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CHAPTER#8

8.1. Why is multiplexing so cost-effective?

Because, the higher the data rate, the more cost effective the transmission facility.

That is, for a given application and over a given distance, the cost per kbps decline

with an increase in the data rate of the transmission facility.

8.2. How is interference avoided by using frequency division multiplexing?

Interference is avoided under frequency division multiplexing by the use of guard

bands, which are unused portions of the frequency spectrum between sub channels.

8.3. What is echo cancellation?

8.4. Define upstream and downstream with respect to subscriber lines?

"Downstream: from the carrier’s central office to the customer’s site. upstream: from

customer to carrier."

8.5. Explain how synchronous time division multiplexing (TDM) works.

Synchronous TDM works by the multiplex or giving exactly the same amount of time to each

device connected to it. This time slice is allocated even if a device has nothing to transmit. ...

Therefore, the use of Synchronous TDM does not guarantee maximum line usage and

efficiency.

8.6. Why is a statistical time division multiplexer more efficient than a synchronous

time division multiplexer?

"A statistical time division multiplexer is more efficient than a synchronous time division

multiplexer because it allocates time slots dynamically on demand and does not dedicate

channel capacity to inactive low speed lines."

CHAPTER#10

10.1. Why is it useful to have more than one possible path through a network

for each pair of stations?

It is advantageous to have more than one possible path through a network for

each pair of stations to enhance reliability in case a particular path fails.

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10.2. What are the four generic architectural components of a public

communications network? Define each term?

Subscribers: the devices that attach to the network, such as telephones and

modems. Subscriber line: the link between the subscriber and the network.

Exchanges: the switching centers in the network. Trunks: the branches between

exchanges. Trunks carry multiple voice-frequency circuits using either FDM or

synchronous TDM.

10.3. What is the principal application that has driven the design of circuit-

switching networks?

Telephone communications

10.4. What are the advantages of packet switching compared to circuit

switching?

(1) Line efficiency is greater, because a single node-to-node link can be

dynamically shared by many packets over time. (2) A packet-switching network

can perform data-rate conversion. Two stations of different data rates can

exchange packets because each connects to its node at its proper data rate. (3)

When traffic becomes heavy on a circuit-switching network, some calls are

blocked; that is, the network refuses to accept additional connection requests

until the load on the network decreases. On a packet-switching network, packets

are still accepted, but delivery delay increases. (4) Priorities can be used. Thus, if a

node has a number of packets queued for transmission, it can transmit the

higher-priority packets first. These packets will therefore experience less delay

than lower-priority packets.

10.5. Explain the difference between datagram and virtual circuit operation?

In the datagram approach, each packet is treated independently, with no

reference to packets that have gone before. In the virtual circuit approach, a

preplanned route is established before any packets are sent. Once the route is

established, all the packets between a pair of communicating parties follow this

same route through the network.

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10.6. What is the significance of packet size in a packet-switching network?

There is a significant relationship between packet size and transmission time. As

a smaller packet size is used, there is a more efficient "pipelining" effect.

However, if the packet size becomes too big, then the transmission is less

efficient.

10.7. What types of delay are significant in assessing the performance of a

packet-switching network?

Transmission, processing, and queuing delays.

10.8. How does frame relay differ from X.25?

Frame relay is based on the older X.25 packet-switching technology that was designed for

transmitting analog data such as voice conversations. Unlike X.25, which was designed for

analog signals, frame relay is a fast packet technology, which means that the protocol does not

attempt to correct errors.

10.9. What are the relative advantages and disadvantages of frame relay

compared to X.25?

Advantages:

Frame relay operates at a higher speed (1.544 mbps & currently 44.376mbps)

by this it can easily be used instead of a mesh of T-1 or T-3 lines.

It allows brusty data. In case of x.25 and t-line there is a fixed data rate.

Frame relay is less expensive than other traditional WAN.

CHAPTER#14

14.1. What geometric shape is used in cellular system design?

Cellular system is designed in a hexagonal shape.

14.2. What is the principle of frequency reuse in the context of a cellular

network?

The same frequency is used in multiple cells with the cell separated far enough

from each other that they do not interfere with each other.

