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UNIT I:Introduction: Network Topologies WAN, LAN, MAN. Reference models- The OSI
Reference Model the TCP/IP Reference Model - A Comparison of the OSI and TCP/IP
Reference Models. Physical Layer –Introduction to physical layer-Data and Signals,
Periodic analog signals, digital signals,transmission impairment, ,Data rate limits,
performance -Introduction to Guided Media- Twisted-paircable, Coaxial cable and Fiber
optic cable and Unguided media: Wireless-Radio waves, microwaves,infrared.
Computer networks
When several computers are connected together and are able to communicate with one
another, it is called a computer network. Computer networks are designed to share data
and information among the computers of the network. Depending on the operating
geographical area, computer networks are of the three major types, namely LAN, MAN,
and WAN.
LAN:
LAN represents Local Area Network. LANs are tiny networks within a single construction
or colleges of up to a few kilometers. When two or more computers are attached in a
limited area, i.e., the inside diameter of not more than a few kilometers, then such a
network is called a local area network. Local area network connects various computers in
a small geographical area such a building so that they can share data, resources,
programs, tools, etc.
LAN has very high data transmission rate because it connects a very limited number of
computers in a single network.
The most common examples of LAN include private computer networks for home, office,
schools, hospitals, colleges, etc.
MAN:
MAN represents Metropolitan Area Network. MAN is a huge version of LAN and uses
similar technology. It uses one or two cables but does not include switching components.
It covers an entire city and can be associated with the local cable TV network.
A typical MAN connects two or more computing devices that reside apart but in the same
or different cities. MAN convers comparatively larger geographical area as compared to a
LAN. But, it is very costly as compared to a LAN. The speed of data transmission in MAN
is of the order of Mbps (Mega Bytes per Second). The most common examples of MAN
are networks of telephone companies that provide high speed digital subscriber line to
cable TV customers.
,WAN:
WAN represents Wide Area Network. WAN supports no limit of distance. A WAN offers
long-distance communication of data, voice, pictures and video data over a large
geographical area consisting of a country, a continent or even a planet.
A WAN uses a relatively very expensive and high-speed technology for data
communication.Based on network structure, WANs are of two types, namely Point-to-
Point WAN and Switched WAN. The most common examples of WANs include the
Internet, the network of bank cash dispensers, etc
Layers of OSI Model:
OSI stands for Open Systems Interconnection. It is a 7-layer architecture with each layer
having specific functionality to perform. All these 7 layers work collaboratively to
transmit the data from one person to another across the globe.
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
,1.Physical Layer:
The lowest layer of the OSI reference model is the physical layer. It is responsible for the
actual physical connection between the devices. The physical layer contains information
in the form of bits. It is responsible for transmitting individual bits from one node to the
next.
The Functions of the Physical Layer:
Bit synchronization: The physical layer provides the synchronization of the bits by
providing a clock. This clock controls both sender and receiver thus providing
synchronization at the bit level.
Bit rate control: The Physical layer also defines the transmission rate i.e. the number of
bits sent per second.
Physical topologies: Physical layer specifies how the different, devices/nodes are
arranged in a network i.e. bus, star, or mesh topology.
Transmission mode: Physical layer also defines how the data flows between the two
connected devices. The various transmission modes possible are Simplex, half-duplex and
full-duplex.
2.Data Link Layer:
The data link layer is responsible for the node-to-node delivery of the message. The main
function of this layer is to make sure data transfer is error-free from one node to
another, over the physical layer.
The Data Link Layer is divided into two sublayers:
1.Logical Link Control (LLC)
2.Media Access Control (MAC)
, The Functions of the Data Link Layer:
Framing: Framing is a function of the data link layer. It provides a way for a sender to
transmit a set of bits that are meaningful to the receiver
Physicaladdressing: After creating frames, the Data link layer adds physical addresses
(MAC addresses) of the sender and/or receiver in the header of each frame.
Errorcontrol: The data link layer provides the mechanism of error control in which it
detects and retransmits damaged or lost frames.
FlowControl: The data rate must be constant on both sides else the data may get
corrupted thus, flow control coordinates the amount of data that can be sent before
receiving an acknowledgment.
Accesscontrol: When a single communication channel is shared by multiple devices, the
MAC sub-layer of the data link layer helps to determine which device has control over
the channel at a given time.
3.Network Layer:
The network layer works for the transmission of data from one host to the other located
in different networks. It also takes care of packet routing i.e. selection of the shortest
path to transmit the packet, from the number of routes available. The sender &
receiver’s IP addresses are placed in the header by the network layer.
The Functions of the Network Layer:
Routing: The network layer protocols determine which route is suitable from source to
destination. This function of the network layer is known as routing.
LogicalAddressing: To identify each device on Internetwork uniquely, the network layer
defines an addressing scheme. The sender & receiver’s IP addresses are placed in the
header by the network layer. Such an address distinguishes each device uniquely and
universally.
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