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COMPUTER NETWORKS

1.1

Course Outcomes:

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CO1: Demonstrate the modern network architectures from a design perspective (Understand- L2)

CO2: Apply various Data Link layer design issues and error detection & correction techniques to solve collisions problems. (Apply- L3)

CO3: Demonstrate the network Layer functionalities (Understand- L2).

CO4: Outline the functions of transport layer protocols (Understand- L2) .

CO5: Examine different application layer protocols. (Understand- L2) .

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Unit-1

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Data Communication Components: Representation of data and its flow Networks, Various Connection Topology, Protocols and Standards, OSI model, Transmission Media, LAN: Wired LAN, Wireless LANs, Connecting LAN and Virtual LAN.

1.3

• COMPUTER NETWORK

Computer Networks are used for data communications

• Communication: There are two modes
• Local : face to face conversation
• Remote: over a distance [e.g.: telegraph]

Data Communication: Exchanging of data b/w the devices via some form of transmission medium.

Computer network:  Two or more computers that are connected with one another for the purpose of communicating data and share resources with each other.

DATA REPRESENTATION

• Data is collection of raw facts which is processed to deduce information. There may be different forms in which data may be represented. Some of the forms of data used in communications are as follows:
• 1. Text Text includes combination of alphabets in small case as well as upper case. It is stored as a pattern of bits. Prevalent encoding system : ASCII, Unicode
• 2. Numbers Numbers include combination of digits from 0 to 9. It is stored as a pattern of bits. Prevalent encoding system : ASCII, Unicode
• 3. Images ―An image is worth a thousand words‖ is a very famous saying. In computers images are digitally stored. A Pixel is the smallest element of an image. To put it in simple terms, a picture or image is a matrix of pixel elements.

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• 4. Audio Data can also be in the form of sound which can be recorded and broadcasted.

Example: What we hear on the radio is a

source of data or information. Audio data is

continuous, not discrete.

• 5. Video Video refers to broadcasting of data in form of picture or movie

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• Characteristics of Data Communication:

Delivery: The system must deliver the data to correct destination. Data must be received by intended device or user.

Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected and unusable.

Timeliness: The system must deliver the data in a timely manner. Data delivered late are useless.

Jitter: It refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio/video packets.

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• Components of Data Communication:

There are 5 components in data communication

Message: It is the information (data) to be communicated. Popular forms of information includes text, numbers, pictures, audio, & videos.

Sender: It is the device that send the message . It can be a computer, workstation, telephone, handset, video camera.

Receiver: It is the device that receives the message.

Transmission Medium: It is the physical path by which message travels from sender to receiver.

Eg: Fiber optic , Twisted pair, Co-axial cable.

Protocol: It is set of rules and regulations b/w two devices that govern data communication.

Without protocol, 2 devices may be connect, but there is no communication b/w two devices

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Types of Dataflow:

How the data is transmitted is called as dataflow. There are 3 types of dataflow.

Simplex: In simplex the communication is uni-directional. Only one of the two devices on a link can transmit, the other can only receive.

Eg: Keyboard & Traditional Monitor

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• Half-Duplex:

In this mode each station can both transmit & receive, but not at the same time. When one device is sending the other can only receive vice-versa.

Ex: Railway Track

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1.11

Full-Duplex or Duplex:

In this mode both stations can transmit and receive simultaneously.

Ex: Telephone conversation

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1.12

Computer network:

A group of computers which are connected to each other for the purpose of sharing their resources is called computer network.

McGraw-Hill

• The McGraw-Hill Companies, Inc., 2000

Characteristics of computer network:-�

• Resource sharing
• Communication speed
• Backup
• Scalability
• Reliability
• S/W & H/W sharing
• Security

Computer Network Topology

The physical arrangement of the computer system which is connected to each other via communication medium is called topology.

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Network really has two shapes or two types of topology.

1. Physical topology
2. Logical topology

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McGraw-Hill

• The McGraw-Hill Companies, Inc., 2000

�1)Physical topology:�

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The physical topology is the physical shape or layout of the wires that can be visible in a network. The Physical topology defines how devices are interconnected with or without wires. Physical topology is further divided into two sections.

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• Point-to-point connections
• Multipoint connections

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McGraw-Hill

• The McGraw-Hill Companies, Inc., 2000

Point-to-point connections

In a point-to-point connection, a communication link is established between two devices with one wire or air (in the case of wireless). A simple example of point-to- point connection is talking over telephone between two persons where anyone else is not allowed to use the phone on either side.

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Multipoint Connection

Multiple devices or machines are involved in a multipoint connection. These devices mainly share cabling but each device needs to have a unique number to identify each other for sending data between them. A good example of a multipoint connection is communication between a group of computers in a network.

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2.Logical topology:-

A logical topology defines how devices communicate with each other across the physical topology. The physical and logical topologies are independent of each other. The physical topology refers to the physical layout of the wires where the logical topology refers to how data moves through the network.

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McGraw-Hill

• The McGraw-Hill Companies, Inc., 2000

Bus Topology:

One long Single Communication Channel & all the devices are Connected to this Cable.

Advantages:

• Easy to add/remove nodes in a N/w
• Required only Cable
• less expensive.
• It broad cast the messages to each device which are connected through the cable.
• It is easy to maintain.
• In Case of any Computer failure, there will be no effect on other devices.

