Chapter 1

16Chapter 6Communications Connections15Chapter 6Communications Connections

Chapter 6Communications ConnectionsTrue-False Questions

1.Communications involves at least two entities a sender and a receiver.Answer: TrueDifficulty:EasyPage Reference: 112

2.When you connect your computer to another over a telephone link, the computers can

communicate with each other simultaneously, each transmitting and receiving at the same

time.Answer: TrueDifficulty:MediumPage Reference: 112

3.The link between a television set and a remote control is an example of a multi-point link.Answer: FalseDifficulty:MediumPage Reference 112

4.Simplex mode indicates that data flows in one direction only.Answer: TrueDifficulty:EasyPage Reference: 112

5.When communicating in half duplex mode, information flows in both directions between the parties, but only in one direction at a time.Answer:TrueDifficulty:MediumPage Reference: 113

6.When communicating in full duplex mode, information flows in both directions at the same time. Answer:TrueDifficulty:EasyPage Reference: 113

7.Full duplex communications are found in some modems and the

TCP protocol used on the Internet, as well as in some local area networks (LANs) and

most high-speed network connections.Answer:TrueDifficulty:MediumPage Reference: 113

8.A point-to-pint connection requires an intermediary between the two devices.Answer:FalseDifficulty:EasyPage Reference: 113

9.When there is single link shared by multiple devices in communicating with another device, this type of connection is referred to as multipoint.Answer:TrueDifficulty:MediumPage Reference: 113

10.With point-to-point connections, link management is complex and the links full bandwidth is available for the use of the attached devices.Answer: FalseDifficulty:MediumPage Reference: 113

11.In centralized management of link sharing, one device controls the access of all the

other devices. This makes multiple access management straightforward and relatively

simple to implement, because the controlling device is a single point of link usage coordination.

Answer: True Difficulty: Hard Page reference: 114

12.A prime example of centralized management is pollingthe controlling device

queries the other attached devices in turn and grants access, one at a time, according to

which ones want to use the link.Answer: True Difficulty:Medium Page Reference: 114

13.A different approach to centralized control is multiplexing. It also is a different approach to sharing, because it focuses directly on the links capacity, which is allocated over the attached devices to use as needed.Answer: True Difficulty:Medium Page Reference: 114

14.A different approach to centralized control is multiplexing. It is a similar approach to sharing, because it focuses directly on the links capacity, which is allocated over the attached devices to use as needed.Answer: False Difficulty:Hard Page Reference: 114

15.Access to the shared link is the province of the multiplexer, which combines transmission requests from the attached devices and sends them out over the shared link, and also distributes incoming data to the appropriate devices.

Answer: False Difficulty:Medium Page Reference: 116

Multiple Choice Questions

16.Compared to contention access methods, token passinga.is less complex.b.is always faster.c.guarantees access within a fixed period of time.d.is less adept at handling large demand volume.Answer: cDifficulty:MediumPage Reference: 116

17.The voice band isa. 100 Hz to 4 kHz.b. 0 Hz to 4 kHz.

c.300 Hz to 3.4 kHz.d.0 Hz to 1 MHz.Answer: cDifficulty:Medium Page Reference: 117

18.A computer connected to a printer by a cable is an example of what type of connection?a.point-to-pointb.multipointc.simplexd.half duplexAnswer: aDifficulty:EasyPage Reference: 113

19.Multiple access protocols have to be considered becausea.a single device will share the link.b.it provides access for a device to use the link. c.rules for managing single access are established.d.it does not manage the link sharing. Answer: bDifficulty:MediumPage Reference: 114

20.In a full mesh network designa.every node has a direct connection to every other node.b.only some of the nodes have a true point-to-point connection.c.only certain nodes have access to all the other nodes..

d.only certain nodes control access to all links.Answer: aDifficulty:MediumPage Reference: 131

21.In a tree structurea.single nodes are connected in a branching manner.b.each node is connected to all of its neighboring modes.c.have direct links that connect all nodes.d.is another variation of a point-to-point network.Answer: dDifficulty:EasyPage Reference: 131

