CS2302-COMPUTERNETWORKS Unit 1

UNIT I

Network architecture – layers – Physical links – Channel Access on links – Hybrid multiple access techniques – Issues in the data link layer – Framing – Error correction and detection – Link-level flow control

Introduction

Computer Network: Definition

          Collection of autonomous computers interconnected by single technology.

Connectivity

          Connectivity occurs between two computers through physical medium like coaxial cable or an optical fiber.

                             Physical Medium – Link

                             Computers  -        Nodes

          When a physical link occurs between a pair of nodes then it is referred as point-to-point.

          When more than two nodes share a single physical link then it is referred as Multiple access.

    Data communication between the nodes is done by forwarding the data from one link to another. The systematic way of organizing these forwarding nodes form a switched network.

Two common types of switched network are

Ø  Circuit switched – e.g. Telephone System

Ø  Packet switched – e.g. Postal System

Packet Switched Network

          In this network nodes send discrete blocks of data to each other. These blocks can be called as packet or message.

Store and forward strategy:

          This network follows this technique. It means “Each node receives a complete packets over the link, stores in internal memory and then forwards to next node”.

Circuit Switched Network

          It first establishes a circuit across the links and allows source node to send stream of bits across this circuit to the destination node

The representation of network is given by cloud symbol

Cloud represents the network

Nodes inside the cloud (Switches) – Implement the network

Nodes outside the cloud (host) -        Use the network

Internetwork

Set of independent network are interconnected to form inter network or internet. Node that is connected to two or more network is called router or gateway. It is responsible for forwarding data between the networks.

Addressing

          The final requirement is that each node must be able to say which of the other node it wants to communicate with.

          This is done by assigning address to each node. when a source node wants to deliver message to destination node, it specifies the address of destination node.

          Switches and Routers use this address to decide how to forward the message. This process based on address is called Routing.

          Unicast – sending message to single node.

          Broadcast – Sending message to all the nodes on the network.

          Multicast – Sending message to some subnet not to all.

Resource Sharing

          Pblm: How do several hosts share the same link when they all want to use it at the same time?

Sol: Multiplexing – System resources are shared among multiple users

Methods:

1.   Synchronous Time Division Multiplexing(STDM)

Divide time into equal sized quanta

2.   Frequency Division Multiplexing(FDM)

Transmit each flow at different frequency

                        3. Statistical Multiplexing

First two methods are limited in 2 ways

Ø  If one flow does not have data to send then its time quantum remains idle, even the other flow has data to transmit.

Ø  No of flows are fixed and known ahead of time, it cannot be resized.

 Statistical methods combine the ideas of both STDM and FDM

Ø  Data from each flow is transmitted on demand so no idle quantum

Ø  It defines upper bound on size of data and it is referred as packet.

Common communication patterns

                   Communication between a pair of processes is done by request / reply basis. The process which sends request is referred as client and the one which honors the request is referred as server.

          This can be done using channels. Two types of channels are

Ø  Request / Reply channels

Ø  Message stream Channels

Reliability

          To get the reliable network, it is necessary to find how network fails.

          Three classes of failures

Ø  Bit error

Ø  Packet loss

Ø  Physical link and node failure

Network Architecture

          Networks do not remain fixed at single point in time, but it must evolve to accommodate changes based on the technologies on which they are based and demands made by application programmer.

          Network architecture guides the design and implementation of network. Two commonly used architecture are

Ø  OSI Architecture

Ø  Internet or TCP/IP architecture

Layering and Protocols

          When the system gets complex, the system designer introduces another level of abstraction. It defines unifying model with important aspects of the system, encapsulated this model in interface objects and hide it from users

          In network, abstraction leads to layering. Layering provides two nice features.

Ø  It decomposes the problem of building a network into more manageable components. Rather than implementing a monolithic piece of software that does everything implement several layers, each of which solves one part of the problem.

Ø  It provides more modular design. To add some new service, it is enough to modify the functionality at one layer, reusing the functions provided at all the other layers.