Disadvantages:

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• If Cable is fail then the entire network will be failed.
• It takes more time to pass the messages from one place to another place.
• The length of Cable is limited
• In this topology data is transmitted only one direction.

Ring topology:

• It is called ring topology because it forms a ring. In this each node is Connected strongly With its adjacent node.

Advantages:

• It forms a strong network.
• Each an every node can share data with

another node connected through a ring topology.

• Transmission rate of data is very speed.
• The data send through ring topology will be

broadcast

Disadvantage:

• It is very difficult task to add some new computer.
• If we want to send data from a source to destination machine then data will un necessary passed to all nodes.
• Single point of failure , that means if a node goes down entire network goes down.
• It is very difficult to recover the ring topology if any particular machine is not working properly.
• We can’t send private messages.

Star Topology:

In star topology all nodes are connected with a central device called HUB. And the sharing of data is only possible through HUB.

McGraw-Hill

• The McGraw-Hill Companies, Inc., 2000

Advantages:

• Easy to add devices as the network expands
• One cable failure does not bring down the entire network (resilience)
• Hub provides centralized management.
• Easy to find device and cable problems.
• Can be upgraded to faster speeds.
• Lots of support as it is the most used

Disadvantages:

• A star network requires more cable than a ring or bus network.
• Failure of the central hub can bring down the entire network .
• Costs are higher (installation and equipment) than for most bus networks.

Mesh Topology�

• In a mesh network, every network device is connected to every other network device with a point-to-point connection.

Advantages:

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• Traffic problem can be avoided.
• It is robust . If any one link get damaged it cannot affect others.
• It gives privacy and security.

Disadvantages:

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• The amount of cabling and the number of I/O ports required are very large.
• Hardware required to connected each device is highly expensive.

Hybrid Topology:

• A hybrid topology is a kind of network topology that is a combination of two or more network topologies, such as mesh topology, bus topology, and ring topology. ....

Tree Topology:

• Alternatively referred to as a star ,bus topology.
• Tree topology is one of the most common network setups that is similar to a bus topology and a star topology.
• A tree topology connects multiple star networks to other star networks.

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TYPES OF NETWORKS

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1. Local Area Network (LAN)

2. Metropolitan Area Network (MAN)

3. Wide Area Network (WAN)

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LAN(Local Area Network):

1.Covers a small region of space, typically a single building.

2. LAN is the smallest network compared to other two

networks.

3. The simplest form of LAN is to connect two computers

together.

4. LAN is operated within a limited physical area such as

at home, school, a single building or several buildings.

5. A network which consists of less than 500 interconnected

devices across several buildings, is still recognized as a

LAN.

6. LAN is very high speed network (from previously 10Mbps)

to 100Mbps,which is faster than MAN and WAN.

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MAN(Metropolitan Area Network):��

• Is collection of LANs with the same geographical area, for instance a city.
• Is a network of computers located at different sites within a large physical area, such as a city.
• MAN often acts as a high speed network(although not as fast as LAN) to allow sharing of regional resources.
• MAN can defined as a group of computers and network devices connected together within a large physical area.
• Companies that have several branches within the vijayawada city such as banks, might find a MAN useful to them.

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MAN

�WAN(Wide Area Network):�

• Is the largest network of all network types.
• The internet is the largest WAN in the world.
• WAN generally covers large distances such as states, countries or continents.
• WAN is group of MANs or LANs or the mixture of both network.
• An example in the society using WAN is the banking organization.

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WAN

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2-1 LAYERED TASKS

We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

Sender, Receiver, and Carrier�Hierarchy

Topics discussed in this section:

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Tasks involved in sending a letter

Layers, Services & Protocols:

• The overall communications process between machines connected across one or more networks is very complex
• Layering partitions related communications functions into groups that are manageable
• Each layer provides a service to the layer above
• Each layer operates according to a protocol
• A protocol is a set of precise & unambiguous rules that governs
• how two or more communicating entities in a layer are to interact
• Messages that can be sent and received
• Actions that are to be taken when a certain event occurs
• The purpose of a protocol is to provide a service to the layer above

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2-2 THE OSI MODEL

Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

Layered Architecture�Peer-to-Peer Processes

Encapsulation

Topics discussed in this section:

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ISO is the organization.�OSI is the model.

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Figure 1.2 Seven layers of the OSI model

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Figure 1.3 The interaction between layers in the OSI model

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Figure 1.4 An exchange using the OSI model

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2-3 LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each layer in the OSI model.

Physical Layer�Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer

Topics discussed in this section:

• The purpose of OSI model is to communicate b/w different systems without changing the s/w and h/w.
• Interface defines that what information & services provided by each layer.
• Layers 1,2,3 are the network support layers. It deals with physical aspects of moving data from one device to another.
• Layers 5,6,7 are the user’s support layers.
• Layer 4 –It links two subgroups.
• The upper OSI layers are always implemented in s/w.
• The lower layers are the combination of h/w and s/w except for physical layer(h/w).

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Figure 1.1 Physical layer

The physical layer is responsible for movements of

individual bits from one hop (node) to the next.

Functionalities of Physical Layer:

• The physical layer function is to carry a bit stream over a physical medium .
• It deals with mechanical & electrical specifications of the interface and transmission media.