22.Multiplexers manage link sharing based ona. time.b. frequency.c. wavelength.d. time and frequency.Answer:bDifficulty:Medium Page Reference: 117

23.In the ring topologya.the attachments do not form a complete loop.b.rings are bidirectional.c.each in a ring is directly attached only to its immediate neighbors.d.nodes do not always pass on all messages intended for another node.Answer:cDifficulty:HardPage Reference: 132

24.In a star structurea.the central device does not create the appropriate path between two nodes.b.the central device may be a simple pass-along hub that send a message to all connected nodes.c.a message from on node has to go through its neighboring node.d.the central device can never be a switch.Answer:bDifficulty:MediumPage Reference: 132

25.A multipoint networka..is also called a multidrop network.b.contains nodes not connected to a common link.c.does not provide decentralized access.d.is structured so that all messages pass through each node.Answer:aDifficulty:MediumPage Reference: 132

26.In a hybrid networka. a tree network and a bus network cannot be combined.b. different topologies are combined.

c. this topology is not used with satellite networks.

d. an uplink or down link cannot be provided between unlike topologies.

Answer: b Difficulty:Medium Page Reference: 132

27.The multi-station access unit (MAU)a. the token ring is physically constructed as a bus but operates logically as a ring. b. causes node faults to be difficult to trace.

c. was used in IBM token ring LANs.

d. creates great difficulty when reconfiguring.

Answer:: c Difficulty:Medium Page Reference: 134

28.Network addressesa. do not have rules for identification which are not consistent.

b. are not enforced for all devices to follow and understand the addressing procedures.

c. do not have a method to convert one systems addresses to anothers.

d. uniquely identifies the communicating nodes.Answer: d Difficulty:Medium Page Reference: 135

29.Decentralized managementa. is very similar to centralized management. b. is not based on any particular protocol for line sharing.

c. cannot be used on WANs.

d. is also called distributed management.

Answer: d Difficulty:Hard Page Reference: 114

30.A star physical topologya. can operate as a logical bus.b. cannot operate as a logical ring.

c. requires more wire than a physical bus.

d. is the most widely used topology for local area networks.

Answer: a Difficulty:Medium Page Reference: 133

Fill In the Blanks

31.Connect your computer to your printer via a cable.There is a direct link between the two devices and the full capacity of that link is dedicated to those devices. In this type of point-to-point connection, there is no intermediary between the devices.Difficulty:Medium Page Reference: 113

32.Twenty terminals in a room are communicating with a mainframe. There is a single link shared by multiple devices in communicating with another device. This type of connection is called multipoint.Difficulty:Hard Page Reference: 113

33.To manage the sharing so as to avoid chaos in the system, we need to consider multiple access protocols multiple because many devices will share the link; access because what needs to be managed is how a device gets to use the link; protocols because what is established are the rules for managing multiple access.

Difficulty:Hard Page Reference: 114

34.In centralized management of link sharing, one device controls the access of all the other devices. This makes multiple access management straightforward and relatively simple to implement, because the controlling device is a single point of link usage coordination. On the other hand, it also means that there is a single point of failureshould the controller fail, the attached devices would not be able to use the link.

Difficulty:Medium Page Reference: 114

35.A different approach to centralized control is multiplexing. It also is a different approach to sharing, because it focuses directly on the links capacity, which is allocated over the attached devices to use as needed. The methods to do this divide link capacity based on frequency, wavelength, or time. Access to the shared link is the province of the multiplexer, which combines transmission requests from the attached devices and sends them out over the shared link, and also distributes incoming data to the appropriate devices.

Difficulty:Hard Page Reference: 114

36.In sharp contrast to centralized management, decentralized management (also called

distributed management) of link sharing is based on protocols that the individual devices

follow to manage themselves when seeking access to the link. Common in LANs and some

wide area networks (WANs), such protocols are found in Ethernet and token ring LANs

among others.Difficulty:Medium Page Reference: 114

37.In polling, one of the connected devices, called the primary or master station, controls

access by having all data transfers go through that station. Further, transfers by the other

devices, called secondary stations, can take place only after getting polled by the primary.

