1、2018/10/16,Computer Network -02-2,1/65,COMPUTER NETWORKS Chapter 02 The Physical Layer Part 2,Shoubao Yang Tel: 3601540 http:/202.38.64.11/syang September 2006,2018/10/16,Computer Network -02-2,2/65,2.5 The Public Switched Telephone Network (PSTN),2.5.1 Structure of the Telephone SystemLocal Loop,
2、Trunks, Switching Office,2018/10/16,Computer Network -02-2,3/65,Major Components of the Telephone System,Local loops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another,2018
3、/10/16,Computer Network -02-2,4/65,2.5.3 The Local Loop: Modems, ADSL, Wireless,The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems (modulator and demodulator) and codecs (coder and decoder).,2018/10/16,Computer Network -02-2,5/65,Modem:
4、 Modulator and Demodulator,Modulation Techniques Problem: How can we encode our signals when we can effectively use only a single frequency (or better: small frequency range)? Answer: Apply modulation techniques Baseband(DC) signaling is unsuitable to be transmitted along a narrow band channel(e.g.
5、a telephone line) due to attenuation, delay distortion, and noise. Modem is introduced to convert baseband (DC) signal into analog signal(AC) to be transmitted along a telephone line, convert analog signal back to digital at the destination.,2018/10/16,Computer Network -02-2,6/65,In order to avoid D
6、C problems to transmit digital signals along a narrow band medium, sine wave carrier is introduced:Asin (t + )A: amplitude : frequency : phase Continuous tone of analog signal 1000-2000Hz,Modem Techniques,2018/10/16,Computer Network -02-2,7/65,Three Modulation Methods,1. Change the amplitude (streng
7、th) of the signal: changing amplitude means a binary 1, constant amplitude a binary 0.Amplitude Shift Keying, ASK 2. Use two frequencies to encode your bits (these frequencies can be put “on top” of your base frequency). FSK 3. Change the phase of the sine wave to do signal encoding. PSK,2018/10/16,
8、Computer Network -02-2,8/65,2018/10/16,Computer Network -02-2,9/65,Observation: Modulation is strongly related to not being able to set a (wide-frequency-ranges) DC signal value on the wire as direct encoding of binary signals:Becomes:,2018/10/16,Computer Network -02-2,10/65,Increasing Transmission
9、Rates,Observation: An important issue is to use low-baud modems for high transmission rates, by increasing the number of signal valuesaccording to Nyquist, c=2Hlog2V. The key issue is how to present more bit information for each sample.,QAM (quadrature amplitude modulation) 3-bit/Baud 4-bit/Baud,201
10、8/10/16,Computer Network -02-2,11/65,Noise may lead to an error, which contains several error bits. So error correction is introduced, so called TCM (Trellis Code Modulation) by adding extra bit (s) for each sample (baud). v.32: uses phase-shifting combined with amplitude modulation, 32 constellatio
11、n points, each baud encodes 4 bits plus 1 parity check, QAM-32. At 2400 baud, The date rate is 9600bps.,Constellation Diagram and Modem Standards,(a) QPSK (b) QAM-16 (c) QAM-64,2018/10/16,Computer Network -02-2,12/65,Constellation Diagram and Modem Standards,v.32bis: each sample transmits 6 data bit
12、s, plus 1 parity check, 128 constellation points, QAM-128, At 2400 baud, date rate is 14.4kbps, such as fax/modem. v.34: each sample transmits 12 data bits, date rate is 28.8kbps v.34bis: each sample transmits 14 data bits, data rate is 33.6kbps Stop at 33.6kbps, because of Shannons limitation. v.90
13、: By Nyquest theorem, 8000 samples per second leads to 56kbps v.92: 56k+,2018/10/16,Computer Network -02-2,13/65,Modem specifications,bit rate, baud rate, modulation method, data format, compression and its ratio, Standard: v.21, v.35, v.22, etc. Transmission: HDX/FDX, 2lines/4lines, dedicated lines
14、.A character frame is as following:,2018/10/16,Computer Network -02-2,14/65,RS-232-c: an Example Protocol of Physical Layer,Physical layer protocol specifies the mechanical, electrical, functional, and procedural interface between DTE/DCE(Data Terminal Equipment/Data Circuit-Terminating Equipment),2
15、018/10/16,Computer Network -02-2,15/65,MechanicalElectrical: Negative logic“0”: +3v +15v, we use +12v“1”: -3v -15v, we use -12v-3v +3v: not defined, illegitimate,RS-232-c,2018/10/16,Computer Network -02-2,16/65,Functional,RS-232-c,2018/10/16,Computer Network -02-2,17/65,Procedural From 25 pin to 9 p
16、in connector RS-449: Comparison of RS-232-c, v.24, RS-449 Asynchronous communication with RS-232-c,RS-232-c,2018/10/16,Computer Network -02-2,18/65,Digital Subscriber Lines - DSL,The trick that makes xDSL work is that when a customer subscribes to it, the incoming line is connected to a different ki
17、nd of switch, one that does not have filter, thus making the entire capacity of the local loop available. The limiting factor then becomes the physics of the local loop, not the artificial 3100Hz band-width created by the filter. Following figure shows bandwidth versus distance over category 3 UTP f