Representation of bits:

The PL data consists of stream of bits(sequence of 0’s and 1’s) with no interpretation. Bits must be encoded into signals i.e electrical and optical. The PL defines type of encoding.

Data Rate:

Number of bits sent per each second is also defined by PL.

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Functionalities of Physical Layer:

• The physical layer function is to carry a bit stream over a physical medium .
• It deals with mechanical & electrical specifications of the interface and transmission media.

Representation of bits:

The PL data consists of stream of bits(sequence of 0’s and 1’s) with no interpretation. Bits must be encoded into signals i.e electrical and optical. The PL defines type of encoding.

Data Rate:

Number of bits sent per each second is also defined by PL.

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Synchronization of bits:

The sender & receiver must not only use the same bit rate but also be synchronized at the bit level.

Line configuration:

It defines the different types of configurations i.e

1. point-point ii) multipoint
1. It defines type of topologies
2. It defines the type of networks
1. It defines the type of transmission media

i) Simplex

ii) Half Duplex

iii) Duplex or Full Duplex

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Figure 1.6 Data link layer

The data link layer is responsible for moving �frames from one hop (node) to the next.

Functionalities of Data Link Layer:

• Framing: The DLL divide the stream of bits received from the n/w layer into manageable data units called frames.
• Physical Address(MAC-Address):

The size of the MAC address is 48 bits

• Flow Control:

The rate at which the data is absorbed by the receiver is less than the rate produced by sender.

• Error Control: The DLL adds reliability to the physical layer by adding mechanisms to detect and re-transmit the damaged and lost frames.
• Access Control: When two or more devices are connected to the same link.DLL protocols are necessary to determine which device has control over the link at any time.

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Figure 1.8 Network layer

The network layer is responsible for the �delivery of individual packets from

the source host to the destination host.

Functionalities of Network Link Layer:

• The network layer is responsible for the �delivery of individual packets from the source host to the destination host.
• Routing:

When independent n/w’s or link’s connected together to create internet or large n/w. The connecting devices called routers, switches, hubs, gate-ways, repeaters.

• Logical Addressing or IP Addressing:

The size of IP address is 32 bits.

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Figure 1.10 Transport layer

The transport layer is responsible for the delivery �of a message from one process to another.

Functionalities of Transport Layer:

• The transport layer is responsible for the delivery of a message from one process to another.
• Segmentation & Reassemble:

A message is divided into transmittable segments with each segment containing sequence numbers .These numbers is used to identify and separate the packets that were lost in the transmission.

• Connection Control :

A transport layer can be either connection oriented an d connection less.

• Service Point Addressing or Port Address:

The size of port address is 16 bits.

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• Flow Control:

The rate at which the data is absorbed by the receiver is less than the rate produced by sender.

• Error Control:

The DLL adds reliability to the physical layer by adding mechanisms to detect and re-transmit the damaged and lost frames.

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1.58

Figure 1.12 Session layer

The session layer is responsible for dialog �control and synchronization.

Functionalities of Session Layer:

• Dialog Control:

The session layer allows 2 systems to enter into a dialog. It allows the communication b/w the two process to take place in either half-duplex or full-duplex.

• Synchronization:

The session layer allows a process to add checkpoints(synchronization points) into a stream of data.

For ex: If a system is sending a file of 2000 pages it is advisable to insert checkpoints after every 100 pages to ensure that each 100 page unit is received and acknowledge independently.

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Figure 1.13 Presentation layer

The presentation layer is responsible for translation, compression, and encryption.

Functionalities of Presentation Layer:

• The presentation layer is responsible for translation, compression, and encryption.
• Translation:

The processes in two systems are usually exchanging information in the form of character string, numbers and so on. The information should be changed into bit streams before being transmitted because different computer systems have different encoding systems.

Encryption:

To carry sensitive information a system must be able to assure privacy. Encryption means that the sender transforms the original information into another form.

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• Decryption: means reverse the original process to transforms the message into original form.
• Compression:

Data compression reduces the number of bits containing the information. It is important in the transmission of multimedia such as text, audio, & video.

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Figure 1.14 Application layer

The application layer is responsible for �providing services to the user.

Functionalities of Application Layer:

• The application layer is responsible for �providing services to the user.
• It provides user interfaces such as e-mail, remote file access and transfer, directory services.
• Mail Services:

The application layer provide for e-mail forwarding & storage.

• Directory Services:

These application provide distributed database sources and access for global information to various objects and services.

• FTAM(File Transfer Access Management):

This application allows a user to access files in a remote host, to retrieve files from remote computer

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Connecting Devices:

• To connect LAN’s, MAN’s, WAN’s we use connecting devices. Connecting devices operates in different layers of internet model.

Different types of connecting devices:

1. Repeater
2. Hub
3. Bridge
4. Switch
5. Router
6. Gateway

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• Repeater: It operates in the physical layer. It carry the information with in a network can travel a fixed distance before attenuation(weak).It receive the signal and before it become to weak or corrupted . It regenerate the original bit pattern. The repeater sends the refreshed signal. It doesn’t connect 2 LAN’s. It connects segments of the same LAN. It forwards every bit but it has filtering capability.
• HUB: It is a multi code repeater. It is a device allows us to connect multiple computer devices together in a network.
• Bridge: It operates in both physical and DLL.
• Switch: It is an intelligent device that connects several nodes to form a n/w and redirects the received information only to the intended nodes.

