the connection, communication, and data transfer between computing endpoints. In its
simplest form, a protocol can be defined as the rules governing the syntax, semantics, and
synchronization of communication. Protocols may be implemented by hardware, software, or a
combination of the two. At the lowest level, a protocol defines the behavior of a hardware
connection.
de facto standard
A protocol that has not been approved by an organized body but adopted as a standard
through widespread use.
de jure standard
A protocol that has been legislated by an officially recognized body.
Common protocols
· IP (Internet Protocol)
· UDP (User Datagram Protocol)
· TCP (Transmission Control Protocol)
· DHCP (Dynamic Host Configuration Protocol)
· HTTP (Hypertext Transfer Protocol)
· FTP (File Transfer Protocol)
· Telnet (Telnet Remote Protocol)
· SSH (Secure Shell Remote Protocol)
· POP3 (Post Office Protocol 3)
· SMTP (Simple Mail Transfer Protocol)
· IMAP (Internet Message Access Protocol)
· SOAP (Simple Object Access Protocol)
· PPP (Point-to-Point Protocol)
· RFB (Remote Framebuffer Protocol)
Protocols
In computer networks, communication occurs between entities in different systems. An
entity is anything capable of sending or receiving information. However, two entities cannot
simply send bit streams to each other and expect to be understxxl. For communication to
occur, the entities must agree on a protocol. A protocol is a set of rules that govern data
communications. A protocol defines what is communicated, how it is communicated, and
when it is communicated. The key elements of a protocol are syntax, semantics, and timing.
Syntax:
The term syntax refers to the structure or format of the data, meaning the order in
which they are presented. For example, a simple protocol might expect the first 8 bits of data
to be the address of the sender, the second S bits to be the address of the receiver, and the rest
of the stream to be the message itself.
Semantics:
The word semantics refers to the meaning of each section of bits. How is a particular
pattern to be interpreted, and what action is to be taken based on that interpretation? For
example, does an address identify the route to be taken or the final destination of the message?
Timing:
The term timing refers to two characteristics: when data should be sent and how fast they
can be sent. For example, if a sender produces data at 100 Mhps hut the receiver can process
data at only 1 Mbps. the transmission will overload the receiver and some data will be lost.
Standards and standards organizations
· American National Standards Institute (ANSI)
· International Electro-technical Commission (IEC)
· International Telecommunication Union (ITU)
· Institute of Electrical and Electronics Engineers (IEEE)
· International Organization for Standardization (ISO)
· Internet Society (ISOC) and the associated Internet Engineering Task Force (IETF)
· Electronic Industries Alliance (EIA) and the associated Telecommunictions Industry
Association (TIA)
· American National Standards Institute (ANSI)
ANSI is a private, non-governmental agency where members are manufacturers, users
and other interested companies. It has nearly 1000 member of the ISO (International Standard
Organization). ANSI has set up the standards for Fiber Distributed Data Interface (FDDI) and
for local are networks using optical fiber. ANSI has also set up the American Standard Code
for Information Interchanged (ASCII), used by many computers for storing information.
· International Electro-technical Commission (IEC)
IEC is a non-governmental agency devising standards for data processing and
interconnections and safety in office equipment. It was involved in the development of the
Joint Photographic Experts Group (JPEG), a group that devised compression standard for
images.
· International Telecommunication Union (ITU)
ITU is an agency of the United Nations and has three sectors.
1) ITU-R deals with radio communications.
2) ITU-D is a development sector.
3) ITU-T deals with telecommunications
International Telecommunications Union sets standards for modems, e-mail, and digital
telephone systems. The ITU has contributed to the following standards.
· Institute of Electrical and Electronics Engineers (IEEE)
The IEEE is the largest professional organization in the world and consists of
computing and engineering professionals. It is involved in developing standards for computing,
communication, and for processes in electrical engineering, and electronics. It sponsored an
important standard for local area networks called Project 802.
· International Organization for Standardization (ISO)
The ISO is a non-governmental organization based in Geneva, Switzerland, in which
over 100 countries participate. One of ISOs most significant activities is its work on open
systems, which define the protocols that would allow any two computers to communicate
independent of their architecture. Open Systems Interconnections (OSI) model, contains seven
layer protocols for network communications.