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14.3. List five ways of increasing the capacity of a cellular system?

Adding New Channels, Frequency Borrowing, Cell Splitting, Cell Sectoring and

Microcells

14.4. Explain the paging function of a cellular system?

14.5. What is fading?

Fading is the loss of signal over a certain propagation media. Types of fading

include fast fading, slow fading, flat fading and selective fading.

14.6. What is the difference between diffraction and scattering?

Diffraction is what you get when waves interfere with each other to produce some sort of

pattern. Scattering is what happens when a wave or particle is kicked off in some direction. The

term "scattering" is most often applied when the results of the interaction produce results that

go off in all directions.

14.7. What is the difference between fast and slow fading?

Fast fading refers to changes in signal strength between a transmitter and receiver as the

distance between the two changes by a small distance of about one-half a wavelength. Slow

fading refers to changes in signal strength concerning a transmitter and receiver as the

distance between the two changes by a larger distance, well in excess of a wavelength.

14.8. What is the difference between flat and selective fading?

"Flat fading, or nonselective fading, is that type of fading in which all frequency components of

the received signal fluctuate in the same proportions simultaneously. Selective fading affects

unequally the different spectral components of a radio signal."

14.9. What are the key differences between first- and second-generation

cellular systems?

second- generation systems, it is a relatively simple matter to encrypt all of the

traffic to prevent eavesdropping. All second-generation systems provide this

capability, whereas first generation systems send user traffic in the clear, providing

no security.

14.10. What are the advantages of using CDMA for a cellular network? 1. Efficient practical utilization of fixed frequency spectrum.

2. Flexible allocation of resources.

3. Many users of CDMA use the same frequency, TDD or FDD may be used

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4. Multipath fading may be substantially reduced because of large signal bandwidth

5. No absolute limit on the number of users, Easy addition of more users.

6. Impossible for hackers to decipher the code sent

7. Better signal quality

14.11. What are the disadvantages of using CDMA for a cellular network? 1. As the number of users increases, the overall quality of service decreases

2. Self-jamming

3. Near- Far- problem arises

CHAPTER #15

15.1. How do the key requirements for computer room networks differ from

those for personal computer local networks?

Computer room networks require very high data rates and usually are concerned

with transfer of large blocks of data.

15.2. What are the differences among backend LANs, SANs, and backbone

LANs?

Backend LAN: Backend networks are used to interconnect large systems such as

mainframes, supercomputers, and mass storage devices. The key requirement

here is for bulk data transfer among a limited number of devices in a small area.

High reliability is generally also a requirement. SAN: A SAN is a separate network

to handle storage needs. The SAN detaches storage tasks from specific servers

and creates a shared storage facility across a high-speed network. Backbone LAN:

A backbone LAN is a high-capacity LAN used to interconnect a number of lower

capacity LANs.

15.3. What is network topology?

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Network topology refers to the way in which the end parts or stations attached

to the network are interconnected

15.4. List four common LAN topologies and briefly describe their methods of

operation?

Bus: all stations attach, through appropriate hardware interfacing known as a tap,

directly to a linear transmission medium, or bus. Full-duplex operation between

the station and the tap allows data to be transmitted onto the bus and received

from the bus. A transmission from any station propagates the length of the

medium in both directions and can be received by all other stations. At each end

of the bus is a terminator, which absorbs any signal, removing it from the bus.

Tree: a generalization of the bus topology. The transmission medium is a

branching cable with no closed loops. The tree layout begins at a point known as

the headend. One or more cables start at the headend, and each of these may

have branches. The branches in turn may have additional branches to allow quite

complex layouts. Again, a transmission from any station propagates throughout

the medium and can be received by all other stations. Ring: the network consists

of a set of repeaters joined by point-to-point links in a closed loop. Each station

attaches to the network at a repeater and can transmit data onto the network

through the repeater. Star: each station is directly connected to a common

central node. Typically, each station attaches to a central node via two point-to-

point links, one for transmission and one for reception.

15.5. What is the purpose of the IEEE 802 committee?

To develop LAN standards.

15.6. Why are there multiple LAN standards?

No single technical approach will satisfy all requirements. Requirements with

respect to cost, data rate, and range dictate a variety of technical alternatives.