Protocols

A protocol is a set of rules that governs data communication. It defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics and timing.

Each protocol defines two different interfaces.

Ø  Service interface - to the other objects on the same computer that want to use its communication services. This service interface defines the operations that local objects can perform on the protocol.

Ø  Peer interface - to its counterpart (peer) on another machine. It also defines the form and meaning of messages exchanged between protocol peers to implement the communication service.

Except at the hardware level, peer to peer communication is indirect.

We can represent these protocols as protocol graph. Nodes of the graph correspond to protocols, and the edges represent a depends-on relation.

 For example, the file access program on host 1 wants to send a message to its peer on host 2 using the communication service offered by protocol RRP. In this case, the file application asks RRP to send the message on its behalf. To communicate with its peer, RRP then invokes the services of HHP, which in turn transmits the message to its peer on the other machine. Once the message has arrived at protocol HHP on host 2, HHP passes the message up to RRP, which in turn delivers the message to the file application. In this particular case, the application is said to employ the services of the protocol stack RRP/HHP.

Encapsulation

         

          Control information must be added with the data to instruct the peer how to handle with the received message. It will be added into the header or trailer.

Header - Small data structure from few bytes to few kilobytes attached to the front of message.

Trailer – Information will be added at the end of the message

Payload or message body – Data send by the program

In this case data is encapsulated with new message created by

protocol at each level.In this example HHP encapsulates RRP’s message by attaching a header of its own. Then HHP sends the message to its peer over some network, and then when the message arrives at the destination host, it is processed in the opposite order.

Multiplexing and De-Multiplexing

The fundamental idea of packet switching is to multiplex multiple flows of data over a single physical link. This can be achieved by adding identifier to the header message. It is known as demultiplexing or demux key. It gives the address to which it has to communicate.

The messages are demultiplexed at the destination side. In some cases same demux key is used on both sides and in some cases different keys are used.

CS2302-COMPUTERNETWORKS

17. Define Aggregation points?

            It collects and processes the data they receive from neighboring nodes, and then transmit the processed data. By processing the data incrementally, instead of forwarding all the raw data to the base station, the amount of traffic in the network is reduced.

18. Define Beacons?

            Beacon to determine their own absolute locations based on GPS or manual configuration. The majority of nodes can then derive their absolute location by combining an estimate of their position relative to the beacons with the absolute location information provided by the beacons.

19. What is the use of Switch?

            It is used to forward the packets between shared media LANs such as Ethernet. Such switches are sometimes known by the obvious name of LAN switches.

20. Explain Bridge?

            It is a collection of LANs connected by one or more bridges is usually said to form an extended LAN. In their simplest variants, bridges simply accept LAN frames on their inputs and forward them out on all other outputs.

21. What is Spanning tree?

            It is for the bridges to select the ports over which they will forward frames.

22. What are the three pieces of information in the configuration messages?

1.      The ID for the bridge that is sending the message.

2.      The ID for what the sending bridge believes to the root bridge.

3.      The distance, measured in hops, from the sending bridge to the root bridge.

23. What is broadcast?

            Broadcast is simple – each bridge forwards a frame with a destination broadcast address out on each active (selected) port other than the one on which the frame was received.

CS2302-COMPUTERNETWORKS - Unit 2

1. What are the functions of MAC?

            MAC sub layer resolves the contention for the shared media. It contains synchronization, flag, flow and error control specifications necessary to move information from one place to another, as well as the physical address of the next station to receive and route a packet.

2. What are the functions of LLC?

The IEEE project 802 models take the structure of an HDLC frame and divides it into 2 sets of functions. One set contains the end user portion of the HDLC frame – the logical address, control information, and data. These functions are handled by the IEEE 802.2 logical link control (LLC) protocol. 

3. What is Ethernet?

            Ethernet is a multiple-access network, meaning that a set of nodes send and receive frames over a shared link.

4. Define the term carrier sense in CSMA/CD?

            All the nodes can distinguish between idle and a busy-link and “collision detect” means that a node listens as it transmits and can therefore detect when a frame it is transmitting has interfered (collided) with a frame transmitted by another node.

