Public Switched Telephone Network (PSTN)

Overview

Figure 5.19 Modulation/demodulation

Conversions

• Computer (binary data) to analog signals – done by modems – scheme is TCM: modulation schemes like QPSK, QAMs -- Local loop I.e. computer/modem to codec

• Analog to Digital – Codecs – scheme is PCM – done thru sampling (result in noise) -- codec to telephone net to codec

• Digital to Analog – Codecs – Inverse PCM – Codec to modem• Analog to binary – by modem – reverse TCM -- to computer

The Local Loop

• Modems

• ADSL

• Wireless

Telephone line bandwidth

• 300 Hz – 3300 Hz

• For voice the entire range is used because some distortion and noise can be tolerated

• But for data, for integrity of data, edges of this range are not used. The range used for data is 600 Hz – 3000 Hz = 2400 Hz bw.

Figure 5.18 Telephone line bandwidth

Modem : Modulator-demodulator

• Modulator : converts the binary data into band-pass analog signal.

• Demodulator : recovers the binary data from the modulated signal

• To convert binary data into analog signals,

A sine wave is used and one of the characteristics (amplitude, phase or frequency) is modulated to carry the binary information. The sine wave is called the carrier wave.

Modems

(a) A binary signal

(b) Amplitude modulation(c) Frequency modulation(d) Phase modulation

• Fig b uses 2 values : 0 amplitude to represent 0 and non-zero amp. To rep. 1

• Fig c uses 2 values : f1 to rep. 0 and f2 to rep. 1

• Fig d uses phase : phase shift represents change in value and no phase shift rep. No change in value

Limitations of ASK and FSK

• Limitations of ASK ; very much susceptible to noise

• Limitations of FSK ; bandwidth requirement

• For detailed study : refer to Forouzan.

Modems (2)

(a) QPSK.(b) QAM-16.(c) QAM-64.

Trellis Coded Modulation

• With a dense constellation diagram, a small amount of noise in amplitude or phase can result in an error. To reduce the chance of an error, higher speed modems do error correction by adding extra bit(/s) to each sample. This scheme is called TCM.

Few terms

• Bit rate : number of bits transmitted per second.

• Baud rate : number of signal units (or samples) transmitted per second.

• Bit rate = Baud rate x number of data bits per sample.

Some traditional modem standards

Earlier modems used the QAM schemes. For QAM schemes the baud rate is equal to the bandwidth (to be shown by …or refer to Forouzan pg117 to 126). Hence for a telephone line of 2400 Hz bw, baud rate of 2400 was used.

• V.32 : 4 data bits + 1 error bit : 2400 x 4 = 9600 bps• V.32bis : 6 data bits + 1 error bit : 2400 x 6 = 14,400 bps• V.34 : 12 data bits : 2400 x 12 = 28,800 bps• V.34bis : 14 data bits : 2400 x 14 = 33,600 bps

Limitations of traditional modems

When the analog signal is digitized at the telephone company switching station (using codecs) noise is introduced in the signal. Hence the data rate is limited according to the Shannon’s capacity.

• In traditional modems, data exchange is between two computers A and B,( thru the digital telephone network.), Hence this sampling exists in both the directions.

• Thus the maximum data rate is 33.6 kbps in either direction.

Figure 5.22 Traditional modems

Some faster modems

• V.90 offers 56kbps download and 33.6 kbps upload speeds.

• This is possible because communication today is via ISPs (Internet Service Providers). We still use modem to upload and download. But, in uploading, the analog signal must be sampled at the switching stations which means the data rate for uploading is limited to 33.6 as earlier. But, there is no sampling in the downloading, hence no noise , hence no Shannon’s limit (theoretically at least).

Figure 5.23 56K modems

The 56Kbps speed for downloading in V.90

• The telephone companies (at their switching offices) generate 8000 samples per second with 8 bits per sample. One bit is for control giving a data rate of 8000 x 7 = 56Kbps

The V.92

• Adjusts its speed and depending upon the noise present can even upload at 48kbps.

• Download is at 56kpbs.

ADSL: Asymmetric Digital Subscriber Line

• ADSL uses a frequency spectrum of 1.1 MHz. Divides it into 256 channels each of size roughly 4312.5 Hz.

