Gunawan Wibisono Dept electrical engineering university of indonesia

1. Chapter 3 Digital Subscriber Line Technology GunawanWibisono Dept electrical engineering university of indonesia

2. Agenda • Cable Modem • DSL

3. Cable Modem Broadband Network

4. CABLE TV FOR DATA TRANSFER Cable companies are now competing with telephone companies for the residential customer who wants high-speed data transfer. In this section, we briefly discuss this technology. Topics discussed in this section: Bandwidth SharingCM and CMTS Data Transmission Schemes: DOCSIS

5. Division of coaxial cable band by CATV

6. Downstream data are modulated using the 64-QAM modulation technique. The theoretical downstream data rateis 30 Mbps. Upstream data are modulated using the QPSK modulation technique. The theoretical upstream data rate is 12 Mbps.

7. The cable network was designed to deliver TV signals in one direction from the Head-End to the subscribers homes To provide TV services Cable Operators had to recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line Operators had to upgrade the cable network so that signals could flow in both directions Changes in the Cable Network

8. Cable Operators assign a spectrum of signal frequencies to the cable network One spectrum is used for the signals that move from the Head-End towards the cable subscriber Another spectrum of signal frequencies are used for the signals that move from the cable subscriber towards the Head-End Changes in the Cable Network

9. By replacing existing one way amplifiers with two way amplifiers Cable Operators are able to separate the upstream and downstream signals and amplify each direction separately in the right frequency range Changes in the Cable Network

10. Changes in the Cable Network A Traditional Cable network

11. Changes in the Cable Network A Modern Cable network

12. What is a Cable Modem?

13. Figure 9.17 Cable modem (CM)

14. Figure 9.18 Cable modem transmission system (CMTS)

15. Cable modem speeds vary widely Depends on the cable modem system Cable network architecture Traffic load. In the downstream direction (from the network to the computer), network speeds can be up to 27 Mbps BUT, this is an aggregate amount of bandwidth that is shared by users. How Fast is a Cable Modem?

16. Few computers will be capable of connecting at such high speeds or have exclusive access to the network A more realistic number is 1 to 3 Mbps. In the upstream direction (from computer to network), speeds can be up to 10 Mbps. However, most modem producers have selected a more optimum speed between 500 Kbps and 2.5 Mbps AND, many cable operators limit the upstream bandwidth to 128 or 384kbs How Fast is a Cable Modem?

17. An asymmetric cable modem scheme is most common. The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream, primarily because Internet applications tend to be asymmetric in nature. Activities such as World Wide Web (http) navigating and newsgroups reading (nntp) send much more data down to the computer than to the network. How Fast is a Cable Modem?

18. Mouse clicks (URL requests) and e-mail messages are not bandwidth intensive in the upstream direction. Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction. How Fast is a Cable Modem?

19. What is a Cable Modem & how does it work? • A Cable Modem is a digital modem that uses a coaxial cable connection for the data transmission. • This data connection is received by a cable modem that decodes the signal into your PC. http://www.cable-modems.org/tutorial/01.htm http://www.cable-modems.org/tutorial/02.htm MORE INFO...

20. How fast is a Cable Modem? • Cable modems are up to 10-20Mbps downloads. Typical downloads are over 300Kbps, or close to 600Kbps, but the speed of the cable modem depends on a few things. • First it depends on how many users are on the system since the cable technology is a "shared" bandwidth. Too many users using too much throughput can drain this “shared” technology. • The second factor to cable modem speed is a limit on the cable modem itself. Some cable providers will limit the upload or download speed on the cable modem, and this could affect your connection speed.

21. How secure is a Cable Modem? • Cable connections arenot 100% secure in any instance like many other connections on the Internet. Even though most cable providers block ports 137-139, cable modems are likely to be generated in any case where a user has file and print sharing turned on, or possibly other services like SMTP (Simple mail transfer protocol), Web Servers and Telnet services. A general rule is to keep passwords long and turn off any service that you don't absolutely need running. A firewall type application should be used to keep a network as secure as possible.

22. The theoretical performance of a Cable Modem is based upon all other devices being able to work at the same speed and performance as the Cable Modem However, in a similar way that the actual usable bandwidth on a 10Mbps Ethernet connection reduces to a 4Mbps, so too will the performance of a Cable Modem connection be reduced Real-world performance

23. The Cable network itself will suffer the same problems of Internet performance as any other Internet Service Provider (ISP) Although performance to services on the cable network itself can be amazingly fast, access to 'the outside world' will be slowed down by the performance of other connections on the way. Real-world performance

24. As usage on your segment grows (as more customers are added) the bandwidth must be shared by more people Adding more cable network segments is very expensive for the cable operator If you connect to a remote Internet site that itself has a connection speed equivalent to a T1 connection (1.5Mbps), then that is as fast as the data can be served to you, no matter how fast your receiving equipment is Real-world performance

25. 3Com, Cisco Systems, Com21, General Instrument, Motorola, Nortel Networks, Phasecom, Samsung, Terayon, Toshiba, Zenith And many others Who Makes Cable Modems?

