Set of devices connected by communication
No specific taxonomy available in which all
computer networks fit, still two are of vital
Classification on transmission
Network topology based on area
Ring Topology :
Categories of Network
LAN -Privately owned and links the devices in
a single office, building or campus.
MAN -Designed to extend over an entire city
either by cable television network or connecting
many LANs into a larger network.
WAN -Provides long-distance transmission of
data, voice, image etc. over large geographic
outline may be country or continent or the
Components of LAN
A Local Area Network, LAN is a combination of
hardware and software.
The physical component of the network system
is hardware. The invisible part, that is programs
enabling the network to function properly is the
Ethernet is the most dominant LAN in the
Lower layers of OSI model
IEEE 802 reference model, is a standardized
protocol architecture for LANs, which describes:
Logical link control (LLC) sub-layer,
Media access control (MAC) sub-layer.
Data Link Layer
• Provides for reliable transfer of
IEEE 802 Reference Model
information across the physical link
• Sends blocks (frames) with necessary
• Error control, flow control
IEEE 802 committee developed, revises, and
Use a three-layer protocol hierarchy: physical,
medium access control (MAC), and logical link
• Concerned with transmission of bit
• Deal with mechanical, electrical
• Procedural characteristics to 9
access physical medium
Most widely used data communication standard
Physical Medium – Carry signal between
MAC rules embedded in each interface that
allow multiple computers to access shared
Ethernet frame of standardized set of bits used
to carry data over the system.
Uses Star topology and 1-persistent CSMA/CD
Logical Link Control
Acts as interface between MAC and upper
Accountable for flow control.
Manages error control and error detection.
Independent of topology and medium.
Specifies method of addressing whether
unicast multicast or broadcast.
Governs the operation of access method and
frames data from upper layer and passes them
for encoding. Access method is 1-persistent
Encoding/decoding of signals mostly using
Specification of the transmission medium.
Transmission medium and topology
Data Transfer through Ethernet
Twisted pair or fiber optics with RJ-45 plug or sockets.
Data rate range from 100Mbits/s to 1000Mbits/s
To network multiple devices, switch is required where
devices are connected to it using regular network
Registers MAC address of each devices connected to
it. When switch receives data, it forwards it only to the
port that is connected to the device with appropriate
destination MAC address.
Token Ring Network
Originally developed by IBM in 1970’s
Still IBM’s primary LAN technology
In cases of heavy traffic, the token ring
network has higher throughput than
Ethernet due to the deterministic (nonrandom) nature of the medium access
Is used in applications in which delay
when sending data must be predictable
Is a robust network i.e. it is fault
tolerant through fault management
Can support data rates of around 16
Typically uses twisted pair
FDDI (Fiber Distributed Data
FDDI is a standard developed by the
American National Standards Institute
(ANSI) for transmitting data on optical
Supports transmission rates of up to 200
Uses a dual ring
First ring used to carry data at 100
Mbps. Second ring used for primary
backup in case first ring fails
If no backup is needed, second ring
can also carry data, increasing the data
rate up to 200 Mbps
Supports up to 1000 nodes
Wide Area Network
Contains collection of machines called hosts running
user program and the host are connected by subnet
which carries message from host to host.
Host owned by customers and subnet by ISP.
Subnet consists of transmission lines and switching
Switching computers are called routers
Wide Area Network
Packets sent from receiver to sender may be stored at
intermediate router till the required output line is busy.
This principle is known as “Store-n-Forward” which is
implemented by almost all WAN except using Satellite.
Sender cuts the data into packets bearing a number
and injected into network one at a time and at the
receiver they are reassembled into original message.
Devices on subscriber premises are called Customer
Premises Equipment owned by the subscriber.
Copper or fiber cable connects CPE to Central
Office (CO) which is known as local loop.
WAN Link Options
When a subscriber makes a telephone call, the dialed
number is used to set switches in the exchanges
along the route of the call so that there is a continuous
circuit from the originating caller to that of the called
party where the internal path is shared by a number of
Time division multiplexing (TDM) is used to give each
conversation a share of the connection in turn.
An alternative is to allocate the capacity to the traffic
only when it is needed. Share the available capacity
between many users
Huge data converted into packets which passes from
exchange to exchange through network.
Allows same data path to be shared between various
users in the network.
A point-to-point link provides a pre-established WAN
communications path from the customer premises
through the provider network to a remote destination.
Point-to-point lines are usually leased from a carrier
and are called leased lines.
Leased lines provide direct point-to-point connections
between LANs and connect individual branches to a
Many WANs are connected to the Internet to provide
alternative for inter-branch connections.
Since the Internet probably exists everywhere that the
enterprise has LANs, there are two principal ways that
this traffic can be carried.
Each LAN can have a connection to its local ISP, or
there can be a single connection from one of the core
routers to an ISP.
The advantage is that traffic is carried on the Internet
rather than on the enterprise network, possibly leading
to smaller WAN links.