15.7. List and briefly define the services provided by LLC?

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Unacknowledged connectionless service: This service is a datagram-style

service. It is a very simple service that does not involve any of the flow- and

error-control mechanisms. Thus, the delivery of data is not guaranteed.

Connection-mode service: This service is similar to that offered by HDLC. A

logical connection is set up between two users exchanging data, and flow

control and error control are provided.

Acknowledged connectionless service:This is a cross between the previous

two services. It provides that datagrams are to be acknowledged, but no

prior logical connection is set up.

15.8. List and briefly define the types of operation provided by the LLC protocol?

Type 1 operation supports unacknowledged connectionless service. There is no

acknowledgment, flow control, or error control. Type 2 operation supports

connection-mode service, using mechanisms similar to HDLC. Type 3 operation

supports acknowledged connectionless service. Each transmitted PDU is

acknowledged using a stop-and-wait technique.

15.9. List some basic functions performed at the MAC layer?

(1) On transmission, assemble data into a frame with address and error-detection

fields.

(2) On reception, disassemble frame, and perform address recognition and error

detection.

(3) Govern access to the LAN transmission medium.

15.10. What functions are performed by a bridge?

For a bridge that connects LANs A and B:

(1) Read all frames transmitted on A and accept those addressed to any station

on B.

(2) Using the medium access control protocol for B, retransmit each frame on

B.

(3) Do the same for B-to-A traffic.

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15.11. What is a spanning tree?

For any connected graph, consisting of nodes and edges connecting pairs of

nodes, there is a spanning tree of edges that maintains the connectivity of the

graph but contains no closed loops.

15.12. What is the difference between a hub and a layer 2 switch?

With a hub, only one attached station may transmit at a time. A switch can

accommodate multiple simultaneous transmissions.

15.13. What is the difference between a store-and forward switch and a cut-

through switch?

Store-and-forward switch: The layer 2 switch accepts a frame on an input line,

buffers it briefly, and then routes it to the appropriate output line. Cut-through

switch: The layer 2 switch takes advantage of the fact that the destination

address appears at the beginning of the MAC (medium access control) frame. The

layer 2 switch begins repeating the incoming frame onto the appropriate output

line as soon as the layer 2 switch recognizes the destination address.

CHAPTER #16

16.1. What is a server farm?

A server farm or server cluster is a collection of computer servers - usually maintained by an

organization to supply server functionality far beyond the capability of a single machine.

16.2. Explain the three persistence protocols that can be used with CSMA?

Nonpersistent: If the medium is idle, transmit; if the medium is busy, wait an amount of time drawn from a probability distribution and then check the medium.

1-persistent: If the medium is idle, transmit; if the medium is busy continue to listen until the channel is sensed idle; then transmit immediately.

p-persistent: If the medium is idle, transmit with probability p, and delay one time unit with probability (1 – p); if the medium is busy, continue to listen until the channel is idle and repeat.

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16.3. What is CSMA/CD?

Carrier sense multiple access with collision detection (CSMA/CD) is a form of medium access control in which a station listens to the medium to try to see if another transmission is in progress. If the medium appears idle, the station transmits. If a collision occurs, the workstations wait a random amount of time before trying again.

16.4. Explain binary exponential backoff?

A station will attempt to transmit repeatedly in the face of repeated collisions. For the first 10 retransmission attempts, the mean value of the random delay is doubled. This mean value then remains the same for 6 additional attempts. After 16 unsuccessful attempts, the station gives up and reports an error.

16.5. What are the transmission medium options for Fast Ethernet?

transmission medium options for Fast Ethernet are:

Shielded twisted pair

high quality unshielded twisted pair (Category 5)

low quality unshielded twisted pair (Category 3)

optical fiber

16.6. How does Fast Ethernet differ from 10BASE-T, other than the data rate? Fast Ethernet differs from 10BASE-T in two important ways:

Two physical links are used between nodes, one for transmitting, one for receiving (four links are used for Fast Ethernet using lower quality unshielded twisted pair (Category 3)).

A different coding scheme 4B/5B-NRZI is used rather than Manchester Coding.

