1. NET 221D:Computer Networks Fundamentals
Networks and Communication Department
Lecture 1: Introduction

2. Introduction
The merging of computers and communications has had a profound influence on the way computer systems are organized.
Computer Network: a collection of autonomous computers interconnected by a single technology.
Networks are usually connected together to make larger networks.

3. Uses of Computer Networks
Business Applications.
Home Applications.
Mobile Users.

4. Business Applications of Networks
Goals: sharing physical resources such as printers or sharing information (product information, inventories, financial statements, …).
Model: one or more databases with company information and some number of employees who need to access them remotely.

5. Business Applications of Networks
A network with two clients and one server.

6. Business Applications of Networks
A computer network can provide a powerful communication medium among employees : .
Telephone calls between employees may be carried by the computer network instead of by the phone company: IP telephony or Voice over IP (VoIP).
Doing Business electronically: E-commerce.

7. Home Network Applications
Home users can access information, communicate with other people, and buy products and services with e-commerce.
Much of this information is accessed using the client- server model.
A different model for accessing information is peer- to-peer communication.

8. Home Network Applications
Peer-to-peer communication is often used to share music and videos.
Social network applications: the flow of information is driven by the relationships that people declare between each other.
Groups of people can work together to create a content: a wiki.
Many people already pay their bills, manage their bank accounts, …

9. Home Network Applications
Peer to peer model:
Some forms of E-commerce:

10. Mobile Network Users
Combinations of wireless networks and mobile computing.

11. Physical Structures: Types of connections
1. Point –to-point
Dedicated link between two devices. Most of them uses an actual length of wire or cable to connect the two ends but other options ,such as microwave satellite are possible.
Behrouz A. Forouzan” Data communications and Networking

12. Physical Structures: Types of connections
2. Multipoint
A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link.
In a multipoint environment, the capacity of the channel is shared as:
spatially: If several devices can use the link simultaneously
Temporally: If users must take turns, it is a timeshared connection.

13. Physical Structures: physical topology
The way in which a network is laid out physically.
It is the geometric representation of the relationship of all the links and linking devices (nods) to one another.
Behrouz A. Forouzan” Data communications and Networking

14. Fully connected mesh topology (for five devices)
Every device has a dedicated point-to-point link to every other devices.
Fully connected mesh network has n(n-1)/2 physical connection to link n devices.
Every device on the network must have n-1 input/output (I/O) ports.
Behrouz A. Forouzan” Data communications and Networking

15. Mesh topology Advantages:
Privacy or security: every message travels along a dedicated line, and only the intended recipient sees it.
Eliminating the traffic problems: the use of dedicated links guarantees that each connection can carry its own data load; that can occur when links must be shared by multiple devices.
A mesh is robust. If one link becomes unusable, it does not incapacitate the entire system.
Fault identification and fault isolation easy. This enables the network manager to discover the precise location of fault and aids in finding its cause and solution.
Behrouz A. Forouzan” Data communications and Networking

16. Mesh topology Disadvantages
Every device must be connected to every other device, installation and reconnection are difficult .
The sheer bulk of the wiring can be greater than the available space can accommodate.
The H.W required to connect each link (I/O ports and cable) expensive.
Behrouz A. Forouzan” Data communications and Networking

17. Start topology
Each device has a dedicated point-to-point link only to a central controller (hub).
Unlike a mesh , a star topology does not allow direct traffic between devices.
If one device want to send data to another , it send it to the hub, which send it to other device.
Behrouz A. Forouzan” Data communications and Networking

18. Start topology Advantages
Easy to install and to reconfigure and it is less expensive.
Robustness: if one link fails, only that link affected and other links remain active.
Identification and fault isolation.
Disadvantages
The dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.
Behrouz A. Forouzan” Data communications and Networking

19. Tree topology
Not every device plugs directly into the central hub. The majority of devices connect to secondary hub that in turn is connected to the central hub.
The advantages and disadvantages of tree topology are generally the same as those of star .
Behrouz A. Forouzan” Data communications and Networking

20. Bus topology
Nodes are connected to the bus cable by drop lines and taps.
A drop line is a connection running between the device and the main cable.
A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core.
Behrouz A. Forouzan” Data communications and Networking