In this process, the primary polls one of the secondaries. If the polled station has no data

to send, it responds with a NAK (negative acknowledgment) message. If it does have data

to send, it transmits the data to the primary, which will ACK (acknowledge) receipt of the

data. The primary forwards that data to the appropriate secondary.Difficulty:Hard Page Reference: 114

38.Another form of central access control is the queuing model. When data packets

(ordered groups of bits) arrive at a network device faster than they can be processed, they

queue to wait for processing. The network device employs various schemes to prevent the

queues from growing too long. Hence, though not central in the sense that there is just one

device controlling all access, the queuing model is central in the sense that for the portionDifficulty:Medium Page Reference: 113

39.Multiplexing can be classified as a centralized control scheme in the same sense as

queuing; because of its importance and prominence in communications.Difficulty:Medium Page Reference: 114

40.With token passing, individual stations also manage their own access, but by very different, non-contention, rules. A special frame called a token is passed from station to station.

The token is like an admission ticket to the shared link, and because only one station

at a time can hold a token, only one at a time can use the link. Thus, access is controlled

and collisions are prevented.Difficulty:Hard Page Reference: 114

41.Each end of the link has a multiplexer (mux) to which the communicating devices are attached. On transmission, the mux merges multiple signals onto a single line; at the other end, the receiving mux separates the combined signal into its original components, a process called de-multiplexing.Difficulty:Medium Page Reference: 115

42.The first successful multiplexing technique, introduced in about 1925, was frequency

division multiplexing (FDM).Difficulty:Medium Page Reference: 115

43.To avoid interference from frequency overlap of adjacent conversations. The extra bandwidth between each conversations allocation is called a guard band.

Difficulty:Medium Page Reference: 116

44.The modulating signal s those that have the information we want to transmitare called baseband signals. So the transmit multiplexing process takes each input baseband signal and uses it to modulate individual carriers, thereby recreating the patterns of the baseband signals in the higher frequency ranges.Difficulty:Hard Page Reference: 116

45.The carrier modulation process, carried out by the mux, is what divides the bandwidth

of the line into discrete partitions called channels, each of which carries a separate conversation. For transmission, the mux combines all the signals by adding them together, creating the single composite signal that is sent over a single wire pair, thus transmitting all conversations at once. (Because the separate conversations are combined in the composite signal, the partitions actually are channels)..Difficulty:Medium Page Reference: 118

46.The process is reversed at the receiving end. First, filters re-create the separate channels

(see Technical extension: bandpass filters). For example, if we apply a filter that will

pass signals only in the 8-kHz to 12-kHz range, that channel is re-created.Difficulty:Medium Page Reference: 118

47.The idea is to use different wavelengths of light as carriers of the data transmissions that

we want to multiplex, a technique called wavelength division multiplexing (WDM).

Conceptually, WDM is similar to FDM. We divide the bandwidth of our fiber-optic

link into sub-bandwidths centered on particular wavelengths (our carriers).Difficulty:Medium Page Reference: 122

48.FDM is an analog technique, not efficient for digital transmission. Yet for digital signals

the goal is the sametransmitting multiple digital signal streams over a single transmission

path. To that end, another technique was developed: time division multiplexing (TDM),

also is known as synchronous TDM.Difficulty:Hard Page Reference: 122

49.Carrying information in frames requires that frames be properly demarcated. That is, for proper transport of the frame, receiving devices must be able to determine precisely when the frame begins. This is called frame synchronization.Difficulty Medium Page Reference: 124