· Internet Society (ISOC) and the associated Internet Engineering Task Force
(IETF)
Internet Society and the associated Internet Engineering Task Force are concerned with
expediting the growth and in the evaluation of Internet communications. The Internet Society
concentrates on users issues, including enhancements to the TCP/IP protocol suite. IETF
focuses on technical Internet issues (hardware and software). Important contributions include
the development of Simple Network Management Protocol (SNMP)
· Electronic Industries Alliance (EIA) and the associated Telecommunictions
Industry Association (TIA)
EIA is responsible to develop network cabling standards. EIA has made significant
contributions by defining physical connection interfaces and electronic signaling specifications
for data communications. TIA was created as a separate body within the EIA to develop
telecommunications and cabling standards.
Line Configuration
Line configuration refers to the way two or more communication devices attached to a
link. Line configuration is also referred to as connection. A link is a communication medium
through which data is communicated between devices. For communication to occur between
two devices, they must be connected to the same link at the same time. There are two possible
types of line configurations or connections. These connections are.
1. Point-to-point connection
2. Multipoint connection
Point-to-Point Connector
The point-to-point connection provides a dedicated link between two communication
devices. The entire link or channel is reserved for two devices for data communication, and no
other devices can use the dedicated link. Usually, in this type of connection, the two devices are
connected together with a cable.
It must be noted that microwave and satellite dedicated links are also possible. Two
computers connected together (point-to-point) through microwave link.
When you change the television channel by remote control, you are establishing a pointto-
point connection between the remote control and the television’s control system.
Multipoint Connection
Multipoint connection is also referred to as multidrop connection. This type of
connection allows multiple devices (more than two devices) to share a single link. The
multipoint connection or line configuration is shown below.
Topology
In networking, the term topology is the way of connecting computers or nodes on a
network. There are many ways in which computers are connected together in a computer
network. Therefore network topology is defined as: the schemes of joining a number of
computers in the form of a network are called Network Topologies.
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.
Network topology signifies the way in which devices in the network see their logical
relations to one another. The use of the term "logical" here is significant. That is, network
topology is independent of the "physical" layout of the network. Even if networked computers
are physically placed in a linear arrangement, if they are connected via a hub, the network has a
Star topology, rather than a bus topology. In this regard the visual and operational
characteristics of a network are distinct; the logical network topology is not necessarily the
same as the physical layout. Networks may be classified based on the method of data used to
convey the data, these include digital and analog networks.
We know that two or more devices are connected to a link for data communication.
Similarly, two or more links form a topology. The topology of a network is the geometric
representation of the relationship of all the links and the nodes (communication devices) to
one another.
There are three commonly used network topologies. These are:
1. Star topology
2. Ring topology
3. Bus topology
1. Star Topology
In a star network, each node (computer or other device) is directly connected to the
central computer or Hub that provides connection points for nodes on the network. The star
topology is the most common topology in use today. In star network, information or data is
communicated from one computer to another through Hub. This form of network
configuration looks like a star as shown in figure below.
Advantages:
The main advantages of star topology are:
· It is easy to install and to maintain.
· You can easily add and remove nodes to and from the network without affecting the
network.
· If any node fails, other nodes are not affected.
Disadvantages
The main disadvantages of star topology are:
· This type of network depends upon the central Hub. If Hub fails the entire network is
failed.
· Each computer is directly connected to the Hub through a cable, so it becomes more
costly.
2. Ring Topology
In ring network, each node is connected to two adjacent nodes in the form a closed ring
or loop. In ring topology, the last node connects to the first node to complete the ring. In ring
topology, each node has a dedicated point-to-point connection only with the two devices on
either side of it.
In this network, data is communicated in one direction from node to node around the
entire ring. When a computer in ring network sends message to another computer on the
network, the message travels to each node or computer until it reaches its destination. The ring
network configuration is shown in figure below.
Advantages
The main advantages of ring topology are:
· It is less expensive than star topology.