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• Router: Router is a 3 layer device. It operates in PL,DLL, N/W layer. It connects different independent networks together to form internet.

A router can connect LAN’s together, A router can connect MAN’s together.

• Gateway: It connects different types of networks. It is capable of understanding address architectures used in different networks and translated them and it can communicate properly.

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2-4 TCP/IP PROTOCOL SUITE

The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.

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Figure 1.16 TCP/IP and OSI model

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Figure 1.18 Relationship of layers and addresses in TCP/IP

• ARP(Address Resolution Protocol): It is used to find the physical address of a node. When its IP address is known.
• RARP(Reverse Address Resolution Protocol): It is used to find the IP address of a node. When its Physical address is known.
• SMTP(Simple Mail Transfer Protocol): It is used for mail transfers
• DNS(Domain Name System): it is used for mapping host names on to their n/w address.
• HTTP(Hyper Text Transfer Protocol): It is used for fetching the web pages on the WWW.
• FTP(File Transfer Protocol): It provides a way to move the data efficiently from one machine to another machine.
• TELNET : It is a virtual terminal protocol allows a user on one machine to log on to different machines and work there.
• IGMP(Internet Group Message Protocol): It is used to facilitate the simultaneous transmission of a message to a group of receivers.
• ICMP(Internet Control Message Protocol): It sends query and error reporting messages.

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Transmission media

• The electromagnetic signals can travel through various transmission media. The transmission media is broadly classified into two categories
• Guided media.
• Unguided media

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A Transmission System

Transmitter

• Converts information into signal suitable for transmission
• Injects energy into communications medium or channel
• Telephone converts voice into electric current; Modem converts bits into tones

Receiver

• Receives energy from medium
• Converts received signal into form suitable for delivery to user
• Telephone converts current into voice; Modem converts tones into bits

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Transmission Impairments

Communication Channel

• Pair of copper wires
• Coaxial cable
• Radio
• Light in optical fiber
• Infrared

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Transmission Impairments

• Signal attenuation
• Signal distortion
• Spurious noise
• Interference from other signals

Guided vs Unguided

The basic difference between guided and unguided media is that,

• In the guided media, the signal travels through a physical medium whereas,
• In unguided media, the signal travel through the air.

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1. Magnetic media
2. Twisted pairs
3. Coaxial cable
4. Power lines
5. Fiber optics

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1. Electro magnetic spectrum
2. Radio Transmission
3. Microwave Transmission
4. Infrared Transmission
5. Satellite Communication

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Guided Transmission Media

• Magnetic media
• Twisted pairs
• Coaxial cable
• Power lines
• Fiber optics

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Magnetic Media:-

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• Write data onto magnetized media
• Several types of magnetized media are used in computer systems,
• including

magnetic tape, floppy disks and hard disk drives.

Advantages:

• Very fast access to data.
• Data can be read directly from any part of the hard disk.
• The access speed is about 1000kb/s.
• Some are very cheap for example floppy disks.
• Store very large amounts of data. Disadvantages:-
• Dust, mechanical problems.
• Gradually lose their charge over time - data lost.
• Hard disks eventually fail -stops the computer working.
• Regular crashes can damage the surface of the disk, leading to loss of data in that sector.
• The disk is fixed inside the computer.

Twisted Pair:-

• It is one of the oldest and most commonly used in transmission media is Twisted pair.
• A twisted pair consists of two insulated copper wires, typically about 1mm thick; twisted together to reduce the susceptibility to interference.
• More twists per cm leads to less crosstalk and better quality over longer distance.
• Application -telephone system.
• Used -transmitting either analog or digital signals.
• Twisted pairs are widely used because high bandwidth and low cost.
• There are 2 types
1. UTP (unshielded Twisted Pair)
2. STP (Shielded Twisted Pair)

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(a) Category 3 UTP (16 MHz).

(b) Category 5 UTP (100 MHz).

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Advantages :

1. Cheaper and far easier to splice
2. Less susceptible to electrical interference caused by nearby equipment or wires.
3. In turn are less likely to cause interference themselves.
4. Because it is electrically "cleaner", STP wire can carry data at a faster speed.

Disadvantages :

1. STP -physically larger and more expensive
2. STP -difficult to connect to a terminating block.

Coaxial Cable:-

• A good combination of high bandwidth and excellent interference immunity
• Higher bandwidth than twisted pair
• Cable TV distribution;
• Long distance telephone transmission
• Used in the original Ethernet LAN medium

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• Two kinds of coaxial cables are widely used

-50 ohm cable is most commonly used for digital transmission.

-75 ohm cable is commonly used for analog transmission.

• Coaxial cables consists of a
• stiff copper wire as the core
• surrounded by an insulating material.
• The insulator is encased by a cylindrical outer conductor (braided mesh).
• The outer conductor is covered in a protective plastic sheath.
• Coaxial cable have high bandwidth and excellent noise immunity.
• The bandwidth depends upon cable quality, length & signal to noise ratio of the data signal.
• Used - in cable television n/w’s and telephone systems and MAN’s.

