Datacommchapter 3

ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING CHAPTER 3 : Characteristics of Data Communication Networks

3.1 Transmission media – guided & unguided 3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM, SPREAD SPECTRUM Saiful Jumaat Osman, SUPELEC France

FACULTY OF ENGINEERING, CYBERJAYA YEAR 2008

ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING

3.1 Transmission media – guided & unguided

3.1 Transmission media – guided & unguided

Figure 1

Transmission medium and physical

Signals travel along the media, directed and contained by the physical limits of the medium 3

3.1 Transmission media – guided & unguided

Figure 2

Classes of transmission

4

3.1 Transmission media – guided & unguided

GUIDED MEDIA Guided media, which are those that provide a conduit from one device to another, include twistedpair cable, coaxial cable, and fiber-optic cable.

Topics discussed in this section: 3 main types of transmission medium used for wired LANs: - Twisted pair - Coaxial cable [coax] - Optical Fiber

5

3.1 Transmission media – guided & unguided

Figure 3

Twisted-pair

●Two type: shielded and unshielded ●Used primarily in star and tree/hub network ●Unshielded twisted pair [UTP]: - does not include any extra shielding around the wire pair - ordinary telephone line and commonly used for LAN - least expensive, easy to work [less rigid], simple to install - subject to external electromagnetic interference - limited length 6

3.1 Transmission media – guided & unguided

Figure 4

UTP and STP cables

Shielded twisted pair [STP]: - Covered with foil shield [polyester covered with aluminum on both sides] to reduced interference and crosstalk - Better performance, but more expensive and difficult to work then UTP [heavy and bulky] 7

3.1 Transmission media – guided & unguided

Table 1

Categories of unshielded twisted-pair cables

8

3.1 Transmission media – guided & unguided

Figure 5

UTP connector

9

3.1 Transmission media – guided & unguided

Figure 6

Coaxial

10

3.1 Transmission media – guided & unguided ●Coaxial Cable ●Used primarily in bus networks ●Operating with either baseband or broadband Baseband: - all available bandwidth is used to derive a single transmission channel Broadband: - available bandwidth is divided to derived a number of lower bandwidth subchannels on one cable

Table 2

Categories of coaxial cables

11

3.1 Transmission media – guided & unguided

Figure 7

BNC

12

3.1 Transmission media – guided & unguided

OPTICAL FIBER : Provides a medium for signals using light instead of electricity

Figure 8

Bending of light

13

3.1 Transmission media – guided & unguided

Figure 9

Optical

14

3.1 Transmission media – guided & unguided

Figure 10

Propagation

Basically two modes of transmission in fiber: (a) Single mode fiber - a light ray in one direction only (b) Multi-mode fiber - a number of path in which light ray may travel 15

3.1 Transmission media – guided & unguided

Figure 11

Modes

●Further classified by reflective index profile of their core ●They can be either step index or graded index ●Three main types of fiber are: (a) Single mode fiber (b)Multimode stepped index (c)Multimode graded index

16

3.1 Transmission media – guided & unguided

Figure 12

Fiber

17

3.1 Transmission media – guided & unguided

Figure 13

Fiber-optic cable

18

3.1 Transmission media – guided & unguided

UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Topics discussed in this section: Radio Waves Microwaves Infrared 19

3.1 Transmission media – guided & unguided

20

3.1 Transmission media – guided & unguided

Signals travel from source to destination in several ways: i. Ground propagation ii. Sky propagation iii. line-of-sight propagation Figure 14

Propagation

21

3.1 Transmission media – guided & unguided

Table 3

Bands

22

3.1 Transmission media – guided & unguided

Figure 15

Wireless transmission

23

3.1 Transmission media – guided & unguided

24

3.1 Transmission media – guided & unguided

Note

Radio waves are used for multicast communications, such as radio and television, and paging systems.

25

3.1 Transmission media – guided & unguided

Figure 16

Unidirectional

26

3.1 Transmission media – guided & unguided

27

3.1 Transmission media – guided & unguided

Note

Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

28

3.1 Transmission media – guided & unguided

29

3.1 Transmission media – guided & unguided

Note

Infrared signals can be used for short-range communication in a closed area using lineof-sight propagation.