· Nodes can be easily added or removed.
Disadvantages
The main disadvantages of ring topology are:
· It is more difficult to install and maintain.
· If a node fails, it affects the entire network.
3. Bus Topology
In bus network, all nodes are connected to a common communication medium or
central cable. The central physical cable that connects the nodes is called Bus. The data is
communicated between nodes in both directions through bus. A bus topology uses the
multipoint connection. The central single cable (or bus) acts as backbone to link all the devices
to the network.
In bus network, when a computer sends a message to another computer it also attaches
the address of the destination computer. In bus topology, a special device called a terminator is
attached at the cable’s start and end points. A terminator stops the network signals.
In LAN, bus topology is mostly used. In this topology, each computer is assigned a
unique address. The bus network configuration is given in figure image.
Complex topologies
1. Mesh Topology
In the mesh topology, separate cables are used to connect individual devices on the
network. This topology is expensive because of the number of cables used in the network.
The mesh topology is of two types,
full-mesh and
partial-mesh.
a) Full mesh:
In this topology, each device is interconnected with all the devices on the network, by a
dedicated cable. If one device fails, the data traveling along the network can be routed through
another device attached to the active device. The structure of the network is complex because
the devices in the network are interconnected.
Fig () Full Mesh Fig () Partial Mesh
b) Partial Mesh
In this topology, each device on the network is not connected to other devices. Only a
few devices on the network are connected using the full-mesh topology, and the others are
connected to one or more devices on the network.
Hybrid Topology:
This topology is the combination of bus, star, and ring networks. In other words, this
topology combines multiple topologies to form a large topology. The hybrid topology is widely
implemented in WANs.
a) Hybrid Star-Bus Topology:
Fig shows two networks, A and B, on a star topology. However, the connection between
the two networks is established using the bus topology. In a star-bus topology, the star
topology of each network is linked to the bus topology.
Hybrid Star-Bus Topology
Fig () Hybrid star bus topology Fig () Hybrid star ring topology
Transmission mode
A transmission mode defines the way in which group of bits goes from one device to
another. In transmission mode data flows in 3 ways.
1. Simplex 2. Half-duplex 3.Full-duplex.
There are 2 categories of transmission modes
1. Parallel transmission
2. Serial transmission
Parallel Transmission
Binary data 1s and 0s are organized into groups of n bits each. Bits are transmitted
simultaneously by using a separate line (wire) for each bit. Multiple bits are sent with each clock
tick.
Advantages:
· It is commonly used for data transmission
· Distances between two devices are short. (eg: communication between computer and
peripheral devices.
Disadvantages:
· Limited to short distances
· Very expensive
Fig () Parallel transmission
Serial Transmission:
Group of bits is transmitted one by one using line (wire) for all bits.
Advantages:
The advantage of serial over parallel transmission is that with only one communication
channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of
n.
Since communication within devices is parallel, conversion devices are required at the
interface between the sender and the line (parallel-to-serial) and between the line and the
receiver (serial-to-parallel).
We can communicate to long distance and it is less expensive.
It has 2 ways to provide communications
1. Asynchronous
2. Synchronous
Fig () Serial Transmission
Asynchronous transmission mode
Bits are divided into small groups (bytes) and sent independently. The sender can send
the groups at any time and the receiver never knows when they will arrive.
We send one start bit (0) at the beginning and one stop bit (1) at end of each byte.
There may be a gap between each byte. When the receiver detects a start bit, it sets a
timer and begins counting bits as they come in after receiving stop bit, it ignores any received
pulses.
Synchronous transmission mode:
Bit stream is combined into larger “frames”, which may contain multiple bytes.
We send bits one after another without start / stop bits or groups.
It is the responsibility of the receiver to group the bits.
Fig () synchronous transmission
Classification of Network
Types of networks
Below is a list of the most common types of computer networks in order of scale.
Personal area network
A personal area network (PAN) is a computer network used for communication among
computer devices close to one person. Some examples of devices that are used in a PAN are
personal computers, printers, fax machines, telephones, scanners, and even video game
consoles. Such a PAN may include wired and wireless connections between devices. The reach
of a PAN is typically at least about 20-30 feet (approximately 6-9 meters), but this is
expected to increase with technology improvements.
