NETWORKING

Given the character 0110101, what bit will be added to support even parity?

0

What is the ratio of check bits to data bits for simple parity?

1 parity bit for every character (7 to 8 bits)

What are the advantages of FireWire and Universal Serial Bus?

1. Advanced, 2. fast, 3. modern, 4. can daisy-chain, 5. can auto-configure (with aid from OS)

List three common examples of frequency division multiplexing.

1. Broadcast television, 2. radio, 3. cable television.

List as many reasons as possible for a company using a modem pool.

1. Do not have to place a modem into each machine. 2. Do not have to have one telephone line for each machine. 3. For incoming calls, the next call automatically goes to the next available modem. 4. All modems are kept in one location making security and maintenance easier.

What are the four components of an interface?

1. Functional, 2. electrical, 3. mechanical, 4. procedural.

What is the primary advantage of a modem pool?

1. Greatly reduce the number of dedicated modems the organization requires. 2. Simplify administration. 3. Increase network security.

What are the alternatives to traditional modems?

1. ISDN (Integrated Services Digital Network) 2. DSL(Digital Subscriber Line) 3. cable modems.

In asynchronous connections, what additional bits are added to a character to prepare it for transfer?

1. Start, 2. stop, 3. parity (optional)

Why don't the 56 Kbps modems transmit at 56 Kbps?

1. Too much noise, 2. lowered power levels, and 3. analog / digital conversions.

What are the basic characteristics of modem?

1. Transmission rates, 2. standard telephone operations, 3. connection negotiation, 4. compression, 5. error correction, 6. facsimile transmission, 7. security, 8. loop-back testing, and 9. internal versus external.

What three circuits are needed, at minimum, to create an EIA-232E connection?

1. Transmit, 2. receive, 3. ground

How many separate channels does a T-1 multiplexor multiplex into one stream?

24 separate voice/data channels of 64kbps

What are the two window sizes found in most sliding window formats?

7 and 127.

Explain the relationship between twisted pair wires and crosstalk.

A signal traveling through one wire creates a magnetic field around that wire. If a second wire runs parallel to the first, a magnetic field passing through the second wire will induce a current in the second wire.

In a stop-and-wait ARQ system, Station A sends packet 0, and it is lost. What happens next?

A timeout followed by a retransmission.

What is the function of an ACK in the stop-and-wait ARQ?

ACK stands for Positive Acknowledgement, it is sent to indicate no error and continue sending packets.

Advantages and disadvantages of dense wavelength division multiplexing

Advantage: Can support up to 80 lambdas, different transmission types & transmission speeds Disadvantage: cost

Advantages and disadvantages of synchronous time division multiplexing

Advantage: Less noise, can apply digital techniques. Disadvantage: never-ending stream is not an efficient use of medium.

Advantages and disadvantages of statistical time division multiplexing

Advantage: less noise, can apply digital techniques, more efficient use of medium. Disadvantage: may not be able to support all inputs at the same time.

Advantages and disadvantages of frequency division multiplexing

Advantage: simple, popular, all receivers don't have to be in the same place. Disadvantage: analog signals and noise

What are the advantages and disadvantages of synchronous communication?

Advantages - Fast, Efficient, Good Error Checking Disadvantages - Complex

What are the advantages and disadvantages of asynchronous communication?

Advantages - Simple Disadvantages - Slow, Inefficient, Poor Error Checking

Which of the noises introduced in this chapter are continuous and which are non-continuous?

All are continuous except for impulse noise.

A benefit of frequency division multiplexing is that all the receivers do not have to be at the same location. Explain what this benefit means and give an example.

All people watching television do not have to be in the same place. Their "de-multiplexors" (television sets) can be located anywhere.

What is a generating polynomial?

An industry-approved bit string used to create the cyclic checksum remainder.

Is frequency division multiplexing associated with analog signals or digital signals?

Analog signals

What are the primary differences between asynchronous connections and synchronous connections?

Asynchronous - Sends one character at a time with stop, start and optional parity bit. Synchronous - Sends multiple characters at one time with beginning and ending flags, control, address info, and cyclic checksum.

Suppose you want to send 1000 characters of data. How many check bits will you need using asynchronous transmission? How many check bits will you need using synchronous transmission? Assume that all 1000 characters will fit within one synchronous transmission frame.

Asynchronous: 1000 characters * 3 check bits per character (start, stop, parity) = 3000 check bits Synchronous: Start flag (8 bits) + Address (8 bits) + Control (8 bits) + CRC (16 bits) + End flag (8 bits) = 48 check bits

Why is addressing of the individual data streams necessary for statistical multiplexing?

Because the order of data streams can vary depending on demand.

What is the significance of the Bell standards and the Hayes standards?

Bell and Hayes standards were the first modem communication protocols which became the foundation for modern modems.