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ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Multiplexing allows several transmission sources to share a larger transmission capacity – to make efficient use of high speed telecommunications lines MUX combines (multiplexes) data from the n input lines and transmits over a higher-capacity data link DEMUX accepts the multiplexed data stream, separates (demultiplexes) the data according to channel, and delivers them to the appropriate output lines

32

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic. Topics discussed in this section: Frequency-Division Multiplexing Wavelength-Division Multiplexing Synchronous Time-Division Multiplexing Statistical Time-Division Multiplexing 33

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 1

Dividing a link into

34

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 2

Categories of

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3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 3

Frequency-division

• Possible when useful bandwidth of medium exceeds required bandwidth(BW) of channel • Each signal is modulated to a different carrier frequency • Carrier frequencies separated so signals do not overlap (guard bands) • e.g. broadcast radio • Channel allocated even if no data 36

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

FDM is an analog multiplexing technique that combines analog signals. • 6 signal sources are fed into a MUX- modulates each signal onto different frequencies (f1,f2,..f6) • Each modulated signal requires a channel: a certain BW centered on its carrier frequency

37

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 4

FDM

38

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 5

FDM demultiplexing

39

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

FDM System In Figure b, the spectrum of signal mi is shifted to centered at fi. fi must be chosen so that the BWs of various signals do not significantly overlap.

Example 1 Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands. Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure 6.6. 41

Example

42

Example 2 Example 2 Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?

Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x 100 + 4 x 10 = 540 KHz, as shown in Figure 43

Figure 6

Analog

44

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM Wavelength Division Multiplexing – Wave-Division multiplexing(WDM) is designed to use the high data rate capability of fiber-optic cable. – The optical fiber data rate is higher than the data rate of metallic transmission cable. Using fiber-optic cable for one single line wastes the available bandwidth. – WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve multiple beams of light at different frequency

45

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM • Same general architecture as other FDM • Number of sources generating laser beams at different frequencies • MUX consolidates sources for transmission over single fiber. DEMUX separates channels at the destination • Optical amplifiers amplify all wavelengths simultaneously -typically tens of km apart • Mostly operate in the 1550nm wavelength range • Was 200MHz per channel but Now 50GHz

46

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM Synchronous Time Division Multiplexing • In a TDM, the data rate of the link is n times faster, and the unit duration is n times shorter. • Multiple digital signals can be carried on a single transmission path interleaved in time • Interleaving- may be at bit level of blocks. Time slots preassigned to sources and fixed. Time slots are allocated even if no data. And time slots do not have to be evenly distributed amongst sources

47

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

– Multiple digital signals (or analog signals carrying digital data) are carried out on a signal transmission path by interleaving portions of each in time

48

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM Figure a) – Each buffer is typically one bit or one character in length Figure b)- Format of transmitted data: frames Channel: the sequence of slots dedicated to one source, from frame to frame Figure c)- The interleaved data are demultiplexed and routed to the appropriate destination buffer.

TDM System

Example 3 In Figure 7, the data rate for each input connection is 3 kbps. If 1 bit at a time is multiplexed (a unit is 1 bit), what is the duration of (a) each input slot, (b) each output slot, and (c) each frame?

Figure 7

Synchronous time-division multiplexing

Solution We can answer the questions as follows: a. The data rate of each input connection is 1 kbps. This means that the bit duration is 1/1000 s or 1 ms. The duration of the 50 input time slot is 1 ms (same as bit duration).

Example 3 (continued) b. The duration of each output time slot is one-third of the input time slot. This means that the duration of the output time slot is 1/3 ms. c. Each frame carries three output time slots. So the duration of a frame is 3 × 1/3 ms, or 1 ms. The duration of a frame is the same as the duration of an input unit.

51

Example 4 Figure 8 shows synchronous TDM with a data stream for each input and one data stream for the output. The unit of data is 1 bit. Find (a) the input bit duration, (b) the output bit duration, (c) the output bit rate, and (d) the output frame rate.

Figure 8

Example 6

Solution We can answer the questions as follows: a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs. b. The output bit duration is one-fourth of the input bit 52 duration, or ¼ μs.