50.The multiplexing techniques we have been discussing until now all have one purposeto combine several low-speed channels into one high-speed channel, so that data streams from those multiple channels can share a smaller number of common connections. What can we do if we have a high-speed data source but only low-speed channels for transmission? We may do the inversethat is, band together several low-speed channels so that they act as one high-speed channel, thereby allowing transmission at a much higher data rate than would be possible with any of the low-speed channels alone. Appropriately, the device to do this is called an inverse multiplexer (or inverse mux).Difficulty:Medium Page Reference: 128

Essay Questions

51.List and describe the different modes of data flow.What we are describing here are different modes of data flow. In the first instance, data

flows in one direction onlyfrom the radio station to the listener. This is called a simplex

mode. Other examples of simplex communications are the links between a fire alarm or

security alarm and the fire station or police station, between a television set and a remote

control, and between a thermostat and furnace.

In the second instance, information flows in both directions between the parties, but in

only one direction at a time. This is called a half duplex mode. Other examples of half

duplex communications are radio traffic between a pilot and the control tower, and the

interplay between a computer and an attached DVD recorder.

In the last instance, information flows in both directions at the same timethis is

called a full duplex mode. Full duplex communications are found in some modems and the

TCP protocol used on the Internet, as well as in some local area networks (LANs) and

most high-speed network connections.

Difficulty Medium Page Reference: 112

52.Describe managing shared links.

All shared link methods have one goal in commonto reduce the amount of wiring

required by point-to-point connections. There are two possibilities for link sharing. In one,

the entire communications channel is taken by a single device but for a limited time (time

sharing), after which it is available to other devices. In the other, channel capacity is divided over several devices that can then use the channel at the same time. However sharing is accomplished, one hurdle must be overcome before it can take place: managing the sharing so as to avoid chaos in the system. That is, we need to consider multiple access protocolsmultiple because many devices will share the link; access because what needs to be managed is how a device gets to use the link; protocols because what is established are the rules for managing multiple access.

In centralized management of link sharing, one device controls the access of all the other devices. This makes multiple access management straightforward and relatively simple to implement, because the controlling device is a single point of link usage coordination. On the other hand, it also means that there is a single point of failureshould the controller fail, the attached devices would not be able to use the link. A prime example of centralized management is pollingthe controlling device queries the other attached devices in turn and grants access, one at a time, according to which ones want to use the link. There are versions of polling that operate as first come, first served and versions that incorporate priorities. There also are limits as to how long a device can use the link before having to relinquish it, usually controlled by limiting the amount of data that can be sent in one link access period.

A similar control technique is a reservation system. In a restaurant, a manager keeps

track of patron requests for dining reservations, which gives them access to tables; in a

transmission system, the attached devices request permission to use the link and a central

controller manages the requests, granting access to the communications link according to

various algorithms.

A different approach to centralized control is multiplexing. It also is a different approach to sharing, because it focuses directly on the links capacity, which is allocated over the attached devices to use as needed. The methods to do this divide link capacity based on frequency, wavelength, or time. Access to the shared link is the province of the multiplexer, which combines transmission requests from the attached devices and sends them out over the shared link, and also distributes incoming data to the appropriate devices.In sharp contrast to centralized management, decentralized management (also called

distributed management) of link sharing is based on protocols that the individual devices

follow to manage themselves when seeking access to the link. Common in LANs and some

wide area networks (WANs), such protocols are found in Ethernet and token ring LANs.Difficulty Hard Page Reference: 114

53.Describe centralized access methods.In polling, one of the connected devices, called the primary or master station, controls

access by having all data transfers go through that station. Further, transfers by the other

devices, called secondary stations, can take place only after getting polled by the primary.

In this process, the primary polls one of the secondaries. If the polled station has no data

to send, it responds with a NAK (negative acknowledgment) message. If it does have data

to send, it transmits the data to the primary, which will ACK (acknowledge) receipt of the

data. The primary forwards that data to the appropriate secondary. Polling is a common control method used for mainframes and minicomputers. In those setups, secondary stations typically talk only to the primary (the mainframe or mini) and not to each other. When data is destined for the primary, the process ends there. If the data is meant for (addressed to) a different secondary station, the primary selects that station and forwards the data to it. The primary also can send messages to all stations. As noted, the main advantage of polling is that it is straightforward and simple to implement, although the master is a single point of failure. A significant drawback to polling is high overhead, a situation that worsens as the number of attached stations increases, which causes the number of polling messages to grow. Overhead is measured by the number of bits of control data sent compared to user data and the amount of time spent on controlling compared to the time spent transmitting user data. Because the ultimate purpose of communications networks is to send user data, control bits and control time are deemed to be overhead.