16.7. In the context of Ethernet, what is full-duplex operation?

With full-duplex operation, a station can transmit and receive simultaneously.

CHAPTER #17

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17.1. List and briefly define four application areas for wireless LANs?

LAN Extension:

Cross-Building Interconnect: In this case, a point-to-point wireless link is used

between two buildings. The devices use are typically bridges or routers.

Nomadic Access: Nomadic access provides a wireless link between a LAN hub and

a mobile data terminal equipped with an antenna, such as a laptop computer or

notepad computer.

Ad Hoc Networking: An ad hoc network is a peer-to-peer network (no centralized

server) set up temporarily to meet some immediate need.

17.2. List and briefly define key requirements for wireless LANs?

The wireless LANs must meet the same sort of requirements typical of any LAN,

including high capacity, ability to cover short distances, full connectivity among

attached stations and broadcast capability. In addition there are a number of

requirements specific to wireless LANs. They are: Throughput (MAC protocol

should make as efficient use as possible of the wireless medium to maximize

capacity), number of nodes (Wireless LANs may need to support hundreds of

nodes across multiple cells), connection to backbone LAN (In most cases,

interconnection with stations on a wired backbone is required.

17.3. What is the difference between a single-cell and a multiple-cell wireless

LAN?

Single-cell wireless LAN: all of the wireless end systems are within range of a single control

module. Multiple-cell wireless LAN: there are multiple control modules interconnected by a wired

LAN; each control module supports a number of wireless end systems within its transmission

range."

17.4. What are some key advantages of infrared LANs?

1) The spectrum for infrared is virtually unlimited, which presents the possibility of achieving

extremely high data rates. (2) The infrared spectrum is unregulated worldwide, which is not true

of some portions of the microwave spectrum. (3) Infrared light is diffusely reflected by light-

colored objects; thus it is possible to use ceiling reflection to achieve coverage of an entire

room. (4) Infrared light does not penetrate walls or other opaque objects. This has two

advantages: First, infrared communications can be more easily secured against eavesdropping

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than microwave; and second, a separate infrared installation can be operated in every room in a

building without interference, enabling the construction of very large infrared LANs. (5) Another

strength of infrared is that the equipment is relatively inexpensive and simple."

7.5. What are some key disadvantages of infrared LANs?

Many indoor environments experience rather intense infrared background

radiation, from sunlight and indoor lighting. This ambient radiation appears as

noise in an infrared receiver, requiring the use of transmitters of higher power

than would otherwise be required and also limiting the range. However, increases

in transmitter power are limited by concerns of eye safety and excessive power

consumption.

17.6. List and briefly define three transmission techniques for infrared LANs.

Directed-beam IR can be used to create point-to-point links. In this mode, the

range depends on the emitted power and on the degree of focusing. An

omnidirectional configuration involves a single base station that is within line of

sight of all other stations on the LAN.Typically,this station is mounted on the

ceiling. In a diffused configuration,all of the IR transmitters are focused and

aimed at a point on a diffusely reflecting ceiling.

17.7. What is the difference between an access point and a portal?

An access point functions as a bridge to enable the linking of multiple separate 802.11

wireless LANs. A portal provides an interconnection point between an 802.11 wireless LAN

and a wired LAN.

17.8. Is a distribution system a wireless network?

It may or may not be. A distribution system can be one of three things: 1) wired network, 2) wireless

network, or 3) a switch.

17.9. List and briefly define IEEE 802.11 services.

1. Authentication: This is used to define an exchange between a user and an AS that provides

mutual authentication and generates temporary keys to be used between the client and the AP

(access point) over the wireless link.

2. Access Control: This enforces the use of the authentication function by routing messages

properly and facilitating key exchange. It can work with a variety of authentication protocols.

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3. Privacy with message integrity: MAC-level data are encrypted along with a message integrity

code that ensures that the data have not been altered.

17.10. How is the concept of an association related to that of mobility?

Association is to agree on a set of security capabilities to be used. It allows a mobile node that

has made a transition to identify itself to the access point (AP) within a basic service set (BSS)

so that the node can participate in data exchanges with other mobile nodes.