Example 4 (continued) c. The output bit rate is the inverse of the output bit duration or 1/(4μs) or 4 Mbps. This can also be deduced from the fact that the output rate is 4 times as fast as any input rate; so the output rate = 4 × 1 Mbps = 4 Mbps. d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second. Because we are sending 4 bits in each frame, we can verify the result of the previous question by multiplying the frame rate by the number of bits per frame.

53

Example 5 Four 1-kbps connections are multiplexed together. A unit is 1 bit. Find (a) the duration of 1 bit before multiplexing, (b) the transmission rate of the link, (c) the duration of a time slot, and (d) the duration of a frame. Solution We can answer the questions as follows: a. The duration of 1 bit before multiplexing is 1 / 1 kbps, or 0.001 s (1 ms). b. The rate of the link is 4 times the rate of a connection, or 4 kbps. 54

Example 5 (continued) c. The duration of each time slot is one-fourth of the duration of each bit before multiplexing, or 1/4 ms or 250 μs. Note that we can also calculate this from the data rate of the link, 4 kbps. The bit duration is the inverse of the data rate, or 1/4 kbps or 250 μs. d. The duration of a frame is always the same as the duration of a unit before multiplexing, or 1 ms. We can also calculate this in another way. Each frame in this case has four time slots. So the duration of a frame is 4 times 250 μs, or 1 ms.

55

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 9

56

Example 6 Four channels are multiplexed using TDM. If each channel sends 100 bytes /s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link.

Figure 10 Example 8

Solution The multiplexer is shown in Figure 6.16. Each frame carries 1 byte from each channel; the size of each frame, therefore, is 4 bytes, or 32 bits. Because each channel is sending 100 bytes/s and a frame carries 1 byte from each channel, the frame rate must be 100 frames per second. The bit rate is 100 × 32, or 57 3200 bps.

Example 7 A multiplexer combines four 100-kbps channels using a time slot of 2 bits. Show the output with four arbitrary inputs. What is the frame rate? What is the frame duration? What is the bit rate? What is the bit duration?

Figure 11

Example 9

Solution Figure 6.17 shows the output for four arbitrary inputs. The link carries 50,000 frames per second. The frame duration is therefore 1/50,000 s or 20 μs. The frame rate is 50,000 frames per second, and each frame carries 8 bits; the bit rate is 50,000 × 8 = 400,000 bits or 400 kbps. The bit duration is 1/400,000 s, or 2.5 μs. 58

Pulse Stuffing – Problem - Synchronizing data sources – If each source has a separate clock, any variation among clock could cause loss of synchronization – Data rates from different sources not related by simple rational number – Solution - Pulse Stuffing • Outgoing data rate (excluding framing bits) higher than sum of incoming rates • Stuff extra dummy bits or pulses into each incoming signal until it matches local •

clock Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

Figure 12

Pulse stuffing

59

Framing – No flag or SYNC characters bracketing TDM frames – Must provide synchronizing mechanism – Added digit framing • One control bit added to each TDM frame – Looks like another channel - “control channel”

• Identifiable bit pattern used on control channel • e.g. alternating 01010101…unlikely on a data channel • Can compare incoming bit patterns on each channel with sync pattern

Figure 13

Framing

60

Example 8

We have four sources, each creating 250 characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source, (b) the duration of each character in each source, (c) the frame rate, (d) the duration of each frame, (e) the number of bits in each frame, and (f) the data rate of the link.

Solution We can answer the questions as follows: a. The data rate of each source is 250 × 8 = 2000 bps = 2 kbps.

61

Example 8 (continued) b. Each source sends 250 characters per second; therefore, the duration of a character is 1/250 s, or 4 ms. c. Each frame has one character from each source, which means the link needs to send 250 frames per second to keep the transmission rate of each source. d. The duration of each frame is 1/250 s, or 4 ms. Note that the duration of each frame is the same as the duration of each character coming from each source. e. Each frame carries 4 characters and 1 extra synchronizing bit. This means that each frame is 4 × 8 + 1 = 33 bits. 62

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Example 9

Two channels, one with a bit rate of 100 kbps and another with a bit rate of 200 kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link? Solution We can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel. The bit rate is 100,000 frames/s × 3 bits per frame, or 300 kbps. 63

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 14

Digital

64

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Table 1

DS and T line rates

65

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 15

T-1 line for multiplexing telephone lines

66

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 16

T-1 frame structure

67

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Table 2

E line rates

68

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Figure 17

TDM slot

69

3.2 Bandwidth utilization: multiplexing – FDM, TDM & WDM

Statistical TDM • In Synchronous TDM many slots are wasted • Statistical TDM allocates time slots dynamically based on demand • Multiplexer scans input lines and collects data until frame full • Data rate on the multiplexed line is less than the sum of the data rates of the attached devices

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.