Local Area Networks
Local area networks, generally called LANs, are privately-owned networks within a
single building or campus of up to a few kilometers 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. In this book, we will adhere to tradition and
measure line speeds in megabits/sec (1 Mbps is 1,000,000 bits/sec) and gigabits/sec (1 Gbps is
1,000,000,000 bits/sec). Various topologies are possible for broadcast LANs. Fig 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 a
random time and tries again later.
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 21 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.
Broadcast networks can be further divided into static and dynamic, depending on how
the channel is allocated. A typical static allocation would be to divide time into discrete
intervals and use a round-robin algorithm, allowing each machine to broadcast only when its
time slot comes up. Static allocation wastes channel capacity when a machine has nothing to
say during its allocated slot, so most systems attempt to allocate the channel dynamically (i.e.,
on demand). Dynamic allocation methods for a common channel are either centralized or
decentralized. In the centralized channel allocation method, there is a single entity, for example,
a bus arbitration unit, which determines who goes next. It might do this by accepting requests
and making a decision according to some internal algorithm. In the decentralized channel
allocation method, there is no central entity; each machine must decide for itself whether to
transmit. You might think that this always leads to chaos, but it does not. Later we will study
many algorithms designed to bring order out of the potential chaos.
Metropolitan Area Networks
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.
Starting when the Internet attracted a mass audience, the cable TV network operators
began to realize that with some changes to the system, they could provide two-way Internet
service in unused parts of the spectrum. At that point, the cable TV system began to morph
from a way to distribute television to a metropolitan area network. To a first approximation, a
MAN might look something like the system shown in Fig.. In this figure we see both television
signals and Internet being fed into the centralized head end for subsequent distribution to
people's homes.
Figure 1-8. A metropolitan area network based on cable TV
Campus area network
A campus area network (CAN) is a computer network made up of an interconnection of
local area networks (LANs) within a limited geographical area. It can be considered one form
of a metropolitan area network, specific to an academic setting.
In the case of a university campus-based campus area network, the network is likely to
link a variety of campus buildings including; academic departments, the university library and
student residence halls. A campus area network is larger than a local area network but smaller
than a wide area network (WAN) (in some cases).
The main aim of a campus area network is to facilitate students accessing internet and
university resources. This is a network that connects two or more LANs but that is limited to a
specific and contiguous geographical area such as a college campus, industrial complex, office
building, or a military base. A CAN may be considered a type of MAN (metropolitan area
network), but is generally limited to a smaller area than a typical MAN. This term is most often
used to discuss the implementation of networks for a contiguous area. This should not be
confused with a Controller Area Network. 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.
Metropolitan area network
A metropolitan area network (MAN) 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. Routers, switches and hubs are connected to create a metropolitan area
network.
Wide area network
A wide area network (WAN) is a computer network that covers a broad area (i.e. any
network whose communications links cross metropolitan, regional, or national boundaries [1]).
Less formally, a WAN is a network that uses routers and public communications links.
Contrast with personal area networks (PANs), local area networks (LANs), campus area
networks (CANs), or metropolitan area networks (MANs), which are usually limited to a room,
building, campus or specific metropolitan area (e.g., a city) respectively. The largest and most
well-known example of a WAN is the Internet. A WAN is a data communications network that
covers a relatively broad geographic area (i.e. one city to another and one country to another
country) and that often uses transmission facilities provided by common carriers, such as
telephone companies. WAN technologies generally function at the lower three layers of the
OSI reference model: the physical layer, the data link layer, and the network layer.
Global area network
A global area networks (GAN) (see also IEEE 802.20) specification is in development
by several groups, and there is no common definition. In general, however, a GAN is a model
for supporting mobile communications across an arbitrary number of wireless LANs, satellite
coverage areas, etc. The key challenge in mobile communications is "handing off" the user
communications from one local coverage area to the next. In IEEE Project 802, this involves a
succession of terrestrial WIRELESS local area networks (WLAN).