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Advantages :

1. Support multiple channel - allows for much greater throughput.
2. lower error rates.(Because the inner conductor is in a shield, noise immunity is improved)
3. It has better shielding as compared to twisted pair cable.

Disadvantages:

1. More expensive - to install compare to twisted pair cable.
2. The thicker the cable, the more difficult to work with.

Fiber optics:-

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• Optical communication system has three key components:
1. Light source: A pulse of light
2. Transmission medium: An Ultra thin fiber of glass
3. Detector: Generates an electrical pulse when light falls on it.
• Optical fibers works on the principle of “Total internal reflection”
• Unidirectional data transmission system is used in optical fiber that means it accepts the electrical signal converts and transmits it by light pulses and then reconverts the o/p to an electric signal at the receiving end.
• Transmission of Light through Fiber:

Fiber optic cable transmits light signals which are converted from electrical to light with the help of devices like LED’s(Light Emitting Diode) or LASER’s (Light Amplification Stimulated Emitted Radiations).

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• Optical fibers are smaller and lighter than copper wire. One optical fiber is approximately

the same diameter as human hair. It may be single mode or multi mode.

• Each fiber has an
• inner core of glass or plastic that conducts light.
• The inner core is surrounded by cladding a layer of glass that reflects the light back to

the core.

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• Single mode fiber allows a single light path and use lasers for converting electrical signal into light. Lasers are chosen because they emit only a single wavelength.

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• Multi-mode fibers use multiple light paths. The multi-mode fibers use LED’s for converting electrical signal into light signal. LED’s emit multiple wavelengths.
• Step Index
• Graded Index

Fiber optics

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• Advantages
1. Greater Bandwidth
2. Low Power consumption
3. Interference - Fibre optic cables are immune to electromagnetic interference.
4. Small Size - comparison to copper
5. Light Weight metal wires.
6. High signal Security - Optical fibres are difficult to tap
7. Flexibility - An optical fibre has greater tensile strength than copper or steel fibres of the same diameter.
• Disadvantages
• System installation is very costly
• It is difficult to bend the cable into a small enough radius to break the fibre.
• Protection - Optical fibres require more protection around the cable compared to copper.

Applications:-

1. Computers - Due to higher bandwidth and less risk interference, fiber optics are used in computer networking.

of signal

2. Broadcasting -Fiber optic cables are used in broadcasting to carry data for lots of channels. Higher bandwidth provides better connections and performance.

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• Medical Instruments - These are fiber optics that have enabled medical practitioners to peer inside our bodies without cutting off.

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• Military and Space - missile launch sites and radar tracking stations. Since these cables don't give off electromagnetic radiation, enemies can't detect presence.

Unguided Media:-

• Wireless communication.
• No physical conductor.
• Signals are broadcast through air.

• Terrestrial Micro wave
• Satellite microwave transmission

Radio wave:-

• Radio waves are the electromagnetic waves that are transmitted in all the directions of free space.
• Radio waves are omnidirectional, i.e., the signals are propagated in all the directions.
• The range in frequencies of radio waves is from 3Khz to 1 Ghz.
• In the case of radio waves, the sending and receiving antenna are not aligned, i.e., the wave sent by the sending antenna can be received by any receiving antenna.
• An example of the radio wave are FM radio, television, cordless phones .

Micro wave:-

• Terrestrial Micro wave
• Satellite microwave transmission

Micro wave:-

• Terrestrial Microwave Transmission
• Microwaves are the electromagnetic waves having the frequency in the range from 1GHz to 300 GHz.
• Microwaves are unidirectional as the sending and receiving antenna is to be aligned, i.e., the waves sent by the sending antenna are narrowly focused.
• In this case, antennas are mounted on the towers to send a beam to another antenna

which is km away.

• It works on the line of sight transmission, i.e., the antennas mounted on the towers are the direct sight of each other.
• Satellite Microwave Communication
• A satellite is a physical object that revolves around the earth at a known height.
• Satellite communication is more reliable nowadays as it offers more flexibility than cable and fibre optic systems.
• We can communicate with any point on the globe by using satellite

communication.

Infrared:-

• An infrared transmission is a wireless technology used for communication over short ranges.
• The frequency of the infrared in the range from 300 GHz to 400 THz.
• It is used for short-range communication such as data transfer between two cell phones, TV remote operation, data transfer between a computer and cell phone resides in the same closed area.

• Infrared waves cannot penetrate the walls. Therefore, the infrared

communication in one room cannot be interrupted by the nearby rooms.

• Infrared communication is unreliable outside the building because the sun rays will interfere with the infrared waves.

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17-2 VIRTUAL LANS

A station is considered part of a LAN if it physically belongs to that LAN.

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The criterion of membership is geographic. What happens if we need a virtual connection between two stations belonging to two different physical LANs?

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We can roughly define a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring.

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Figure 17.10: A switch connecting three LANs

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But what would happen if the administrators needed to move two engineers from the first group to the third group, to speed up the project being done by the third group?

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The LAN configuration would need to be changed.

The network technician must rewire.

The problem is repeated if, in another week, the two engineers move back to their previous group.

In a switched LAN, changes in the work group mean physical changes in the network configuration.

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Figure 17.11: A switch using VLAN software

Moving stations from one group to another through software is easier than changing the configuration of the physical network.