Topics discussed in this section: Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS) 71

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 18

Spread

72

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

●Signals are sent on 19 Frequency hopping spread different carrier frequencies Figure spectrum (FHSS) using a pseudorandom sequence known to both sender and receiver ●Unauthorized person who tunes his/her receiver to one carrier frequency (subband) may only receive part of the transmitted signal 73

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 20

Frequency selection in

74

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 21

FHSS

75

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 22

Bandwidth

76

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 32

DSSS

77

3.2 Bandwidth utilization: multiplexing – SPREAD SPECTRUM

Figure 23

DSSS

78

ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING CHAPTER 3 : Characteristics of Data Communication Networks

3.3 Circuit switched data networks 3.4 Packet-switched data networks

Saiful Jumaat Osman, SUPELEC France

FACULTY OF ENGINEERING, CYBERJAYA YEAR 2008

ECP 2056 : DATACOMMUNICATIONS AND COMPUTER NETWORKING

3.3) Packet-switched data networks 3.4) Circuit switched data networks

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

What is WAN? − −

−

Covers large geographical areas (towns, cities, states, countries,..) or usually across an area of multiple km radius. LAN depends on their own hardware or equipment but WANs may use public, leased, or private communication equipments (combined). Usually consists of several interconnected switching nodes ● The nodes provide a switching facility that will move the data from one node to another until they reach their destination

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Switching Networks Long distance transmission is typically done over a network of switching devices called nodes − Nodes do not concern with the content of data − Data routed by being switched from node to node − End devices are called stations −

●e.g. Computer, terminal, phone, etc

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 1

Switched

network

83

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Nodes − − − −

Nodes may connect to other nodes only (e.g. node 5), or to stations and other nodes (e.g. node 6) Node-to-node links usually multiplexed (FDM or TDM) Some redundant connections (alternative paths) are desirable for reliability Two general types of switching technologies ●Circuit switching ●Packet switching

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 2 Taxonomy of switched

networks

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

CIRCUIT-SWITCHED NETWORKS A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

Topics discussed in this section: Three Phases Efficiency Delay Circuit-Switched Technology in Telephone Networks

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

−

Dedicated communication path between two stations

●The path is a connected sequence of links between network nodes ●On each physical link, a logical channel is dedicated to the connection

The nodes must have switching capacity and channel capacity to establish connection − The nodes must have intelligence to work out routing −

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.

Figure 3 A trivial circuit-switched network

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. – Circuit switching involves 3 phases: » I) Circuit Establishment  Before any signal can be transmitted, an end to end (station-to-station) circuit must be established » II) Data Transfer  Information is transferred  Generally full duplex and digital nowadays » III) Circuit Disconnect  The connection can be terminated by either one of the two stations  If requested by node A, signal for deallocating dedicated resources is propagated to nodes 4,5 and 6

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Disadvantages −

−

Inefficient ●Channel capacity dedicated for duration of connection hence if no data, capacity wasted especially for terminalto-computer connection. Delay ●Set up connection takes time

Applications

Public telephone network − Private branch exchange − Data switch −

Example 1 As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4-kHz voice channels. We assume that each link uses FDM to connect a maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure 4 shows the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course the situation may change when new connections are made. The switch controls the connections.

Figure 4

Circuit-switched network used in Example 8.1

91

Example 2 As another example, consider a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices. Figure 5 shows the situation.

Figure 5

Circuit-switched network used in Example 8.2

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 6

Delay in a circuit-switched

network

93

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Limitations of Circuit Switching −

Circuit switching is originally designed to handle voice traffic

●Dedicated resources (e.g. channel/connection) is allocated to a particular call

−

As the networks began to handle more and more datatype traffic, two shortcomings became apparent: ●Resources are wasted −

In a typical user/host data connection, much of the time the line is idle.

