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8/10/2019 GTUanswerkey

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A bus topology connects each computer (node) to a single segment trunk. A trunk is a

communication line, typically coax cable, which is referred to as the bus. The signal travelsfrom one end of the bus to the other. A terminator is required at each end to absorb the signal soit does not reflect back across the bus.


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The bus topology is passive. In other words, the computers on the bus simply listen for asignal; they are not responsible for moving the signal along.

A bus topology is normally implemented with coaxial cable.

Advantagesof bus topology:

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Disadvantagesof bus topology:

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Star Topology :-

All of the stations in astar topologyare connected to a central unit called ahub.


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Star topologies are normally implemented using twisted pair cable, specifically unshieldedtwisted pair (UTP). The star topology is probably the most common form of network topologycurrently in use.

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- Physical layer is the lowest layer of the OSI model.-

The physical layer is concerned with transmitting raw bits over a communication

channel. It deals with the electrical and mechanical specifications of interface and

transmission media. It also deals with procedures and functions required for transmission.

- Functions of Physical layer:1.

Physical characteristics of interface and Media- The design issue of

physical layer considers the characteristics if interface between devices and

transmission media.2.

Representation of bits- Physical layer encodes the bit stream into electrical

or optical signal.3. Data rate- Physical layer defines the duration of bit which is called the data

rate.

4. Synchronization of bits- physical layer synchronizes the transmission rate

and receiving rate. D

- The data link layer transforms the physical layer, a raw transmission facility, to a reliable

link. It makes the physical layer appear error-free to the network layer.-

Data link layer implements physical addressing.

-

Other responsibilities\functions of data link layer:

1.

Framing- The frames received from network layer is divided into manageable dataunits called frame.

2. Physical addressing- If the frames are to be sent to different systems in the network,

the data link later adds a header to the frame to define the sender and/or receiver.

3. Flow Control- When the rate of data reception is less than the rate of data producedby the sender, the data link layer imposes control mechanism to avoid overwhelming

the receiver.

4. Error control- The data link layer adds reliability to the physical layer by addingmechanisms to detect and retransmit damaged or lost frames. Error control is

normally achieved through a trailer to the end of the frame.

5. Access control- When multiple devices are connected to the same link, data link

layer will determine which device has the control over the link at any given time.

- The network layer is responsible for the source-to destination delivery of the packet,possibly across multiple networks.

- If two systems are connected to the same link, there is usually no need for a network

layer. However if two systems are attached to different networks with connecting devices


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between the networks, there is often a need for the network layer to accomplish source-

to-destination delivery.-

Functions of Network layer:

1. Logical addressing- The physical addressing implemented by the data link layer

handles the addressing problem locally. When a network passes the network

boundary, an addressing system is needed to distinguish source and destination,network layer performs this function. The network layer adds a header to the packet

coming from the upper later that, among other things, includes the logical the logicaladdresses of the sender and receiver.

2. Routing- Network layer route or switch the packets to its final destination in an

internetwork.

- The transport layer is responsible for process-to-process delivery of the entire message.

- The transport layer ensures that the whole message arrives complete and in order,overseeing both error control and flow control at source-to-destination level.

-

Functions of transport layer:

1.

Service-point/port addressing- computer performs several processessimultaneously. For this reason, source-to-destination delivery means from a specific

process on one computer to a specific process on the other. The transport layer header

must therefore include a type of address called a service-point address. Thus the

transport layer gets the entire message to the correct process of the system.2.

Segmentation and recovery- A message is divided into segments, each segment

contains a sequence number which enables transport layer to resemble at the

destination.3.

Connection control- the transport layer can be either connectionless or connection-

oriented. In connectionless transport each segment is treated as an independent

packet. A connection-oriented transport layer makes connection with transport layerat the destination.

4. Flow Control- transport layer performs flow control in end to end manner rather

across single link as in data link layer.5.

Error Control-trans port layer performs error control in process-to-process manner

rather across a single link as in data link layer. Transport layer ensures error free

transmission.

-

The session layer is the network dialog controller. It establishes, maintains, and

synchronizes the interaction among communicating systems.- Functions of Session layer

1. Dialog control- Communication between two processes take place in either halfduplex or full duplex mode. The session layer manages dialog control for this

communication.