Virtual private network
A virtual private network (VPN) is a computer network in which some of the links
between nodes are carried by open connections or virtual circuits in some larger network (e.g.,
the Internet) instead of by physical wires. The data link layer protocols of the virtual network
are said to be tunneled through the larger network when this is the case. One common
application is secure communications through the public Internet, but a VPN need not have
explicit security features, such as authentication or content encryption. VPNs, for example, can
be used to separate the traffic of different user communities over an underlying network with
strong security features.
A VPN may have best-effort performance, or may have a defined service level
agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN
has a topology more complex than point-to-point.
A VPN allows computer users to appear to be editing from an IP address location other
than the one which connects the actual computer to the Internet.
Internet work
An Internetwork is the connection of two or more distinct computer networks or
network segments via a common routing technology. The result is called an internetwork
(often shortened to internet). Two or more networks or network segments connect using
devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a
router. Any interconnection among or between public, private, commercial, industrial, or
governmental networks may also be defined as an internetwork.
In modern practice, interconnected networks use the Internet Protocol. There are at
least three variants of internetworks, depending on who administers and who participates in
them:
· Intranet
· Extranet
· Internet
Intranets and extranets may or may not have connections to the Internet. If connected
to the Internet, the intranet or extranet is normally protected from being accessed from the
Internet without proper authorization. The Internet is not considered to be a part of the
intranet or extranet, although it may serve as a portal for access to portions of an extranet.
Intranet
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as
web browsers and file transfer applications, that is under the control of a single administrative
entity. That administrative entity closes the intranet to all but specific, authorized users. Most
commonly, an intranet is the internal network of an organization. A large intranet will typically
have at least one web server to provide users with organizational information.
Extranet
An extranet is a network or internetwork that is limited in scope to a single organization
or entity and also has limited connections to the networks of one or more other usually, but
not necessarily, trusted organizations or entities (e.g., a company's customers may be given
access to some part of its intranet creating in this way an extranet, while at the same time the
customers may not be considered 'trusted' from a security standpoint). Technically, an extranet
may also be categorized as a CAN, MAN, WAN, or other type of network, although, by
definition, an extranet cannot consist of a single LAN; it must have at least one connection
with an external network.
Internet
The Internet consists of a worldwide interconnection of governmental, academic, public,
and private networks based upon the networking technologies of the Internet Protocol Suite. It
is the successor of the Advanced Research Projects Agency Network (ARPANET) developed
by DARPA of the U.S. Department of Defense. The Internet is also the communications
backbone underlying the World Wide Web (WWW). The 'Internet' is most commonly spelled
with a capital 'I' as a proper noun, for historical reasons and to distinguish it from other generic
internetworks.
Participants in the Internet use a diverse array of methods of several hundred
documented, and often standardized, protocols compatible with the Internet Protocol Suite
and an addressing system (IP Addresses) administered by the Internet Assigned Numbers
Authority and address registries. Service providers and large enterprises exchange information
about the reachability of their address spaces through the Border Gateway Protocol (BGP),
forming a redundant worldwide mesh of transmission paths.
OSI Model
The OSI model is a reference model which most IT professionals use to describe
networks and network applications.
The OSI model was originally intended to describe a complete set of production
network protocols, but the cost and complexity of the government processes involved in
defining the OSI network made the project unviable. In the time that the OSI designers spent
arguing over who would be responsible for what, TCP/IP conquered the world.
The Seven Layers of the OSI Model
The seven layers of the OSI model are:
Layer Name
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
Layers of OSI Model
Layer Seven of the OSI Model
The Application Layer of the OSI model is responsible for providing end-user services,
such as file transfers, electronic messaging, e-mail, virtual terminal access, and network
management. This is the layer with which the user interacts.
Layer Six of the OSI Model
The Presentation Layer of the OSI model is responsible for defining the syntax which
two network hosts use to communicate. Encryption and compression should be Presentation
Layer functions.
Layer Five of the OSI Model
The Session Layer of the OSI model is responsible for establishing process-to-process
commnunications between networked hosts.
