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Lesson 04
Topology of a LAN means its physical lay out. There are three main types of Network Topology, namely,
Bus Topology, Star Topology and Ring Topology as shown in Figures 1-3 below. There are two different
approaches to networking ­ Server-based and Peer-to-Peer approach. In server-based approach there is a
centralized server machine that can serve users' requests for the sharing of resources on a network.
Contrarily, in peer-to-peer, the machines on the network have an equal capability which means that they can
act both as a client and a server.
Fig. 1
Fig. 2
Fig. 3
Networking devices
These provide central connection point for a LAN. They organize cables and relay data signals to all
computers. There are ports on the back of a hub used to connect computers. They come in different sizes
and shapes.
Repeaters regenerate signals. A repeater would amplify the entire electric signal it receives. However, it has
no capabilities of directing network traffic.
A Bridge is a combination of hardware and software. It has its own operating system. It helps to conserve
the bandwidth of a network. Bandwidth is the speed of a network. It indicates how many bits can be
transmitted across a network in a given time. In case of a bridge, the larger network is physically chopped
into smaller segments. A bridge can read the MAC (Media Access Control) or physical address of a
computer on data packets. MAC address is printed on the Network Interface Card. A bridge matches this
MAC address with the one stored in its table and judges that which particular segment does a data packet
belong to and accordingly sends that packet to such a segment. It does not allow other packets belonging to
other segments to spread to a particular segment and hence conserves the bandwidth. (See Fig. 4 below)
Fig. 4
A switch is also a combination of hardware and software having its own operating system. Like bridges, the
switches are also used to increase the bandwidth of a network. However, in case of a switch, a network is
virtually divided into small segments called Virtual LANs or VLANs. Similar type of users can be grouped
into a VLAN despite that they have no physical proximity or closeness. A switch would read the MAC
address on a data packet and match it with the list of MAC addresses of the user machines contained in it. It
then sends data packets only to that machine on a VLAN to which these packets relate. Packets belonging
to other VLANs are not allowed to enter into a particular VLAN, hence bandwidth increases.
Routers use a combination of hardware and software to forward data packets to their destination on the
internet. They are more efficient and sophisticated than bridges and switches. They can divide large
networks into logical segments called Subnets on the basis of IP addressing scheme. A router can
communicate with another router on the internet with the help of Routing Information Protocol, and thus
it can build a routing table. This routing table contains information about different network addresses and
different routes for delivery of data packets to a host machine on a network. A router is a decision making
device. When a data packet arrives at a router it reads the network addresses from the IP address on the
data packet and determines the shortest possible path through which this packet can be delivered to its
Following diagram (Fig. 5) explains how routers on the internet backbone forward data packets by
determining the shortest possible path for the destination. In this example a client machine `A' sitting on
network no. 140.57 sends data to a web server `B' hosted at network no. 100. `R1' is the router attached to
network no. 140.57. After consulting its routing table it can find that the shortest possible path for sending
data to machine `B' is to forward data packets to `R2', the router attached to network no. 100. In case there
is a blockade then`R1' can look for the alternative shortest path. Here, the alternative shortest path for the
delivery of data packets to `B' can be R1-R3-R4-R2.
Fig. 5
Cabling options
Cooper-based cables
They are quite commonly used for connecting computers. They have two main types ­ coaxial and twisted
pair. In twisted pair there are two further options ­ unshielded twisted pair (UTP) and Shielded Twisted
Pair (STP).
Fiber optic cables
They are used on the internet. They deliver data at a very high speed. They use glass or plastic filaments and
the pulses of light as a data transfer method.
Telephone and electrical wire networks
They can also be used for connectivity purpose.
Wireless options
Wireless options include radio connectivity (wi fi, bluetooth), infrared connectivity and Satellite microwave
Address resolution protocols (ARP) and address resolution protocol (RARP)
Each computer on the internet prepares a list of its IP address and corresponding MAC address using ARP.
Through RARP this information is forwarded to a network server. When a data packet arrives at the
destination router, it inquires about the corresponding MAC address form the network server, inserts it and
delivers the packet to that MAC address. A data packet therefore needs both IP and MAC addresses to
reach its destination. (See Fig. 6 below)
Fig. 6
Role of ISPs on internet
Internet Service Providers (ISPs) are those organizations which can provide internet connections, offer
services of web hosting, newsgroups and internet e-mail etc. They work in a hierarchy. The ISP at the top of
the hierarchy which is connected with internet backbone is called Internet Access Provider or IAP. An
internet trunk line or backbone passing through a metropolitan area is also called Metropolitan Area
Ethernet (MAE). IAPs are connected to MAEs at points called Network Access Points (NAPs). Trunk lines
allow IAPs to move data. The actual routing of data packets takes place on internet backbone (see Fig. 7
Fig. 7