2. Synchronization- the session layer allows a process to add checkpoints orsynchronization points , to the stream of data.


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- The presentation layer is concerned with the syntax nd semantics of the information

exchange between two systems.-

Functions of presentation layer

1. Translation- the processes on different systems exchange information. Different

computers use different encoding systems. The presentation layer maintains

interoperability between the two encoding system.2. Encryption- To carry sensitive information, a system must be able to ensure privacy.

Encryption means that the sender transforms the original information to another formand sends the resulting message out over the network. While decryption is the reverse

process.

3. Compression- Data compression reduces the number of bits contained in theinformation. Data Compression becomes particularly important in the transmission of

multimedia such as text, audio, and video. A

-

The application layer enables the user , whether the human or software, to access the

network. It provides user interfaces and support for services such as electronic mail.

Remote file access and transfer, shared database management, and other types ofdistributed information services.

-

Specific services provided by application layer:

1. Network Virtual Terminal- It is the software version of physical terminal that

allows a user to log onto a remote host. To do so, the application creates a softwarecopy of a terminal at the remote host. The remote host believes that it is

communicating with one of its own terminal and allows the user to log on.

2. File transfer, access and management- it allows user to access files in remote hosts,to retrieve files and to manage the files in remote computer.

3. Mail services- E-mail forwarding, storage are the services under this category.

4. Directory services- Directory services include access for global information anddistributed database.


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Q. 3

Explain Service Primitives

A service is formally specified by a set of primitives (operations) available to a user process toaccess the service. These primitives tell the service to perform some action or report on an action

taken by a peer entity. If the protocol stack is located in the operating system, as it often is, the

primitives are normally system calls. These calls cause a trap to kernel mode, which then turnscontrol of the machine over to the operating system to send the necessary packets.

The set of primitives available depends on the nature of the service being provided. Theprimitives for connection-oriented service are different from those of connectionless service. As

a minimal example of the service primitives that might be provided to implement a reliable byte

stream in a client-server environment, consider the primitives listed here

Five service primitives for implementing a simple connection-oriented service.

Five service primitives for implementing a simple connection-oriented service.

primitive meaning

LISTEN Block waiting for an incoming connection

CONNECT Establish a connection with a waiting peer

RECEIVE Block waiting for an incoming message

SEND Send a message to the peerDISCONNECT Terminate a connection

These primitives might be used as follows. First, the server executes LISTEN to indicate that it isprepared to accept incoming connections. A common way to implement LISTEN is to make it ablocking system call. After executing the primitive, the server process is blocked until a request

for connection appears.


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Next, the client process executes CONNECT to establish a connection with the server. The

CONNECT call needs to specify who to connect to, so it might have a parameter giving theserver's address. The operating system then typically sends a packet to the peer asking it to

connect, as shown in fig. The client process is suspended until there is a response. When the

packet arrives at the server, it is processed by the operating system there. When the system sees

that the packet is requesting a connection, it checks to see if there is a listener. If so, it does twothings: unblocks the listener and sends back an acknowledgement (2). The arrival of this

acknowledgement then releases the client. At this point the client and server are both running andthey have a connection established. It is important to note that the acknowledgement (2) is

generated by the protocol code itself, not in response to a user-level primitive. If a connection

request arrives and there is no listener, the result is undefined. In some systems the packet maybe queued for a short time in anticipation of a LISTEN.

Packets sent in a simple client-server interaction on a connection-oriented network.

The obvious analogy between this protocol and real life is a customer (client) calling a

company's customer service manager. The service manager starts out by being near the telephone

in case it rings. Then the client places the call. When the manager picks up the phone, theconnection is established.

The next step is for the server to execute RECEIVE to prepare to accept the first request.Normally, the server does this immediately upon being released from the LISTEN, before the

acknowledgement can get back to the client. The RECEIVE call blocks the server.

Then the client executes SEND to transmit its request (3) followed by the execution of

RECEIVE to get the reply.