26. It MOdulates and DEModulates signals Much more complicated than their telephone counterparts Cable modems can be part modem, part tuner, part encryption/decryption device, part bridge, part router, part network interface card, part SNMP agent, and part Ethernet hub Cable Modem Technology

27. Typically, a cable modem sends and receives data in two slightly different fashions In the downstream direction he digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750 MHz 64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals. Cable Modem Technology

28. The upstream channel is more tricky Typically, in a two-way activated cable network, the upstream (also known as the reverse path) is transmitted between 5 and 42 MHz This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment The drawback is that QPSK is "slower" than QAM. Cable Modem Technology

29. The dominant service is high-speed Internet access This enables the typical array of Internet services to be delivered at speeds far faster than those offered by dial-up telephone modems Other services will include access to streaming audio and video servers, local content (community information and services) access to CD-ROM servers a wide variety of other service offerings. New service ideas are being developed daily. Cable Modem Services

30. In North America, cable operators are packaging high-speed data services much like they do basic cable television service Typically charging $40 - $60 per month for an Internet service package Includes software, unlimited Internet access, specialized content and rental of a cable modem Cost of Cable Modem Service

31. At the low end of this pricing scale, a very robust Internet service is available to consumers for about the cost of a dial-up account with a local Internet service provider and a second telephone line Even at $60 per month, cable is a far better value than ISDN. Cost of Cable Modem Service

32. Not really a cable technology Used more often with Direct Satellite video systems Satellite down link is used for fast downstream transmission A telephone modem handles upstream communication over the public telephone network. "Telco-Return" Modems

33. A cable modem can provide Intenet access to multiple PCs, if they are connected via a local area network (LAN) Cable modems typically have an Ethernet output, so they can connect to the LAN with a standard Ethernet hub or router Each PC must have an assigned IP address The cable ISP usually sells at a premium of $5-$10 a month per PC NAT (Network Address Translation) can allow multiple PCs to "hide" behind a single IP Address Support for Multiple PCs

34. Cable Modems vs. ADSL • There is one major advantage that ADSL has over cable modems. Cable modems use a shared networking technology where all the cable modems share a single pipe to the Internet. This pipe speed will fluctuate depending on the number of subscribers on the network. • When ADSL is used, the pipe to the Internet is solely "yours", and is not shared along the way to a central office. This allows for a more consistent speed, and this speed does not typically fluctuate like cable modem networks. http://www.whatis.com/adsl.htm MORE INFO...

35. Pro: Secure. “Point to point connectivity” of ADSL ensures the security of the service. Cable, by contrast, is shared media and is not secure at all. Bigger coverage area. Cheap. ADSL uses existing twisted pair, hence is cheap in installation and also cheap in monthly payment. Cons: Bandwidth. ADSL has about 1.1MHz BW due to loop limitations, while cable modem has about 745MHz BW. Bridge taps, DLCs, load coils can lead to problems. Mutual noise among different DSL lines, T1 lines. ADSL vs. cable modem

36. Introduction toDSL Yaakov J. Stein Chief ScientistRAD Data Communications

37. PSTN

38. Original PSTN UTP UTP Manual switching directly connected two local loops Due to microphone technology, audio BW was 4 kHz

39. Analog switched PSTN Invention of tube amplifier enabled long distance Between central offices used FDM spaced at 4 kHz (each cable carrying 1 group = 12 channels) Developed into hierarchical network of automatic switches (with supergroups, master groups, supermaster groups)

40. UTP modem modem Data supported viavoice-grade modems • To send data, it is converted into 4 kHz audio (modem) • Data rate is determined by Shannon's capacity theorem • there is a maximum data rate (bps) called the "capacity" • that can be reliably sent through the communications channel • the capacity depends on the BW and SNR • In Shannon's days it worked out to about 25 kbps • today it is about 35 kbps (V.34 modem - 33.6 kbps)

41. Digital PSTN CO SWITCH “last mile” TDM analog digital PSTN TDM “last mile” Subscriber Line CO SWITCH LP filter to 4 kHz at input to CO switch (before A/D)

42. Digital PSTN Sample 4 kHz audio at 8 kHz (Nyquist) Need 8 bits per sample = 64 kbps Multiplexing 64 kbps channels leads to higher and higher rates Only the subscriber line (local loop) remains analog (too expensive to replace) Can switch (cross connect) large number of channels Noise and distortion could be eliminated due to Shannon's theorems 1. Separation theorem 2. Source coding theorem 3. Channel capacity theorem

43. network/ ISP router Voice-grade modemsstill work over new PSTN CO SWITCH PSTN UTP subscriber line modem CO SWITCH But data rates do not increase ! Simulate analog channel so can achieve Shannon rate < native 64 kbps rate modem Internet

44. Where is the limitation ? The digital network was developed incrementally No forklift upgrades to telephones, subscriber lines, etc. Evolutionary deployment meant that the new network needed to simulate pre-existing analog network So a 4 kHz analog channel is presented to subscriber The 4 kHz limitation is enforced by LP filter at input to CO switch (before 8 kHz sampling) The actual subscriber line is not limited to 4 kHz Is there a better way to use the subscriber line for digital transmissions ?

45. UTP

46. What is UTP? The achievable data rate is limited by physics of the subscriber line The subscriber line is an Unshielded Twisted Pair of copper wires • Two plastic insulated copper wires • Two directions over single pair • Twisted to reduce crosstalk • Supplies DC power and audio signal • Physically, UTP is • distributed resistances in series • distributed inductances in series • distributed capacitances in parallel so the attenuation increases with frequency • Various other problems exist (splices, loading coils, etc.)

47. UTP characteristics • Resistance per unit distance • Capacitance per unit distance • Inductance per unit distance • Cross-admittance (assume pure reactive) per unit distance

48. UTP resistance Influenced by gauge, copper purity, temperature Resistance is per unit distance • 24 gauge 0.15 W/kft • 26 gauge 0.195 W/kft Skin effect: Resistance increases with frequency Theoretical result R ~ f 1/2 In practice this is a good approximation

49. UTP capacitance Capacitance depends on interconductor insulation About 15.7 nF per kft Only weakly dependent on gauge Independent of frequency to high degree

50. UTP inductance Higher for higher gauge 24 gauge 0.188 mH per kft 26 gauge 0.205 mH per kft Constant below about 10 kHz Drops slowly above