varinder mc0075 part b

8/3/2019 Varinder Mc0075 Part b

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Q.1. Discuss the physical description of the different transmission mediums.Ans. Transmission mediumTransmission media: Transmission media is the physical path between thetransmitter and receiver. It can be guided or unguided.Ans.Guided media provides a guided (by a solid medium) path for propagation of

signals such as twisted pairs, coaxial cables, optical fibers etc. Unguided mediaemploy an antenna for transmitting through air, vacuum or water. This form oftransmission is referred to as wireless transmission. For example Broadcastradio, satellite etc.Selection of transmission Media depends on the characteristics and quality ofdata transmission are in turn determined by characteristics of the medium andsignal. For guided media the medium itself in determining the limitations oftransmission. For Unguided media BW of the signal produced at the transmittingantenna is more important than characteristics of the transmissioncharacteristics.

Bandwidth: Greater the BW of the signal, the higher data rate can be

achieved.Transmission impairment: these limit the distanceTwisted pair suffers more impairment than coaxial cable which in turn suffersmore than optical fiber.

Interference: overlapping frequency bands can distort/wipeout a signal.It is more concern for unguided media than guided.For guided it can be caused due to nearby cables. Proper shielding of cables canminimize this problem.

Number of receivers: A point to pint links are used or shared link is used with multiple attachments are used.In shared link each attachment introduces some attenuation and distortion on the

line limiting the distance and/or data rate.For guided the transmission capacity depends on data rate or BW and depends critically on the distance (whether medium is p-p or multipoint)Twisted pairThey are least expensive and most widely used. They are easier to work with but limitedin terms of data rate and distance.Physical Description

These were designed to support voice traffic using analog signaling. However itcan handle digital data traffic at modest data rates. It is also commonly used fordigital signaling with the use of a digital switch or digital PBX with data rate of64kbps commonly. It is for LAN supporting PCs with commonly 10Mbps (now adays may 1Gbps also is possible). For long distance Twisted pair with 4Mbps ormore is used.


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Q. 2. Describe the following Medium Access Control Sub Layers Multiple accessprotocols: A) Pure ALOHA B) Slotted ALOHAAns. The ALOHA network was created at the University of Hawaii in 1970 under theleadership of Norman Abramson. The Aloha protocol is an OSI layer 2 protocol for LANnetworks with broadcast topology.The first version of the protocol was basic:

If you have data to send, send the data

If the message collides with another transmission, try resending later

So if N>1 the users are generating the frames at higher rate than the channel canhandle. Hence all frames will suffer collision.

Hence the range for N is0>1, many retransmissions and hence G>N.

Under all loads: throughput S is just the offered load G times the probability of successful transmission P0


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The probability that k frames are generated during a given frame time is given byPoisson distribution

We get for G = 0.5 resulting in a maximum throughput of 0.184, i.e. 18.4%.Pure Aloha had a maximum throughput of about 18.4%. This means that about 81.6% ofthe total available bandwidth was essentially wasted due to losses from packetcollisions.Slotted or Impure ALOHA

An improvement to the original Aloha protocol was Slotted Aloha. It is in 1972, Robertspublished a method to double the throughput of an pure ALOHA by uses discretetimeslots. His proposal was to divide the time into discrete slots corresponding to oneframe time. This approach requires the users to agree to the frame boundaries. Toachieve synchronization one special station emits a pip at the start of each intervalsimilar to a clock. Thus the capacity of slotted ALOHA increased to the maximumthroughput of 36.8%.The throughput for pure and slotted ALOHA system is as shown in figure 7.5. A stationcan send only at the beginning of a timeslot, and thus collisions are reduced. In this case,the average number of aggregate arrivals is G arrivals per 2X seconds. This leverages the lambdaparameter to be G. The maximum throughput is reached for G = 1.

With Slotted Aloha, a centralized clock sent out small clock tick packets to the outlyingstations. Outlying stations were only allowed to send their packets immediately after

receiving a clock tick. If there is only one station with a packet to send, this guaranteesthat there will never be a collision for that packet. On the other hand if there are twostations with packets to send, this algorithm guarantees that there will be a collision, andthe whole of the slot period up to the next clock tick is wasted. With some mathematics,it is possible to demonstrate that this protocol does improve the overall channelutilization, by reducing the probability of collisions by a half.It should be noted that Aloha's characteristics are still not much different from thoseexperienced today by Wi-Fi, and similar contention-based systems that have no carriersense capability. There is a certain amount of inherent inefficiency in these systems. It is


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typical to see these types of networks' throughput break down significantly as thenumber of users and message burstiness increase. For these reasons, applicationswhich need highly deterministic load behavior often use token-passing schemes (suchas token ring) instead of contention systemsFor instance ARCNET is very popular in embedded applications. Nonetheless,contention based systems also have significant advantages, including ease of

management and speed in initial communication. Slotted Aloha is used on lowbandwidth tactical Satellite communications networks by the US Military, subscriberbased Satellite communications networks, and contact less RFID technologies.

Q. 3. Discuss the different types of noise with suitable example .Ans. Noise is a third impairment. It can be define as unwanted energy from sources other than thetransmitter. Thermal noise is caused by the random motion of the electrons in a wire and is

unavoidable. Consider a signal as shown in figure 3.5, to which a noise shown in figure 3.6, is

added may be in the channel.

At the receiver, the signal is recovered from the received signal and is shown in figure 3.7. That is

signals are reconstructed by sampling. Increased data rate implies "shorter" bits with highersensitivity to noise

Source of NoiseThermal:Agitates the electrons in conductors, and is a function of the temperature. It is oftenreferred to as white noise, because it affects uniformly the different frequencies.

The thermal noise in a bandwidth Wis kTW N

Where T=temperature, andk= Boltzmann's constant = 1.38 10-23 Joules/degrees Kelvin.

Signal to noise ratio: dB NS ) dB(SNR

Power Noise Power signal log* 10 dB SNR 10 Typically measured at the receiver, because it is the point where the noise is to beremoved from the signal.Intermodulation:Results from interference of different frequencies sharing the same medium. It is causedby a component malfunction or a signal with excessive strength is used. For example,the mixing of signals at frequencies f1 and f2 might produce energy at the frequency f1 +f2 . This derived signal could interfere with an intended signal at frequency f1 + f2 .


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Cross talk: Similarly cross talk is a noise where foreign signal enters the path of thetransmitted signal. That is, cross talk is caused due to the inductive coupling betweentwo wires that are close to each other. Sometime when talking on the telephone, you canhear another conversation in the background. That is cross talk.

Impulse:These are noise owing to irregular disturbances, such as lightning, flawedcommunication elements. It is a primary source of error in digital data.

varinder mc0075 part b

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