Bat Assignmt

8/4/2019 Bat Assignmt

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Ans 1:

The three channels into which the subscriber loops in ISDN are organised are: B channel, D channel and H channel.

B or the Bearer Channel: It is a user channel that can be used to carry digital data at a fraction of 64kbps It can carry any

type of digital info in full duplex mode B Channel carries transmission end to end

D Or data channel:

16 Kbps or 64 Kbps.

D channel is for packet based data transmission – used typically for control and signaling information.

D channel carries the control signaling for all of the Channels. The subscriber uses D channel to connect to the network and

secures B Channel connection.

H channel or hybrid channel:

Available with data rates of 384 Kbps ( H0), 1536 Kbps ( H11) or 1920 Kbps (H12). The user may use such a channel as

high speed trunk or subdivide the channel. Examples of applications include video, teleconfrencing and multiplexed

information streams at lower data rates.

ANS 2:

The simple seven layer architecture of OSI model is not applicable in ISDN. ISDN differs from OSI in its

management needs, primary considerations in global integration. The functionality of various

components of ISDN is limited to the first three layers of OSI.

• Physical-layer (Layer 1)

It Deals with the electrical standards (Line Coding), and hardware connector specs. It defines ITU

protocol I.430 (For BRI) and I.431 (for PRI). It specifies the specifications for NT-1, U interface. The

standard connectors of RJ-11 and RJ-45 are also used for physical connectivity of ISDN components.

Data Link Layer ( Layer 2)

Data link layer typically defines procedures for maintaining communications between two network

components. The protocol defined for this purpose is LAPB / Q.921. It defines the frame structure of

data packets, and the Link Access Procedure, sequence control, flow control

Network Layer ( Layer 3)

Layer 3 is a crucial layer handling addressing schema and issues relating to end – to end connectivity.

It has been defined in protocol specs Q.931.

Ans3:

The ntwrks configuration n protocols for circuit switchin involves both B n D channels. B channel is used for the transparent

exchange of user data. THe communicating users may use any protocol they wish for end to end communication. D channel

is used to exchange control info btwn usser n the ntwrk for call establishment and termination n access to d ntwrk facilities.

B channel is serviced by NT1 or NT2 using layer 1 functions. End users may employ any protocol although layer 3 will be

null. On d channel, a 3 layer ntwrk access protocol is used. Finally the process of establishing a circuit through isdn involves

cooperation of switches internal to ISDN to set up the connection. These switches interact using signaling system number

7.



Ans 4:

Total no. of channels available are 256 – (1+5+2) = 248.

Now ¾ channels are available for downstream link.

So total no. of channels available for downstream link are 248 * 3/4 = 186

ADSL uses 4000 buds and QAM-64 uses 6 bits/baud which gives us 4000 * 6 = 24000 bps.

So for 186 channels bandwidth is 24000 * 186 = 4464000 bps.

Ans 5:

DSL achieves higher data-transfer rates than dial-up modems by utilizing more of the available bandwidth in a local loop.

Ordinary telephone service only makes use of a very limited amount of the lower frequencies; 3400 Hz and below. Although

a bandwidth of 3400 Hz is more than enough for transmitting reasonable-quality analog voice, it is not suitable for

transmitting high-speed data.

DSL, on the other hand, provides at least 256 kit/s to as much as 100 Mbit/s. This allows for the simultaneous transmissionof high-quality digital voice, data and even full motion video (a.k.a. triple-play). Services based on ADSL and ADSL2+ have

been widely deployed and deliver rich-content broadband experiences for residential and business consumers.

Ans 7:

To generate OFDM, the relationship between all the carriers must be carefully controlled to maintain the orthogonality of

the carriers. For this reason, OFDM is generated by firstly choosing the spectrum required, based on the input data, and

modulation scheme used. Each carrier to be produced is assigned some data to transmit. The required amplitude and phase

of the carrier is then calculated based on the modulation scheme (typically BPSK, QPSK, or QAM). The spectrum is

constructed using the calculated amplitudes and phases of the carriers. The required spectrum is then converted back to its

time domain signal using an Inverse Fast Fourier Transform.

The orthogonal carriers required for the OFDM signal can be easily generated by setting the amplitude and phase of each

frequency bin, then performing the IFFT. Since each bin of an IFFT corresponds to the amplitude and phase of a set of

orthogonal sinusoids, the process guarantees that the carriers generated are orthogonal.

Two functions f(x), g(x) are said to be orthogonal in period[a,b] if,

Let T be the period of periodic signal. Consider n and m to be integral.

Since n and m are integral, each term on the RHS is integrated over an integral number of cycles. The

integral of a cos function over a complete cycle is, of course, zero, so the expression must be zero.



Ans 9:

The term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically

configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.

As a guard interval, it eliminates the intersymbol interference from the previous symbol.

As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be

modelled as circular convolution, which in turn may be transformed to the frequency domain using a discrete Fourier

transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.

In order for the cyclic prefix to be effective, the length of the cyclic prefix must be at least equal to the length of the

multipath channel.

( ye abhi incomplete hai…is questn ka 2nd part ka answer bakki hai)

Ans 10:

FDM is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or

wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text,voice, video, etc.). FDM typically requires there to be frequency guard bands between the frequencies so that they do not

interfere with each other.

Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel

into a number of equally spaced frequency bands. Each sub-carrier is orthogonal with every other sub-carrier and hence,

they donot interfere with each other. Thus, by allowing the subcarriers to overlap, the overall amount of spectrum required

is reduced.

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