Propagation time can be a drawback as well. If the polled stations are far apartfor

example, when a satellite is the primaryround-trip propagation time is relatively long.

That means that the time it takes to poll is long, forcing stations to wait until they get permission to transmit. In those cases, reservation systems may be preferred. The reservation process is based on dividing access time over the attached stations. Very small mini-slots of time are set aside by the primary to carry reservation messages from the secondaries. The primary collects reservation requests and then allocates regularsize time slots to the reserving stations. Because reservation requests come only from those stations that need to use the link, stations that have nothing to send do not need to participate in the access process, thus saving time for all. After the slots are used, the process repeats.

One of the reservation schemes, called packet-demand assignment multiple access

(PDAMA), has the secondaries competing for reservation slots via a contention method

for example, first come, first served. This is like announcing concert ticket sales to a group

of people who can reserve tickets by calling in; requests are granted in the order received until no tickets are left.

In another scheme, called fixed priority-oriented demand assignment (FPODA), one

reservation mini-slot is assigned to each secondary station for use as needed; certain

stations may also be given priorityfor example, a server might have priority over a workstation, or a store-file request might have priority over a print request. This is like selling season tickets to a performance series; you can use your tickets or not, and if not, the seats remain empty; those who have purchased season tickets in the past may get priority for the next season over those making new requests Another form of central access control is the queuing model. Difficulty Hard Page Reference: 114

54.What is token passing?With token passing, individual stations also manage their own access, but by very different,

non-contention, rules. A special frame called a token is passed from station to station. The token is like an admission ticket to the shared link, and because only one station at a time can hold a token, only one at a time can use the link. Thus, access is controlled and collisions are prevented. This procedure is followed in the token ring LAN. Each station is linked to two others, called predecessor and successor stations. Data flows only in one direction, from station to station, and the stations form a ring. Additional rules prevent monopolizing the ring.

One problem with any of the token-passing schemes is their complexity, which means that a significant amount of computer time is spent on link management. Another issue can be round-trip timethe time it takes for a token to make a complete trip around the link before becoming available to the next station. On the other hand, as opposed to random access, performance in token passing schemes is deterministic. That is, when we know how many stations are involved, we can calculate how long it will take before the token works its way back to a given station under various conditions, ranging from no station wanting to use the link to all stations wanting access.

Difficulty Medium Page Reference: 116

55.Describe multiplexing.Early in the development of the telephone system the cost pressure of adding and managing an increasing number of telephone wires led to the development of methods by which the phone wires could be shared so that multiple simultaneous conversations could be carried over a single link. Such techniques are called multiplexing, the most widely used of all link-sharing methods.The idea is to combine signals from several slow-speed links into a single signal for transmission over a high-speed link. Why would we want to do that? Simple economics.

Although low-speed links cost less than high-speed links, the total cost of multiple lowspeed

links is greater than the cost of a high-speed link whose capacity equals that of the combined low-speed links.Each end of the link has a multiplexer (mux) to which the communicating devices are attached. On transmission, the mux merges multiple signals onto a single line; at the other end, the receiving mux separates the combined signal into its original components, a process called de-multiplexing. (See Figure 6.2.) Typically, the two functions, multiplexing and de-multiplexing, are combined in a single box, which is simply called a multiplexer.Difficulty Medium Page Reference: 117

56.Describe frequency division multiplexing.The first successful multiplexing technique, introduced in about 1925, was frequency division multiplexing (FDM). At the time, all telephones were analog devices: Human

voices, which are composed of combinations of sinusoidal sound waves, were carried over the wires as their electrical parallels (analogs), combinations of sinusoidal electric waves.