• Channel 0 : POTS• Channels 1-5 ; guard band between voice and data• Two for control channels, one for downstream and one for

upstream• Remaining are partitioned between upstream and

downstream : depends on the service provider; usually it is asymmetric giving 80-90% for download and remaining for upstream – hence the word Asymmetric

Digital Subscriber Lines (2)

Operation of ADSL using discrete multitone modulation.

ADSL contd

• Within each channel, modulation scheme similar to V.34 is used ;

• QAM with 15 bits per baud• 4000 baud instead of 2400• With 224 downstream channels, download speed

13.44 Mbps is theoretically possible• In practice, S/N ratio is never good enough to

achieve this rate, but 8 Mbps is possible on short runs over high quality local loops

Installation requirement of ADSL

A typical ADSL equipment configuration.

Cable broadband Vs DSL

• Cable Broadband is a public network and is shared by several users, hence– Bandwidth reduces as more users log in, and– Less secure

• ADSL is a private network ..works on leased lines from old PSTN, hence– Dedicated bandwidth, and– More secure

Cable broadband Vs DSL : Speeds

• Can’t distinguish on the basis of speeds• Different companies offer different packages• Cable modem speeds vary widely. While cable

modem technology can theoretically support up to about 30 Mbps, most providers offer service with between 1 Mbps and 6 Mbps bandwidth for downloads, and bandwidth between 128 Kbps and 768 Kbps for uploads.

• Both take flat monthly or yearly rents

Cable Vs DSL :speeds

• Very recent announcements from two companies (Dec’05)• Cable : Vietnam Power Telecom (VP Telecom) and Vietnam

Cable Television (VCTV) on Monday officially launched a service that allows users get broadband Internet access via cable television.

The service offers web browsers a chance to download at speeds of 56 megabits per second and upload at a maximum rate of 30 Mbps.

• AT&T DSL Service : Under its Expert Plus S-package, the telecom giant offers a 6mbps DSL service for customers that want to host their own Web site and have a static IP address.

Wireless Local Loops

• MMDS(Multichannel Multipoint Distribution Service) - Uses microwaves 198 MHz band at 2.1 GHz frequency range– Range of about 50km

– Penetrate vegetation and rain moderately well

– Advantage• Technology is well established and equipment readily

available

– Disadv : bandwidth available is not much and must be shared by several users.

WLL - LMDS

• The acronym LMDS is derived from the following:• L (local)?denotes that propagation characteristics of signals in this

frequency range limit the potential coverage area of a single cell site; ongoing field trials conducted in metropolitan centers place the range of an LMDS transmitter at up to 5 miles

• M (multipoint)?indicates that signals are transmitted in a point-to-multipoint or broadcast method; the wireless return path, from subscriber to the base station, is a point-to-point transmission

• D (distribution)?refers to the distribution of signals, which may consist of simultaneous voice, data, Internet, and video traffic

• S (service)?implies the subscriber nature of the relationship between the operator and the customer; the services offered through an LMDS network are entirely dependent on the operator's choice of business

Wireless Local Loops

• LMDS(Local Multipoint Distribution Service) : uses Millimeter waves (because of low bw of MMDS)

• 28-31 GHz band in US and 40GHz band in Europe (both MM wave bands) were not allocated because it was difficult to build silicon integrated circuits that operate so fast. With the invention of Gallium arsenide ICs the speed became achievable and hence people started thinking of using MM waves for communication.

Problems with MM waves

• Highly directional : hence there must be a clear line of sight between the roof top antennas and the tower.

• Rain and trees absorb them

Wireless Local Loops

Architecture of an LMDS system.

Long-Haul Trunks

• The next thing now is to combine the signals received in the end office(switching offices of the telephone co.s) from various local loops into one signal that is transmitted on the long-haul trunk. This is done with the help of various multiplexing schemes :

• FDM• WDM• TDM

Frequency Division Multiplexing

(a) The original bandwidths.(b) The bandwidths raised in frequency.(b) The multiplexed channel.

WDM : Wavelength Division Multiplexing

• In optical fibers, the scheme used is WDM instead of FDM.