Layer Four of the OSI Model
The Transport Layer of the OSI model is responsible for delivering messages between
networked hosts. The Transport Layer should be responsible for fragmentation and
reassembly.
Layer Three of the OSI Model
The Network Layer of the OSI model is responsible for establishing paths for data
transfer through the network. Routers operate at the Network Layer.
Layer Two of the OSI Model
The Data Link Layer of the OSI model is responsible for communications between
adjacent network nodes. Hubs and switches operate at the Data Link Layer.
Layer One of the OSI Model
The Physical Layer of the OSI model is responsible for bit-level transmission between
network nodes. The Physical Layer defines items such as: connector types, cable types,
voltages, and pin-outs.
The OSI model is a layered framework for the design of network systems that allows
communication between all types of computer systems. It Consists of seven separate but
related layers, each of which defines a part of the process of moving information across a
network (see Figure 2.2). An understanding of the fundamentals of the OSI model provides a
solid basis for exploring data communications.
Layers of OSI Model
Layers of OSI Model
Layered Architecture
The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer
2), network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and application
(layer 7). Figure 2.3 shows the layers involved when a message is sent from device A to device
B. As the message traveLs from A to B, it may pass through many intermediate nodes. These
intermediate nodes usually involve only the first three layers of the OSI model.
In developing the model, the designers distilled the process of transmiting data to its most
fundamental elements. They identi6ed which networking ftznctons had related uses and
collected those functions into discrete groups that became the layers. Each layer defines a
family of functions distinct from those of the other layers. By defining and localizing
functionality in this fashion, the designers created an architecture that is both comprehensive
and flexible. Most importantly, the OS1 model allows complete i rileroperabilily between
otherwise incompatible systems.
Within a single machine, each layer calls upon the services of the layer just below it.
Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4
Between machines, layer x on one machine communicates with layer x on another machine.
This communication is governed by an agreed-upon series of rules and conventions called
protocols. The processes on each machine that communicate at a given layer are called peer-topeer
processes. Communication between machines is therefore a peer-to-peer process using
the protocols appropriate to a given layer.
Physical layer
The physical layer is the first or the lowest layer in the OSI reference model. This layer
deals with the actual transmission of data using a transmission medium.
Functions:
· The physical layer is responsible for:
· Interfacing with the physical transmission medium.
· Defining the physical, electrical, and mechanical properties of he involved components.
Services:
· The services offered by the physical layer are:
· Setting up of connection
· Ending the connection
· Transmitting data over a communication channel
· Receiving data from a communication channel
Data link layer:
· The data link layer is the second layer in the OSI reference model.
· Provides for the transfer of frames (block of information) across a transmission link that
directly connects two nodes.
· It uses error detection and correction techniques, to ensure that transmission contains
no errors.
· It uses flow control techniques.
Function:
· Interfacing with the physical layer and network layer.
· It received data from network and passes it to physical layer. Data packets are framed by
adding header and trailer.
Services:
· Correcting errors
· Controlling flow of data
· Framing
Network layer:
It is a heart of the OSI model. It deals with routing strategies, which are responsible for
delivery of a packet from source to destination.
Function:
· It is used to provide internetworking.
· It moves packets from source to destination.
Services:
· Routing
· Accounting
· Packetizing
Transport Layer:
It provide different types of data transmission services.
Function:
· Interfacing with network and session
· Splitting data
· Controlling transmission and sequencing.
· It ensures the packets are delivered correctly.
Services:
· Controlling errors
· Controlling flow
· Both services connection oriented and connection less
Eg: TCP, UDP, SPX.
Session layer:
This layer manages sessions between communicating entities.
Function:
· Interfacing between transport and presentation.
· It accepts data from presentation and passes to transport.
· It manages the information exchange between two communicating systems with the
help of various services.
Services:
·
Conclusion
Application layer:
It is the top most layer.
Function:
· Interfacing between user and presentation
· Accepts input from user and passes to presentation.
· Defining how applications on one computer can communications with application on
other computers.
Services:
It provides underlying network related services.
E.g: FTP, Telnet.a