●Data rate is fixed −

This limits the utility of the network in interconnecting a variety of host computers and terminals

94

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

DATAGRAM NETWORKS In data communications, we need to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol. Topics discussed in this section: Routing Table Efficiency Delay Datagram Networks in the Internet 96

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Packet-switching Principles −

Data are transmitted in small packets ●Typically 1000 octets ●Longer messages are split into a series of packets ●Each packet contains a portion of user data plus some control info for routing purpose

−

Packets are received, stored briefly (buffered) and past on to the next node and finally to the destination  Store and forward

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Advantages over Circuit Switch ●

Greater line efficiency – Single node to node link can be shared by many packets over time – Packets queued and transmitted as fast as possible

●

Data rate conversion – Different stations connect to the local node at their own data rates – Nodes buffer data if required to equalize rates

●

No blocking – Packets are accepted even when network is busy but need to tolerate delay

●

Prioritization can be applied – Packets with high priority will experience less delay than lower-priority packets. Can have different queues with different priorities 98

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Packet Switching Techniques ●

Station breaks long message into packets

●

Packets sent one at a time to the network

●

Packets handled in two ways −

Datagram

−

Virtual circuit

99

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Datagram Approach

●

Each packet treated independently

●

Packets can take any practical route

●

Packets may arrive out of order

●

Packets may go missing

●

Up to receiver to re-order packets and to detect and recover missing packets In a packet-switched network, there is no resource reservation; resources are allocated on demand. 100

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 7

A datagram network with four switches

(routers)

101

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 8

Routing table in a datagram

network

102

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

A switch in a datagram network uses a routing table that is based on the destination address. The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet.

Switching in the Internet is done by using the datagram approach to packet switching at the network layer. 103

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 9

Delay in a datagram

network

104

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

VIRTUAL-CIRCUIT NETWORKS A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. Topics discussed in this section: Addressing Three Phases Efficiency Delay Circuit-Switched Technology in WANs 105

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 10

Virtual-circuit

network

106

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

−

−

−

In virtual circuit networks, a virtual circuit identifier (VCI) is used as the identifier for data transfer. It is used by a frame between two switches. When a frame arrives at a switch, it has one VCI; when it leaves, it has another.

Figure 11

Virtual-circuit identifier

107

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Virtual Circuit Approach (cont.) ●

To communicate, a source and destination need to go through three phases: −

Setup

−

Data transfer

−

Teardown

108

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Setup Phase ●

●

●

●

Preplanned route established before any packets sent. All switches need to have a table entry for this virtual circuit. These switching tables contain information such as incoming port/VCI and outgoing port/VCI. This phase is implemented in two approaches: − −

Permanent virtual circuit (PVC) Switched virtual circuit (SVC) 109

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Permanent Virtual Circuit ●

●

●

●

The corresponding table entry is recorded for all switches by the administrator. An outgoing VCI is given to the source, and an incoming VCI is given to the destination. The source always uses this VCI to send frames to that particular destination. The PVC is like a leased telephone line. 110

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Switched Virtual Circuit ●

●

The SVC creates a temporary, short connection that exists only when data are being transferred between source and destination. Two steps are required to create the virtual circuit: −

Setup request

−

Acknowledgment 111

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks Figure 12 Switch and tables in a virtual-circuit network

112

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks Figure 13 Source-to-destination data transfer in a virtual-circuit network

113

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

−

Setup request

●A setup request frame is sent from the source to the destination.

Figure 14 network

Setup request in a virtual-circuit

114

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

−

Acknowledgment

●An acknowledgment frame can complete the entries in the switching tables.

Figure 15 network

Setup acknowledgment in a virtual-circuit 115

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Data Transfer Phase ●

●

Once established, all the packets will follow the same route. Fixed for the duration of the logical connection.

116

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Teardown Phase ●

●

●

Source A, after sending all frames to B, sends a special frame called a teardown request. Destination B responds with a teardown confirmation frame. All switches erase the corresponding entry from their tables.