• As more and more wavelengths are being discovered in a single fiber WDM is getting denser and now the name DWDM (dense WDM) is being used when the number of channels is vary large in a single fiber.

Growth of WDM

• 1990: 8 wavelengths X 2.5 Gbps 20Gbps

• 1998: 40 X 2.5 Gbps 100Gbps

• 2001: 96 X 10 Gbps 100Gbps : enough to transmit 30 full-length movies per second.

Wavelength Division Multiplexing

Wavelength division multiplexing.

TDM

• WDM : applicable only on optical fiber and not on copper, but a lot of copper is there on the last mile, also analog.

• FDM : used on copper and microwave but requires analog circuitry and cannot be done by a computer,

• Solution : TDM : unfortunately can be used only for digital data. So,

Digital Trunks

• What we need is to convert the analog signals received in the end office(switching offices of the telephone co.s) from various local loops into digital signals and combine them into one signal that is transmitted on the digital trunk. This is done with the help of TDM.

CODEC : PCM (Pulse Code Modulation)

• The codec makes 8000 samples per sec or one sample per 125 microsec. This is because Nyquist theorem says that this is sufficient to capture all the information from the 4KHz ( remember? bit rate = #samples x log L => sample rate = 2B from Nyquist theorem). This technique is called PCM.

• All the time intervals (a pulse) within the telephone system are multiples of 125 microsec.

Time Division Multiplexing : T1 Carrier

• T1 carrier is used on long-haul trunks.

• Supports Codec with 24 Local Loops I.e. 24 channels

• Codec picks signals from these 24 channels on a Round Robin basis to insert 8 bits (7 data + 1 error) for each sample( I.e. for each channel)

T1 Carrier

The T1 carrier (1.544 Mbps).

193 X 8000 = 1.544 Mbps

T1 Carrier

• 193rd bit is used for frame synchronization : a pattern of 010101… is looked for --- analog nodes cannot generate this pattern, digital users can but the chances are less.

•

Signaling(control) information in T1

• Notice : 8000 bps signaling information : too much : two possible approaches to reduce this : – Common channel signaling : use of 193rd bit for

signaling in alternate frames say odd frames and for data in even frames.

– Channel-associated signaling : each channel has its own private signaling subchannel – one of the eight user bits in every sixth frame is used for signaling

E1 Carrier

• 32 channels : 30 for data + 2 for signaling

• Each group of four frames provides 64 bits of signaling : half for channel specific + half for frame sync

• Capacity : 32 X 8 X 8000 = 2.04 Mbps

Differential Pulse Code Modulation

• Instead of digitized amplitude, difference is kept and digitized

• Jumps of the magnitude of more than +-16 are rare in 128 levels. So 5 instead of 8 bits are sufficient.

Delta Modulation

Delta modulation.

Predictive Encoding

• Extrapolate the previous few values to predict the next value.

• Encode the difference between actual and the predicted signal

Time Division Multiplexing (3)

Multiplexing T1 streams into higher carriers.

TDM on optical fiber (for digital data)

Two back-to-back SONET frames.

Time Division Multiplexing (5)

SONET and SDH multiplex rates.

PSTN contd…

• Can be viewed to have two types of componenets:– External (communication medium…last mile,

long haul trunks etc) and,– Internal (Switching Offices)

Switching Offices

• Two types of switching is used:

– Circuit Switching (PSTN)– Packet Switching (Internet)

Circuit Switching

(a) Circuit switching.(b) Packet switching.

Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching

Topics for presentation

• Satellite Networks (2 people) : Explain user to user, where and how they are used etc.

• Mobile Networks (3-5 people), take book from me for reference, rest from net, talk about GSM, GPRS, EDGE, CDMA, their 2nd gen, 3rd gen etc, difference between “use of data card to connect to internet wirelessly anywhere anytime” and GPRS/EDGE enabled mobile phone etc.

Cover in detail: which frequency range, call setup, their switching offices etc, technology used etc

Packet Switching

A comparison of circuit switched and packet-switched networks.

I Acknowledge

Help from the following site

http://www.cs.vu.nl/~ast/

In preparing this lecture.