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Figure 17.12: Two switches in a backbone using VLAN software

This is a good configuration for a company with two separate buildings.

Chapter 5: Outline

13.1 ETHERNET PROTOCOL

13.2 STANDARD ETHERNET

13.3 FAST ETHERNET

13.4 GIGABIT ETHERNET

13.5 10 GIGABIT ETHERNET

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13-1 ETHERNET PROTOCOL

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The data-link layer and the physical layer are the territory of the local and wide area networks.

​

This means that when we discuss these two layers, we are talking about networks that are using them.

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As we see in this and the following two chapters, we can have wired or wireless networks. We discuss wired networks in this chapter and the next.

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Figure 13.1: IEEE standard for LANs

Ethernet:

• It is one of the most widely used wired LAN technologies
• It operates in the DLL and PL
• Family of networking technologies that are defined in the

IEEE 802.2 and IEEE 802.3 standards

• It supports bandwidths of 10,100,1000,10,000, 40,000, and 100,000Mbps((100 Gbps).

Ethernet Standards:

• It define layer 2 protocols and layer 1 technologies.
• Two separate sublayers of the data link layer to operate –Logical Link Control(LLC) and the MAC sublayers.

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13.1.2 Ethernet Evolution

The Ethernet LAN was developed in the 1970s by Robert Metcalfe and David Boggs.

​

Since then, it has gone through four generations:

• Standard Ethernet (10 Mbps)
• Fast Ethernet (100 Mbps)
• Gigabit Ethernet (1 Gbps)
• 10 Gigabit Ethernet (10 Gbps)

​

as shown in Figure. We briefly discuss all these generations.

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Figure 13.2 : Ethernet evolution

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13-2 STANDARD ETHERNET

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We refer to the original Ethernet technology with the data rate of 10 Mbps as the Standard Ethernet.

​

Although most implementations have moved to other technologies in the Ethernet evolution, there are some features of the Standard Ethernet that have not changed during the evolution.

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We discuss this standard version first.

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13.2.1 Characteristics

Let us first discuss some characteristics of the Standard Ethernet.

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Connectionless and Unreliable Service

Ethernet provides a connectionless service, which means each frame sent is independent of the previous or next frame.

Ethernet has no connection establishment or connection termination phases. The sender sends a frame whenever it has it; the receiver mayor may not be ready for it.

​

Ethernet is also unreliable like IP and UDP. If a frame is corrupted during transmission and the receiver finds out about the corruption, which has a high level of probability of happening because of the CRC-32, the receiver drops the frame silently.

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Frame Format

The Ethernet frame contains seven fields

​

Preamble.

• The first field of the 802.3 frame
• contains 7 bytes (56 bits) of alternating 0 s and 1 s
• The pattern provides only an alert and a timing pulse.
• alerts the receiving system to the coming frame
• and enables it to synchronize its input timing.
• The preamble is actually added at the physical layer and is not (formally) part of the frame.

Start frame delimiter (SFD).

• The second field (1 byte: 10101011) signals the beginning of the frame.
• The SFD warns the station or stations that this is the last chance for synchronization.
• The last 2 bits is 11 and alerts the receiver that the next field is the destination address.

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Destination address (DA).

The DA field is 6 bytes and contains the physical address of the destination station.

Source address (SA).

The SA field is also 6 bytes and contains the physical address of the sender of the packet.

Length or type.

This field is defined as a type field or length field.

The original Ethernet used this field as the type field to define the upper-layer protocol using the MAC frame.

The IEEE standard used it as the length field to define the number of bytes in the data field.

Both uses are common today.

​

Data.

This field carries data encapsulated from the upper-layer protocols.

It is a minimum of 46 and a maximum of 1500 bytes.

CRC.

The last field contains error detection information, in this case a CRC-32

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Figure 13.3: Ethernet frame

Frame Length

​

• Ethernet has imposed restrictions on both the minimum and maximum lengths of a frame, as shown in Figure.
• The minimum length restriction is required for the correct operation of CSMA/CD
• An Ethernet frame needs to have a minimum length of 512 bits or 64 bytes.
• Part of this length is the header and the trailer.
• If we count 18 bytes of header and trailer
• 6 bytes of source address,
• 6 bytes of destination address,
• 2 bytes of length or type,
• 4 bytes of CRC),
• Then the minimum length of data from the upper layer is 64 - 18 = 46 bytes.
• If the upper-layer packet is less than 46 bytes, padding is added to make up the difference.

Frame Length

• The standard defines the maximum length of a frame (without preamble and SFD field) as 1518 bytes.
• If we subtract the 18 bytes of header and trailer, the maximum length of the payload is 1500 bytes.
• The maximum length restriction has two historical reasons.
• First, memory was very expensive when Ethernet was designed:
• a maximum length restriction helped to reduce the size of the buffer.
• Second, the maximum length restriction prevents
• one station from monopolizing the shared medium,
• blocking other stations that have data to send.

​

​

Frame length:

Minimum: 64 bytes (512 bits) �Maximum: 1518 bytes (12,144 bits)

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13.2.2 Addressing

Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network interface card (NIC).

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The NIC fits inside the station and provides the station with a link-layer address.