18、or DSL.,2018/10/16,Computer Network -02-2,19/65,xDSL Services,The xDSL services have been designed with certain goals: The services must work over the existing category 3 twisted pair local loops. The must not affect customers existing telephones and fax machine. They must be much faster than 56kbps
19、. They should be always on, with just a monthly charge but no per-minute charge.,2018/10/16,Computer Network -02-2,20/65,Currently, were in ADSL4, which provides 6Mbps one-way (enough for good-quality TV) and 640 kbps two-way. ADSLs background Essence: ADSL splits the bandwidth that can be transport
20、ed across a simple copper wire into four different classes: Higher-bandwidth simplex channel Lower-bandwidth duplex channel Duplex control channel POTS (Plain Old Telephone Service),Asymmetric Digital Subscriber Line,2018/10/16,Computer Network -02-2,21/65,How ADSL Works,Dividing the spectrum availa
21、ble on the local loop, which is about 1.1MHz, into three frequency bands: POTS (Plain Old Telephone Service) upstream (user to end office) and downstream (end office to user), frequency division multiplexing.,Operation of ADSL using discrete multitone modulation.,2018/10/16,Computer Network -02-2,22
22、/65,How ADSL Works,Important: The real low-level bandwidth POTS channel can operate independent from the other channels. In other words, you can phone and transfer data at the same time.,2018/10/16,Computer Network -02-2,23/65,A typical ADSL equipment configuration.,2018/10/16,Computer Network -02-2
23、,24/65,Wireless Local Loops,A fixed telephone using a wireless local loop is a bit like a mobile phone, but there a three crucial differences: WLL user wants high speed Internet connectivity at least equal to ADSL User does not mind to install a large directional antenna on his roof User does not mo
24、ve, eliminating all the problems with mobility and cell handover,Architecture of an local multipoint distribution system.,2018/10/16,Computer Network -02-2,25/65,2.5.4 Trunks and Multiplexing,What is multiplexing? Problem: Considering that the bandwidth of a channel can be huge, wouldnt it be possib
25、le to divide the channel into sub-channels? And how? Multiplexing: the technique of transport multiple data flow over a single channel FDM, TDM, WDM, DWDM, ,2018/10/16,Computer Network -02-2,26/65,Divide the available bandwidth into channels through frequency filtering, and apply modulation techniqu
26、es per channel:,Frequency Division Multiplexing,2018/10/16,Computer Network -02-2,27/65,Looks very much like FDM, but used for fiber optics.Observation: Light waves have their own frequency range; they are simply combined and separated using standard (de)fraction properties DWDM, CWDM,Wavelength Div
27、ision Multiplexing,2018/10/16,Computer Network -02-2,28/65,Wavelength Division Multiplexing,2018/10/16,Computer Network -02-2,29/65,Simply merge/split streams of digital data into a new stream. Data is handled in frames a fixed series of consecutive bits:This is full-digital solution in contrast to
28、FDM and WDM,Time Division Multiplexing,2018/10/16,Computer Network -02-2,30/65,Digitize analog signal into digital signal, send over a digital transmission system, especially for voice transfer. Three steps for PCM : Sampling: 8000 times/per second125 sec/sample Quantitizing: give definite value for
29、 each sample Encoding: binary encoding of the value, say 7-bit or 8-bit,PCM: Pulse Code Modulation,2018/10/16,Computer Network -02-2,31/65,T1 system and others,T1 system is a typical example of the combination with PCM and TDMThe T1 system samples at 8000 Hz, and encodes each sample as a 7-bit numbe
30、r (i.e. 128 different values). With some extra control bits, we merge samples into 193-bit frames, every 125 sec:,2018/10/16,Computer Network -02-2,32/65,T1 supports a total of (7+1) x 24 + 1 bit x 1/125 s x 106= 193 bit x 1/125 s x 106 = 1.544Mbit/sec or 1.544Mbps E1: 2.048Mbps 8 x 32 x 1/125 x 106
31、 = 2.048Mbps T2: 6.312 MbpsT3: 44.736 MbpsT4: 274.176 MbpsE2: 8.848 MbpsE3: 34.304 MbpsE4: 139.264 Mbps,T1 system and others,2018/10/16,Computer Network -02-2,33/65,DPCM(Differential PCM)4 + 1(signaling) x 8000 = 40 kbps Delta Modulation1 x 8000 = 8kbps,2018/10/16,Computer Network -02-2,34/65,TDM al
32、so makes it easy to offer individual senders higher bandwidth, by simply putting more data into a frame, or to combine several trunks into higher-bandwidth trunks:,2018/10/16,Computer Network -02-2,35/65,Synchronous Optical Network (SONET)Synchronous Digital Hierarchy (SDH)Provides a way to multiple
33、x multiple digital channels together.,SONET/SDH,2018/10/16,Computer Network -02-2,36/65,SONET Goals,SONET had to make it possible for different carriers to work collaboratively Some means was needed to unify the U.S., European, and Japanese digital system, which are all based on 64-kbps PCM channels