You are in charge of a highly sensitive application which accepts requests and supplies confidential data. You don't want unauthorized individuals to access the data. But you do want to allow remote dial in using a modem. What security feature will allow an authorized remote user to access the database?

Callback security is probably one of the best security techniques. You can also use a very good password protection system.

FireWire and USB are standards to interconnect what to what?

Computers to different peripheral devices.

What is a DTE, and what is a DCE?

Data terminating equipment (DTE) is the computer or terminal and is the end of an interface. Data communicating equipment (DCE) is the modem which provides a path for communication.

Will proper shielding of a media increase or decrease the chance of errors?

Decrease the chance of errors.

Frame relay practices which form of error control?

Discard frame if error is detected.

Twenty-four voice signals are to be multiplexed and transmitted over twisted pair. What is the bandwidth required (in bps) if synchronous time division multiplexing is used, if we use the standard analog to digital sampling rate, and if each sample is converted into an 8-bit value?

Each voice channel has a bandwidth of 4000 Hz. Samples two times the frequency equals 8000 samples per second. Each sample is then converted into an 8-bit value, so 8000 samples per second times 8-bits per sample = 64,000 bits per second. Twenty-four signals times 64,000 bps each equals 1,536,000 bps.

Twenty-four voice signals are to be multiplexed and transmitted over twisted pair. What is the total bandwidth required if frequency division multiplexing is used?

Each voice channel has a bandwidth of 4000 Hz. Twenty-four times 4000 Hz equals 96000 Hz.

List which of the EIA-232E interface signals are used only between a DTE and its DCE and list which signals travel over the phone line to the remote side.

Essentially all signals are only between a DTE and its DCE.

What is the difference between even parity and odd parity?

Even parity: 0 or 1 added to the string produces even number of binary 1s Odd parity: 0 or 1 added to the string produces odd number of binary 1s

What types of errors will simple parity not detect?

Even-numbered bits of error in exactly the same columns.

What type of medium is required to support dense wavelength division multiplexing?

Fiber-optic line/cable

What is the difference between half duplex and full duplex communications?

Half Duplex - Both sides talk, but one at a time Full Duplex - Both sides can talk at the same time

Is stop-and-wait ARQ a half duplex protocol or a full duplex protocol?

Half-duplex

What is necessary for error correction to be performed?

Hamming code: special check bits are added to the data bits

What is meant by fall forward and fallback negotiation?

If the transmitting modem sends data and data arrives garbled, the receiving modem may ask the transmitting modem to fall back to a slower transmission speed.

With a sliding window protocol and a maximum window size of 7, a three-bit field is used to hold the window size. A three-bit value gives 8 combinations, from 0 to 7. Why then can a transmitter send out only a maximum of 7 packets at one time and not 8?

If you sent all 8 frames 0 - 7, and none arrived you can't distinguish between whether you are waiting for the old transmitted frame or getting a new different transmitted frame.

How many different "things" can prevent a 56 Kbps modem from transmitting at 56 Kbps?

Impairments in the telephone network path, interoperability issues between modems, modem settings.

Which is the most difficult type of noise to remove from an analog signal? Why?

Impulse, because it is noncontinuous and appears as an analog waveform, which might make it difficult to separate from the analog waveform of the data.

What types of errors will cyclic checksum not detect?

In cases where the error burst is greater than or equal to r + 1 bits in length is there a chance.

Which is the most difficult type of noise to remove from a digital signal? Why?

Jitter. Regeneration of the signal, which often reduces other forms of noise, can increase the amount of jitter.

Why is the modem for a laptop different than a modem for another type of computer?

Laptop modems are small and portable and almost as fast as other modems.

Terminals A, B, and C are connected to a mainframe computer. Only terminal C has data to transmit. Show the sequence of messages sent between the mainframe and the three terminals using roll call polling.

Mainframe polls A. >A responds NO. Mainframe polls B. >B responds NO. Mainframe polls C. >C responds with data. >>Mainframe acknowledges data.

What is the function of a NAK in the stop-and-wait ARQ?

NAK stands for Negative Acknowledgement, it is sent back to the sender to reject previously sent message or indicate error.

If you install a 56 Kbps modem into your computer and dial into a remote network that only has 33,600 bps modems, is your modem useless?

No, the 56K modem will fallback to the appropriate speed.

If a statistical multiplexor is connected to 20 devices, does it require a high speed output line that is equivalent to the sum of the 20 transmission streams?

No. Assuming that only a percentage of all input streams will have data to transmit at one given time since statistical multiplexor does not transmit empty time slots.

What is impulse noise?

Noise spike is a noncontinuous noise and one of the most difficult errors to detect because it can occur randomly. Typical analog burst of energy.

In asynchronous connections, how many characters are placed into one frame?

One character per frame.

How many packets can be sent at one time using stop-and-wait ARQ?

One packet at a time

What is the ratio of check bits to data bits for longitudinal parity?

One parity bit to every character plus one parity character to a block of data characters.

