1. Tutor: Dr. Youssef Harrath
Meeting 12 – Module 3
Managing Networks
Book N: Networks
Tutor: Dr. Youssef Harrath

2. Part 1
Local area networks

3. 1. introduction
In this module we describe Computer/Data Communication as apposed to speech communication.
In this module we have chosen to explain the Ethernet standard for LAN (Local Area Network) which is defined in the IEEE 802.
Reasons for linking computers together:
To share hardware resources such as printers or scanners.
Data is stored in a distributed database which is made up from files stored on different computers linked via a network.
To allow people to communicate with each other.

4. 1. introduction: Measuring data
Data communication is a general term used to describe the sending of data over communication networks
The number of bits transferred during a given period of time is called bit-rate, expressed in bits per second (bit/s).
For digital systems, the maximum bit-rate that a channel can handle is called the bandwidth.
The term bandwidth has a different definition for analogue systems, where it is defined as the difference between the highest and lowest frequencies present in a signal.

5. 1. introduction: Measuring data
Communication world
Computer world
kilo bit (k)
103
Kilo Byte (K)
210
Mega bit (M)
106
Mega Byte (M)
220
Giga bit (G)
109
Giga Byte (G)
230

6. 1. introduction: Measuring data
data rate =
Example: the user transfers 100 × 1024 × 8 bits in 5 minutes. The data transfer rate is therefore:
= 2730 bit/s = 2.73 kbit/s

7. 1. introduction: Measuring data
Sometimes you are given the data rate and asked how long the transfer will take. Thinking of this with some simple numbers (for example 1000 bits to be transferred at 10 bit/s must take 1000 ÷ 10 seconds = 100 seconds) leads to:              
time taken for transfer =

8. 1. introduction: Connecting computers
Point-to-point connection: A simple point-to-point connection of two computers with a suitable cable, as shown in figure below  this requires a cable, with appropriate connectors, network interface cards (NIC), and some software to control the data transfer.         
A point-to-point connection

9. 1. introduction: Connecting computers
Local Area Network: A collection of computers which are reasonably closely located (e.g. within a building or a group of buildings on one site) can be connected together to form a local area network (LAN).            
Wide Area Network: If the computers are far apart the network connecting them is described as a Wide Area Network (WAN). WANs can also be used to connect a number of geographically separated LANs.  

10. 1. introduction: Connecting computers
These different kinds of networks – LANs and WANs – can all be connected together to form a ‘network of networks’, described as an inter-network. The Internet (with a capital I) is one internetwork.
Concept of medium:
Many different types of medium are used to connect computers.
We usually think of connections as some kind of ‘wire’, such as optical-fibre, co-axial cable or twisted pair.
Computers can also be connected using ‘wireless’ communication, for instance microwave or infra-red links.

11. 1. introduction: Connecting computers
Concept of Network topology:
Computers can be connected together in different configurations. These arrangements are described as network topologies .
Star topology: In a network of computers with a star topology, each computer has its own link to a central point, called a hub

12. 1. introduction: Connecting computers
Mesh topology: We connect each new PC to each of the other PCs, to form a fully connected mesh. As more computers are added, the number of links required can become very large. But it is not necessary to have point-to-point connections between all the computers, because communication can go between two computers via others.

13. 1. introduction: Connecting computers
Ring topology: Here each computer is connected to just two neighbors. Some Local Area Networks use a ring topology.

14. 1. introduction: Connecting computers
Tree topology: Another arrangement which relies on communication via intermediate computers is a tree

15. 1. introduction: Connecting computers
Bus topology: The final topology we wish to introduce (Figure 3e) is called a bus. You can imagine this as a piece of cable with computers connected to it at intervals. When one computer needs to communicate with another it sends a signal onto the cable (the bus) and that signal is then available to all the other computers.

16. 1. introduction: Connecting computers
for n the number of nodes (computers):
number of links =
if n = 4, number of links = = = 6.
For 100 computers, the number of connections is:
                                    =                = 4950
Connecting a large number of computers in a mesh soon becomes impractical. LANs are designed to support many users, so the idea of sharing a channel is central to the protocols used in local area networks.

17. 2. Ethernet
Packet concept:
The most important principle to have developed out of the interconnection of computers is the organization of data into chunks called packets; hence the term ‘packet networks’.
However, the details of the packets used will differ between a LAN and the Internet.
Frame concept:
This concept is similar to the packet concept in that a frame deals with chunks of data.
The frame concept is mostly used in communication systems (Layer 1) sense and is related to time, for example a time frame.

18. 2. Ethernet
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