34、 in different ways SONET had to provide a way to multiplex multiple digital channels SONET had to provide support for operations, administrations, and maintenance,2018/10/16,Computer Network -02-2,37/65,Time Division Multiplexing,A block of 810 bytes put out every 125s 8X810=6480 bits are transmitte
35、d 8000 time per second for a gross data rate of 51.84Mbps, called STS-1 (Synchronous Transport Signal-1) The followings shows two back-to-back SONET frames.,2018/10/16,Computer Network -02-2,38/65,SONET and SDH Multiplex Rates,The followings shows SONET multiplexing hierarchy. The optical carrier co
36、rresponding to STS-n is called OC-n. SPE is for Synchronous Payload Envelope.,2018/10/16,Computer Network -02-2,39/65,安徽省教育科研网络主干网二期拓扑结构图 2004.9,2.5G,合肥地区城域网光纤接入,安徽省网中心 中国科大,安徽农大,合肥工大,安徽大学,安徽医大,合肥学院,安徽中医学院,建工学院,电子工程学院,合肥市,155M SDH 北环 7个市共享,155M SDH 南环 9个市共享,安庆师范学院,安徽师范大学,淮北市,1G,中国科大,省教育厅,省电教馆,省招办,20
37、18/10/16,Computer Network -02-2,40/65,Weathercast Map,安徽省教育和科研网主干网网络天气图,2018/10/16,Computer Network -02-2,41/65,Switching,Switching: A measure provided by telecom subnet to determine how to send message from source to destination. Circuit switching: Make a true physical connection from sender to rec
38、eiver. This is what happens in traditional telephone systems. Packet switching: store-and-forward,(1) Split any data (i.e. message) into small packets, (2) route those packets separately from sender to receiver, and (3) assemble them again. Message switching: store-and-forward, a message is complete
39、ly received at a router, stored, and then put into an outgoing queue for further routing.,2018/10/16,Computer Network -02-2,42/65,2018/10/16,Computer Network -02-2,43/65,Circuit switching, message switching, and packet switching,2018/10/16,Computer Network -02-2,44/65,Switching: Comparison,2018/10/1
40、6,Computer Network -02-2,45/65,Crossbar Switch,Crossbar Switch, crosspoint switch Essence: Construct a matrix with input lines (rows)and output lines (columns); each entry represents a switch, implements circuit switching by making a direct electronic connection. Expensive and no scalability.,2018/1
41、0/16,Computer Network -02-2,46/65,Time-Division Switch,Essence: Based on TDM, reshuffle the packets in a frame so that packets will be forwarded to the right output line:,2018/10/16,Computer Network -02-2,47/65,Observation: This approach strongly resembles circuits switched networks, without using a
42、ctual physical connections between sender and receiver. Note that the switch has to work as fast as the sampling in TDM, limits the number of in/output lines.,2018/10/16,Computer Network -02-2,48/65,2.6 The Mobile Telephone System,First-Generation Mobile Phones: Analog Voice Second-Generation Mobile
43、 Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data,2018/10/16,Computer Network -02-2,49/65,Advanced Mobile Phone System,(a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used.,First-Generation Mobile Phones: Analog Voice,2018/10/
44、16,Computer Network -02-2,50/65,Channel Categories,The 832 channels are divided into four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fa
45、x, or data,2018/10/16,Computer Network -02-2,51/65,Second-Generation Mobile Phones: Digital Voice,(a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users.,D-AMPS: Digital Advanced Mobile Phone System,2018/10/16,Computer Network -02-2,52/65,GSM: Global System for Mobile Communicati
46、ons,GSM uses 124 frequency channels, each of which uses an eight-slot TDM system,2018/10/16,Computer Network -02-2,53/65,GSM (2),A portion of the GSM framing structure.,2018/10/16,Computer Network -02-2,54/65,CDMA Code Division Multiple Access,(a) Binary chip sequences for four stations (b) Bipolar
47、chip sequences (c) Six examples of transmissions (d) Recovery of station Cs signal,2018/10/16,Computer Network -02-2,55/65,Third-Generation Mobile Phones: Digital Voice and Data,Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace e-mail, fax, SMS, cha
48、t, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.),2018/10/16,Computer Network -02-2,56/65,Cable Television,Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable,2018/10/16,Computer Network -02-2,57/65,Commu
49、nity Antenna Television,An early cable television system.,2018/10/16,Computer Network -02-2,58/65,Internet over Cable (1),Cable television,2018/10/16,Computer Network -02-2,59/65,Internet over Cable (2),The fixed telephone system.,2018/10/16,Computer Network -02-2,60/65,Spectrum Allocation,Frequency allocation in a typical cable TV system used for Internet access,2018/10/16,Computer Network -02-2,61/65,Cable Modems,Typical details of the upstream and downstream channels in North America.,