The Ethernet address is 6 bytes (48 bits), normally written in hexadecimal notation, with a colon between the bytes. For example, the following shows an Ethernet MAC address:

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Figure 13.4: Unicast and multicast addresses

• Unicast, Multicast, and Broadcast Addresses
• A source address is always a unicast address
• the frame comes from only one station.
• The destination address, can be unicast, multicast, or broadcast.
• Figure 13.7 shows how to distinguish a unicast address from a multicast address.
• If the least significant bit of the first byte in a destination address is
• 0, the address is unicast;
• otherwise, it is multicast.

​

​

• A unicast destination address defines only one recipient; the relationship between the sender and the receiver is one-to-one.
• A multicast destination address defines a group of addresses; the relationship between the sender and the receivers is one-to-many.
• The broadcast address is a special case of the multicast address; the recipients are all the stations on the LAN. A broadcast destination address is forty-eight 1s.

​

Example 13.1

• Define the type of the following destination addresses:

a. 4A:30:10:21:10:lA

b. 47:20:1B:2E:08:EE

c. FF:FF:FF:FF:FF:FF

Unicast, Multicast, and Broadcast Addresses

Solution

• we need to look at the second hexadecimal digit from the left.
• If it is even, the address is unicast.
• If it is odd, the address is multicast.
• If all digits are F's, the address is broadcast.
• Therefore, we have the following:

a. This is a unicast address because A in binary is 1010 (even).

b. This is a multicast address because 7 in binary is 0111 (odd).

c. This is a broadcast address because all digits are F's.

• The way the addresses are sent out on line is different from the way they are written in hexadecimal notation. The transmission is left-to-right, byte by byte; however, for each byte, the least significant bit is sent first and the most significant bit is sent last. This means that the bit that defines an address as unicast or multicast arrives first at the receiver.

The least significant bit of the first byte �defines the type of address.�If the bit is 0, the address is unicast;�otherwise, it is multicast.

The broadcast destination address is a special case of the multicast address in which all bits are 1s.

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Figure 13.5: Implementation of standard Ethernet

13.2.4 Efficiency of Standard Ethernet

The efficiency of the Ethernet is defined as the ratio of the time used by a station to send data to the time the medium is occupied by this station.

​

The practical efficiency of standard Ethernet has been measured to be

13.2.5 Implementation

The Standard Ethernet defined several implementations, but only four of them became popular during the 1980s.

​

Table 13.1 shows a summary of Standard Ethernet implementations.

Figure 13.6: Encoding in a Standard Ethernet

Encoding and Decoding

• All standard implementations use digital signaling (baseband) at 10 Mbps.
• At the sender and receiver, data are converted and interpreted to a digital signal using the Manchester scheme;
• Manchester encoding is self-synchronous, providing a

transition at each bit interval.

​

10Base5: Thick Ethernet

• The first implementation is called 10Base5, thick Ethernet, or Thicknet.
• The nick-name derives from the size of the cable, which is roughly the size of a garden hose and too stiff to bend with your hands.
• 10Base5 was the first Ethernet specification
• To use a bus topology with an external transceiver (transmitter/receiver)
• connected via a tap to a thick coaxial cable.

10Base5: Thick Ethernet

• The transceiver is responsible for
• transmitting,
• receiving,
• and detecting collisions.
• The transceiver is connected to the station via a transceiver cable that provides separate paths for sending and receiving;
• collision can only happen in the coaxial cable.
• The maximum length of the coaxial cable must not exceed 500 m, otherwise, there is excessive degradation of the signal.
• If a length of more than 500 m is needed, up to five segments, each a maximum of 500-meter, can be connected using repeaters.

​

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Figure 13.7: 10Base5 implementation

10Base2: Thin Ethernet

• The second implementation is called 10Base2, thin Ethernet, or cheapernet.
• 10Base2
• uses a bus topology,
• the cable is much thinner and more flexible.
• The transceiver is normally part of the network interface card (NIC), which is installed inside the station.

10Base2: Thin Ethernet

• Note that the collision here occurs in the thin coaxial cable.
• This implementation is more cost effective than 10Base5 because
• thin coaxial cable is less expensive than thick coaxial
• and the tee connections are much cheaper than taps.
• Installation is simpler because the thin coaxial cable is very flexible.
• However, the length of each segment cannot exceed 185 m (close to 200 m) due to the high level of attenuation in thin coaxial cable.

​

​

Figure 13.8: 10Base2 implementation

10Base- T: Twisted-Pair Ethernet

• The third implementation is called 10Base-T or twisted-pair Ethernet. 10Base-T
• Uses a physical star topology.
• The stations are connected to a hub via two pairs of twisted cable, shown in figure.

10Base- T: Twisted-Pair Ethernet

• Note that two pairs of twisted cable create two paths (one for sending and one for receiving) between the station and the hub.
• Any collision here happens in the hub.
• Compared to 10Base5 or 10Base2, we can see that the hub actually replaces the coaxial cable as far as a collision is concerned.
• The maximum length of the twisted cable here is defined as 100 m, to minimize the effect of attenuation in the twisted cable.

​

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Figure 13.9: 10Base-T implementation

10Base-F: Fiber Ethernet

• Although there are several types of optical fiber 10-Mbps Ethernet, the most common is called 10Base-F.
• 10Base-F uses a star topology to connect stations to a hub.
• The stations are connected to the hub using two fiber-optic cables, as shown in Figure.