Very early on, phone companies, realizing that it was not necessary to carry the entire

spectrum of the human voice (about 100 Hz to 7,000 Hz), calculated that restricting the

range of vocal sounds transmitted to 300 to 3,400 Hzthe so-called voice bandwould

produce acceptable quality for phone conversations and be less demanding on the communications.FDM is appropriate for any analog system where the total bandwidths of the individual signals plus the guard bands are not more than the overall bandwidth of the system. to drop the frequencies of the signals in each channel down to their original 300-Hz to 3100-Hz range, re-creating the original voices. Figure 6.4 illustrates this process. FDM can combine only analog signals. This is because FDM must limit the bandwidth of the signals it carries so that the links overall bandwidth can be subdivided into bands that can be used separately and simultaneously. Analog signals can be band-limited even if they are not band-limited to begin with. Digital signals cannot be band-limited readily. FDM can be applied to any analog transmission link with suitable bandwidth. Cable television relies on FDM, as do AM and FM radio.Difficulty Medium Page Reference: 117

57.What is wavelength division multiplexing?The unused line capacity issue that we saw for electrical transmission of analog signals

applies to light signals as well: The capacity of optical fiber is much greater than what is

needed to relay one transmission. Unless we can utilize it, most goes to waste. Hence, it is

natural to turn to multiplexing. Because optical fiber bandwidth is so large, dividing that bandwidth with an FDM approach seems appropriatebut with light as the carrier, we focus on wavelength rather than frequency. Recall from Chapter 2, The modern signal carriers, that wavelength and frequency are inversely related, as indicated by the equation , where is wavelength, vm is the speed of light in medium m, and f is its frequency. Also recall that hereas frequency is determined by the light source and does not change, wavelength is determined by the speed of light and does change. Color is determined by wavelength as well.

Thus, the idea is to use different wavelengths of light as carriers of the data transmissions that we want to multiplex, a technique called wavelength division multiplexing (WDM).

Difficulty Medium Page Reference: 120

58.What is time division multiplexing?FDM is an analog technique, not efficient for digital transmission. Yet for digital signals

the goal is the sametransmitting multiple digital signal streams over a single transmission

path. To that end, another technique was developed: time division multiplexing (TDM), also is known as synchronous TDM.

Although TDM operates as simultaneous transmission from the viewpoint of the

senders and receivers, it actually is a sequential transmission technique. Instead of dividing

a broad bandwidth into narrow sub-bands, time on a single connection is sliced into

small, fixed-length, full-bandwidth segments (time slots) that are allocated to the attached

devices in rotation (see Figure 6.6). The combination of slots creates a frame. Frames are transmitted sequentially without delay.

Difficulty Medium Page Reference: 122

59.. What is inverse multiplexing?The multiplexing techniques we have been discussing until now all have one purposeto

combine several low-speed channels into one high-speed channel, so that data streams from those multiple channels can share a smaller number of common connections. What can we do if we have a high-speed data source but only low-speed channels for transmission?

We may do the inversethat is, band together several low-speed channels so that they act as one high-speed channel, thereby allowing transmission at a much higher data rate than would be possible with any of the low-speed channels alone. Appropriately, the device to do this is called an inverse multiplexer (or inverse mux). An inverse mux could, for example, couple two 64-Kbps lines into one 128-Kbps line, as is done in ISDN (Integrated Services Digital Network) systems.

Difficulty Medium Page Reference: 128

60.What is a network and why do we create them?When applied to computers, the term network encompasses many things. The purpose of a

computer network is communicationsthat is, we create networks to provide a vehicle for

transferring information from one place to another. For this to happen, the devices components) of the network must be connected to each other in a systematic way, using agreed upon protocols, so they can understand each other.

A computer network is a system of interconnected, comprehending, communicating hardware and software, designed to facilitate information transfer via of accepted protocols.Difficulty Easy Page Reference: 129