The arrival of the request packet at the server machine unblocks the server process so it can

process the request. After it has done the work, it uses SEND to return the answer to the client(4). The arrival of this packet unblocks the client, which can now inspect the answer. If the client

has additional requests, it can make them now. If it is done, it can use DISCONNECT to

terminate the connection. Usually, an initial DISCONNECT is a blocking call, suspending the

client and sending a packet to the server saying that the connection is no longer needed (5).When the server gets the packet, it also issues a DISCONNECT of its own, acknowledging the

client and releasing the connection. When the server's packet (6) gets back to the client machine,

the client process is released and the connection is broken. In a nutshell, this is how connection-oriented communication works.

Of course, life is not so simple. Many things can go wrong here. The timing can be wrong (e.g.,the CONNECT is done before the LISTEN), packets can get lost, and much more. We will look

at these issues in great detail later, but for the moment, briefly summarizes how client-server

communication might work over a connection-oriented network.

Given that six packets are required to complete this protocol, one might wonder why a

connectionless protocol is not used instead. The answer is that in a perfect world it could be, in


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which case only two packets would be needed: one for the request and one for the reply.

However, in the face of large messages in either direction (e.g., a megabyte file), transmissionerrors, and lost packets, the situation changes. If the reply consisted of hundreds of packets, some

of which could be lost during transmission, how would the client know if some pieces were

missing? How would the client know whether the last packet actually received was really the last

packet sent? Suppose that the client wanted a second file. How could it tell packet 1 from thesecond file from a lost packet 1 from the first file that suddenly found its way to the client? In

short, in the real world, a simple request-reply protocol over an unreliable network is ofteninadequate.

c) Define terms:

i) Data rate:

In 1924, Nyquist proved:

1.

If an arbitrary signal has been run through a filter of bandwidth H, the filtered

signal can be completely reconstructed by making only2Hsamples per second.

2. If the signal consists of Vdiscrete levels, then: maximum

data rate =2Hlog2 V bits/sec

A noiseless 3000 Hz telephone line cannot transmit binary (2-level) signals at a rate exceeding6000 bps.

In 1948, Shannon carried Nyquist's work further to extend it to the case of a channel subject to

random noise. His major result is maximum data rate =Hlog2 (1 + S/N) bits/secwhere S/Ndenotes the signal-to-noise power ratio. For a 3000 Hz telephone line with S/N=

30dB, the upper bound data rate is 30,000 bps.

In practice, it is difficult to even approachthe Shannon limit.

ii) Bandwidth:

` A voice grade line(for telephone) has a cutoff frequency near 3000 Hz. If this

type of lines are used for data transmission, we have the following numbers for some commonlyused data rates:

) E A 802.11.


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The protocols used by all the 802 variants, including Ethernet, have a certain commonality ofstructure. A partial view of the 802.11 protocol stack is given in FIG. The physical layercorresponds to the OSI physical layer fairly well, but the data link layer in all the 802 protocols

is split into two or more sublayers. In 802.11, the MAC (Medium Access Control) sublayer

determines how the channel is allocated, that is, who gets to transmit next. Above it is the LLC

(Logical Link Control) sublayer, whose job it is to hide the differences between the different 802variants and make them indistinguishable as far as the network layer is concerned. We studied

the LLC when examining Ethernet earlier in this chapter and will not repeat that material here.

. Part of the 802.11 protocol stack.

Logical link control

802.11

infrared

802.11

FHSS

802.11

DSSS

802.11a

OFDM

802.11b

HR-DSSS

802.11g

OFDM

The 1997 802.11 standard specifies three transmission techniques allowed in the physical layer.

The infrared method uses much the same technology as television remote controls do. The othertwo use short-range radio, using techniques called FHSS and DSSS. Both of these use a part of

the spectrum that does not require licensing (the 2.4-GHz ISM band). Radio-controlled garage

door openers also use this piece of the spectrum, so your notebook computer may find itself incompetition with your garage door. Cordless telephones and microwave ovens also use this band.All of these techniques operate at 1 or 2 Mbps and at low enough power that they do not conflict

too much. In 1999, two new techniques were introduced to achieve higher bandwidth. These are

called OFDM and HR-DSSS. They operate at up to 54 Mbps and 11 Mbps, respectively. In 2001,a second OFDM modulation was introduced, but in a different frequency band from the first one.

Now we will examine each of them briefly. Technically, these belong to the physical layer and

should have been examined but since they are so closely tied to LANs in general and the 802.11MAC sub layer.

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