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Virtual Circuits v Datagram ●

Virtual Circuits − −

Network can provide sequencing and error control Packets are forwarded more quickly ●

− ●

No routing decisions to make

Less reliable as loss of that node affect all the circuits over that node

Datagram −

No need call setup phase ●

−

More flexible ●

−

Better if few packets Routing can be used to avoid congested parts of the network

More reliable ●

If a node fails, subsequent packets may find an alternative route that bypasses that node.

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand.

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Figure 16

Delay in a virtual-circuit

network

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Circuit Switching Concept ●

Circuit switching elements

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Elements of a Circuit-Switch Node ●

Digital Switch – Provide transparent signal path between devices – Full duplex transmission

●

Network Interface – Functions and hardware needed to connect digital devices to the network – e.g. data processing devices and digital telephones to the network.

●

Control Unit – Carry out three general tasks 1. 2. 3.

Establish connections Maintain connection Disconnect or tear down connection

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Characteristics of a Circuit Switching Device ●

Blocking − −

●

Blocking can happen when a network is unable to connect stations because all paths are in use Acceptable for voice systems since short duration calls, only a fraction of the telephone will be engaged at any time

Non-blocking − −

Permits all stations to connect (in pairs) at once Suited for data connections as a terminal can be connected for long hours at a time 123

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Space Division Switching ●

● ●

The signal paths are physically separated from one another (divided in space) Crossbar switch Limitations: – Number of crosspoints grows as square of number of stations  costly – Loss of crosspoint prevents connection – Inefficient use of crosspoints as even when all stations connected, only a few crosspoints are engaged

●

To overcome these limitations  Multiple-Stage switches

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Input lines

Crossbar Switch

Output lines

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Multistage Switch ●

●

●

●

Example: Three-Stage Division Switch Reduced number of crosspoints compared to the cross-bar cross-points number. More than one path through network  Increased reliability Downside: −

More complex control

−

May be blocking.

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Three-Stage Division Switch

5x2 switches

3x3 switches

2x5 switches

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Comparison: Crosspoints Required

−

The multistage requires only 35 percent as many crosspoints as the single-stage switch

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Time Division Switching

13

24

31

41

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Time-Slot Interchange (TSI) ●

●

●

A TSI consists of random access memory (RAM) with several memory location The RAM fills up with incoming data from time slots in the order received Slots are then sent out in an order based on the decision of a control unit 130

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

TSI (cont.)

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

TDM Bus ●

●

●

●

The input and output lines are connected to a high-speed bus through input and output gates (microswitches) Only one pair of input/output gates is closed for each time slot This pair of gates allows data to be transferred using the bus The control unit opens and closes the gates

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

TDM Bus (cont.)

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Space- and Time-Division Switch Combinations ●

Space-division  Fast The number of crosspoint required

●

Time-division  Needs no crosspoint Processing delays

●

●

Combining the two results in switches that are optimized both physically and temporally Multistage switches of this sort can be designed as time-space-time (TST), time-space-space-time (TSST), space-time-time-space (STTS), or other 134 possible combinations

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

TST Switch

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Performance Analysis ●

Propagation delay − −

●

Transmission time −

●

The time it takes a signal to propagate from one node to the next. It is generally negligible.

The time it takes for a transmitter to send out a block of data.

Node delay −

The time it takes for a node to perform the

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Performance Analysis (cont.)

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CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Performance Analysis – Circuit Switching ●The delay is mainly caused by Call Request. ●A processing delay is incurred at each node during the call request for setting the route. ●There is no delay for Call Accept signal as the connection is already set up. 138

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Performance Analysis – VC Packet Switching ●The delay is similar to circuit switching −

Call Request incurs a delay at each node

●Call Accept experiences node delays because this packet is queued at each node and must wait its turn for transmission. ●Data packets also queued and delayed. 139

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

Performance Analysis – Datagram Packet Switching ●Does not require a call setup hence faster for short messages.

●Because each individual datagram is routed independently, the processing for each datagram at each node may be longer than for VC. ●For long messages VC is better. 140

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

141

CHAPTER 3 Characteristics of Data Communication Networks: Packet-switched data networks & Circuit switched data networks

142