Figure 13.10: 10Base-F implementation

MAC Control Layer

• Standard Ethernet was designed as a connectionless protocol at the MAC sublayer.
• There is no explicit flow control or error control to inform the sender that the frame has arrived at the destination without error.
• When the receiver receives the frame, it does not send any positive or negative acknowledgment.
• To provide for flow and error control in full-duplex switched Ethernet, a new sublayer, called the MAC control, is added between the LLC sublayer and the MAC sublayer.

​

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13-3 FAST ETHERNET(100 Mbps)

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In the 1990s, Ethernet made a big jump by increasing the transmission rate to 100 Mbps, and the new generation was called the Fast Ethernet.

​

The designers of the Fast Ethernet needed to make it compatible with the Standard Ethernet. The MAC sublayer was left unchanged.

​

But the features of the Standard Ethernet that depend on the transmission rate, had to be changed.

MAC Sublayer

• It uses the MAC sublayer untouched.
• It uses the star topology with half duplex and full duplex.
• In the half-duplex approach,
• the stations are connected via a hub;
• in the full-duplex approach,
• the connection is made via a switch with buffers at each port.
• It uses the CSMA/CD for the half-duplex approach;
• for full-duplex Fast Ethernet, there is no need for CSMA/CD.
• However, the implementations keep CSMA/CD for backward compatibility with Standard Ethernet.

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13-4 GIGABIT ETHERNET

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The need for an even higher data rate resulted in the design of the Gigabit Ethernet Protocol (1000 Mbps).

​

The IEEE committee calls it the Standard 802.3z.

​

The goals of the Gigabit Ethernet were to upgrade the data rate to 1 Gbps, but keep the

• address length
• the frame format
• and the maximum and minimum frame length the same.

​

• The goals of the Gigabit Ethernet design can be summarized as follows:

1. Upgrade the data rate to 1 Gbps.

2. Make it compatible with Standard or Fast Ethernet.

3. Use the same 48-bit address.

4. Use the same frame format.

5. Keep the same minimum and maximum frame lengths.

6. To support autonegotiation as defined in Fast Ethernet.

Full-Duplex Mode

• In full-duplex mode, there is a central switch connected to all computers or other switches.
• In this mode, each switch has buffers for each input port in which data are stored until they are transmitted.
• There is no collision in this mode, (Switch uses the destination address)
• This means that CSMA/CD is not used.
• Lack of collision implies that the maximum length of the cable is determined by the signal attenuation in the cable, not by the collision detection process.

​

​

In the full-duplex mode of Gigabit Ethernet, there is no collision;

the maximum length of the cable is determined by the signal attenuation �in the cable.

Figure Gigabit Ethernet implementations

13-5 10-GIGABIT EHTERNET

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In recent years, there has been another look into the Ethernet for use in metropolitan areas.

​

The idea is to extend the technology, the data rate, and the coverage distance so that the Ethernet can be used as LAN and MAN (metropolitan area network).

​

The IEEE committee created 10 Gigabit Ethernet and called it Standard 802.3ae.

• The goals of the Ten-Gigabit Ethernet design can be summarized as follows:

1. Upgrade the data rate to 10 Gbps.

2. Make it compatible with Standard, Fast, and Gigabit Ethernet.

3. Use the same 48-bit address.

4. Use the same frame format.

5. Keep the same minimum and maximum frame lengths.

6. Allow the interconnection of existing LANs into a metropolitan area network (MAN) or a wide area network (WAN).

7. Make Ethernet compatible with technologies such as Frame Relay and ATM

​

13.5.1 Implementation

10 Gigabit Ethernet operates only in full-duplex mode, which means there is no need for contention; CSMA/CD is not used in 10 Gigabit Ethernet.

​

Four implementations are the most common:

• 10GBase-SR
• 10GBase-LR
• 10GBase-EW
• and 10GBase-X4.

​

Table shows a summary of the 10 Gigabit Ethernet implementations.

• Token Ring
• Token ring (IEEE 802.5) is a communication protocol in a local area network (LAN) where all stations are connected in a ring topology and pass one or more tokens for channel acquisition.
• A token is a special frame of 3 bytes that circulates along the ring of stations.
• A station can send data frames only if it holds a token. The tokens are released on successful receipt of the data frame.
• Token Passing Mechanism in Token Ring
• If a station has a frame to transmit when it receives a token, it sends the frame and then passes the token to the next station; otherwise it simply passes the token to the next station.

​

• Passing the token means receiving the token from the preceding station and transmitting to the successor station. 
• The data flow is unidirectional in the direction of the token passing. In order that tokens are not circulated infinitely, they are removed from the network once their purpose is completed.
• Token Bus
• Token Bus (IEEE 802.4) is a standard for implementing token ring over virtual ring in LANs. The physical media has a bus or a tree topology and uses coaxial cables.
• A virtual ring is created with the nodes/stations and the token is passed from one node to the next in a sequence along this virtual ring. Each node knows the address of its preceding station and its succeeding station.
• A station can only transmit data when it has the token. The working principle of token bus is similar to Token Ring.

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​

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Differences between Token Ring and Token Bus

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