5. Define Repeater?

            A repeater is a device that forwards digital signals, much like an amplifier forwards analog signals. However, no more than four repeaters may be positioned between any pairs of hosts, meaning that an Ethernet has a total reach of only 2,500m.

6. Define collision detection?

            In Ethernet, all these hosts are competing for access to the same link, and as a consequence, they are said to be in the same collision detection.

7. Why Ethernet is said to be a I-persistent protocol?

            An adaptor with a frame to send transmits with probability ‘1 ‘whenever a busy line goes idle.

8. What is exponential back off?

            Once an adaptor has detected a collision and stopped its transmission, it waits a certain amount of time and tries again. Each time it tries to transmit but fails, the adaptor doubles the amount of time it waits before trying again. This strategy of doubling the delay interval between each transmission attempt is a general technique known as exponential back off.

CS2302-COMPUTERNETWORKS

29. What is the purpose of hamming code?

A hamming code can be designed to correct burst errors of certain lengths. So the simple strategy used by the hamming code to correct single bit errors must be redesigned to be applicable for multiple bit correction.

30. What is redundancy?

It is the error detecting mechanism, which means a shorter group of bits or extra bits may be appended at the destination of each unit.

31. Define flow control?

Flow control refers to a set of procedures used to restrict the amount of data. The sender can send before waiting for acknowledgment.

32. Mention the categories of flow control?

There are 2 methods have been developed to control flow of data across communication links. a) Stop and wait- send one from at a time. b) Sliding window- send several frames at a time.

33. What is a buffer?

Each receiving device has a block of memory called a buffer, reserved for storing incoming data until they are processed.

34.What is the difference between a passive and an active hub?

An active hub contains a repeater that regenerates the received bit patterns before sending them out. A passive hub provides a simple physical connection between the attached devices.

35.  For n devices in a network, what is the number of cable links required for a

mesh and ring topology?

·         Mesh topology – n (n-1)/2

·         Ring topology – n

CS2302-COMPUTERNETWORKS - Unit 1

1. What are the three criteria necessary for an effective and efficient network?

The most important criteria are performance, reliability and security.

Performance of the network depends on number of users, type of transmission medium, and the capabilities of the connected h/w and the efficiency of the s/w.

Reliability is measured by frequency of failure, the time it takes a link to recover from the failure and the network’s robustness in a catastrophe.

Security issues include protecting data from unauthorized access and viruses.

2. Group the OSI layers by function?

The seven layers of the OSI model belonging to three subgroups.

Physical, data link and network layers are the network support layers; they deal with the physical aspects of moving data from one device to another.

Session, presentation and application layers are the user support layers; they allow interoperability among unrelated software systems.

The transport layer ensures end-to-end reliable data transmission.

3. What are header and trailers and how do they get added and removed?

Each layer in the sending machine adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it. This information is added in the form of headers or trailers. Headers are added to the message at the layers 6,5,4,3, and 2. A trailer is added at layer2. At the receiving machine, the headers or trailers attached to the data unit at the corresponding sending layers are removed, and actions appropriate to that layer are taken.

4. What are the features provided by layering?

            Two nice features:

·         It decomposes the problem of building a network into more manageable components.

·         It provides a more modular design.

5. Why are protocols needed?

In networks, communication occurs between the entities in different systems. Two entities cannot just send bit streams to each other and expect to be understood. For communication, the entities must agree on a protocol. A protocol is a set of rules that govern data communication.

6.  What are the two interfaces provided by protocols?

·         Service interface

·         Peer interface

Service interface- defines the operations that local objects can perform on the protocol.

Peer interface- defines the form and meaning of messages exchanged between protocol peers to implement the communication service.

7. Mention the different physical media?

·         Twisted pair(the wire that your phone connects to)

·         Coaxial cable(the wire that your TV connects  to)

·         Optical fiber(the medium most commonly used for high-bandwidth, long-distance links)

·         Space(the stuff that radio waves, microwaves and infra red beams propagate through)