What is the difference between a point-to-point connection and a multipoint connection?

Point-to-point connection is connection between a computer terminal and mainframe computer that is dedicated to one terminal. Multipoint connection is a shared connection between more than one computer terminal and a mainframe computer.

How does a mainframe computer ask a terminal to send it data?

Polling: Roll-call polling and hub polling

In what order does synchronous time division multiplexing sample each of the incoming signals?

Round-robin fashion

What is the difference between go-back-N ARQ and selective-reject ARQ?

Selective reject - No need to wait for individual ACK from the reciever. The reciever may selectively reject a single frame, which may be retransmitted alone. The receiver accepts out-of-order frames and buffers them. The sender individually retransmits frames that have timed out. Go-back-N - retransmits frames from last reported ACK.

What is the basic block diagram of a synchronous frame?

Sender [10100011][11111011][11111010][11010101] Receiver

List two examples each of simplex, half duplex, and full duplex connections not mentioned in the book.

Simplex - Security system & Fire Alarms Half Duplex - Fax-machines, ATM Full Duplex - Telephone system, cable modem

Show the sequence of start, data, and stop bits that are generated during asynchronous transmission of the character string "LUNCH."

Start, L, stop, start, U, stop, start, N, stop, start, C, stop, start, H, stop

In a sliding window system with go-back-N ARQ, Station A sends packets 4, 5, 6, and 7. Station B receives them and wants to acknowledge all of them. What does Station B send back to Station A?

Station B ACK 4 Station B ACK 5 Station B ACK 6 Station B ACK 7

In a sliding window system with go-back-N ARQ, Station A sends packets 0, 1, 2, 3, 4, 5, and 6. Packet 0 arrives corrupted. What does Station B send back to Station A? What is Station A's response?

Station B doesn't send any ACK because there is no correct in-order frame. Station A resends the frames after a timeout period.

In a sliding window system with Selective-reject ARQ, Station A sends packets 0, 1, 2, 3, 4, 5, and 6. Packet 0 arrives corrupted. What does Station B send back to Station A? What is Station A's response?

Station B sends a NAK, Station A continues with transmission. It retransmits the erroneous code at the end of initial transmission.

What are the main differences between statistical time division multiplexing and synchronous time division multiplexing?

Synchronous: continuous bit stream, never out of order, waste unused transmission space Statistical: packets or frames of data, order may vary, does not transmit empty time slots

What would happen if a synchronous time division multiplexor sampled the incoming signals out of order?

The demultiplexor would not know what is what.

Using statistical time division multiplexing with 24 stations inputting data at 9600 bps, and assuming that only 60 percent of those stations transmit at one time, what is the minimal speed necessary for the high speed link connecting multiplexor and demultiplexor?

The number of stations transmitting is 60% of 24 = 14.4 = 15. The bit rate for each station is 9600. To be able to transmit at least 15 signals at one time the minimal speed necessary Is 15 * 9600 bps = 144,000 bps.

What is echo?

The reflective feedback of a transmitted signal as the signal moves through a medium.

What is jitter?

The result of small timing irregularities that become magnified during the transmission of digital signals as the signals are passed from one device to another.

Devise a code set for the digits 0 to 9 that has a Hamming distance of 2.

There are many possible answers. The simplest would be to count from 0 to 9 using a 4-bit binary number, and add a simple parity bit to the end of each such as 00 or 11 to make it even or odd then check for errors between each numbers.

How does a synchronous time division multiplexor stay synchronized with the demultiplexor on the receiving end?

There are synchronous bits inserted in the transmission stream at regular intervals.

What is white noise?

Thermal noise or Gaussian noise is a relatively continuous type of noise and is much like the static you hear when a radio is being tuned between two stations.

Why are the 56 Kbps modems faster than the older 33,600 bps modems?

They use digital signaling techniques.

How many channels does Basic Rate ISDN support over a single connection?

Three (2 B channels and 1 D channel)

In communication systems what does timeout mean?

Timeout is an amount of time when no information is sent because an ACK message was never received.

Create a table that compares the advantages and disadvantages of the Universal Serial Bus to the RS-232 interface.

USB is faster, simpler, and easier to install, lower power level

What is crosstalk?

Unwanted coupling between two different signal paths which can be electrical or electromagnetic and might occur between two sets of twisted pair wire.

What are the primary differences between X.21 and EIA-232E?

X21 - Pin has multiple functions EIA-232E - Each pin has a single function

When data is transmitted using a statistical multiplexor, the individual units of data must have some form of address that tells the receiver the intended recipient of each piece of data. Instead of assigning absolute addresses to each piece of data, is it possible to incorporate relative addressing? If so, explain its benefits.

Yes, relative addressing can be used. You only need to create an address for those devices connected to this pair of multiplexors. This should be a smaller address space than all the devices in the entire system.

How many different wavelengths can dense wavelength division multiplexing place onto one connection?

up to 100 lambdas