21. Bus topology Advantages
Easy of insulation: use less cabling than mesh or star.
Disadvantages
A fault in bus cable (break) stops all transmissions even between devices on the same side of the problem.
Reconnection
It can be difficult to add new devices (adding more require modification or replacement of the backbone).
Behrouz A. Forouzan” Data communications and Networking

22. Ring Topology
Each device has a dedicated point-to-point connection only with the two devices on either side of it.
A signal is passed along the ring in one direction from device until it reaches its destination.
Each device in the ring incorporate as repeater.
Repeater :regenerates the signal. It receives a weakened signal, creates a copy, bit for bit, at the original strength.
Behrouz A. Forouzan” Data communications and Networking

23. Ring Topology Advantages:
Easy to install and reconfigure: Each device is linked only to its immediate neighbors.
Fault isolation is simplified : A signal is circulating at all times (token) if one device does not receive a signal within specified period, it can issue an alarm.
Disadvantages
Unidirectional traffic: A break in the ring (such as disabled station) can disable the entire network. This can be solved by use dual ring.
Behrouz A. Forouzan” Data communications and Networking

24. A Hybrid topology
Behrouz A. Forouzan” Data communications and Networking

25. Personal Area Network
Bluetooth is used to connect wirelessly a computer with its peripherals.
PANs can also be built with other technologies that communicate over short ranges, such as RFID.

26. Local Area Networks
A LAN is a privately owned network that operates within and nearby a single building like a home, office or factory.
Every computer has a radio modem and an antenna that it uses to communicate with other computers.
Each computer talks to a device in the ceiling called an AP (Access Point), wireless router, or base station.
IEEE , popularly known as WiFi is a communication standard for LANs. It runs at speeds anywhere from 11to hundreds of Mbps.

27. Local Area Networks

28. Local Area Networks
Most of wired LANs use copper wires, but some use optical fiber.
Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps, have low delay (microseconds or nanoseconds), and make very few errors.
It is just easier to send signals over a wire or through a fiber than through the air.
The topology of many wired LANs is built from point-to-point links.
IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN.

29. Metropolitan Area Networks
A metropolitan area network based on cable TV.

30. Wide Area Networks
WAN (Wide Area Network) spans a large geographical area, often a country or continent.
Consist of hosts and communication subnets.
In most WANs, the subnet consists of two distinct components: transmission lines and switching elements.
When data arrive on an incoming line, the switching element must choose an outgoing line on which to forward them.

31. Wide Area Networks
The hosts and subnet are owned and operated by different people.
The routers will usually connect different kinds of networking technology.
What is connected to the subnet? This could be individual computers, as was the case for connecting to LANs, or it could be entire LANs.

32. Wide Area Networks
Relation between hosts on LANs and the subnet.

33. Wide Area Networks
A stream of packets from sender to receiver.

34. Internetworks
People connected to one network often want to communicate with people attached to a different one.
A collection of interconnected networks is called an internetwork or internet.
The Internet uses ISP networks to connect enterprise networks, home networks, and many other networks.
Gateway: connects two or more networks and provides the necessary translation, both in terms of hardware and software.

35. Network Software
Most networks are organized as a stack of layers or levels, each one built upon the one below it.
The purpose of each layer is to offer certain services to the higher layers.
A protocol is an agreement between the communicating parties on how communication is to proceed.
The interface defines which primitive operations and services the lower layer makes available to the upper one.

36. Network Software Protocol Hierarchies
Layers, protocols, and interfaces.

37. Protocol Hierarchies
The philosopher-translator-secretary architecture.

38. Protocols Set of rules that governs data communications.
Protocol defines :
What is communicated?
How it communicated?
When it is communicated?
Key elements of a protocol: Syntax, semantics and timing Syntax:
Structure or format of the data, meaning the order in which they are presented. Example: A simple protocol might expect the first byte of data to be the address of the sender, the second byte to be the address of the receiver and the reset of the stream to be the message itself.
Behrouz A. Forouzan” Data communications and Networking

39. Protocols Semantics: •Refers to the meaning of each section of bits.
• Example: does an address identify the route to be taken or the final destination of the message.
• Timing:
•When data to should be sent?
•How fast they can be sent?
Example: If a sender produces data at 100Mpbs but the receiver can process data at only 1Mpbs, transmission will overload the receiver and data will be largely lost.
Behrouz A. Forouzan” Data communications and Networking