1. Computer Networks Digital Access Technologies

2. Spring 2006Computer Networks2 How Computer Networks are Built?  LANs (Local Area Networks) are relatively cheap and easy to built.  WANs are expensive and difficult to built  PSTN (Public Switched Telephone Network) is ”an older brother” to the computer networks.  Computer still need to use infrastructure built by PSTN, esspecially when it comes to WAN

3. Spring 2006Computer Networks3 Core, Distribution and Access Network  Core Network  Combination of switching centers and transmission systems connecting switching centers.  Distribution Network  Network in between the access and core network  Access Network  The portion of public network that connects individuals or companies to some access node through which they can reach the core network (directly or via some distribution network)

4. Spring 2006Computer Networks4 Example Fiber-opticOC12 DPT HFC OC48 SONET ISP1 CoreDistribution Access HFC HFC Regional Core ISP2 Switch

5. Spring 2006Computer Networks5 Various Access Options  Access at the customer premises  Narrow band  PSTN based access  ISDN based access  Broad band  xDSL  Cable modem  Fiber to the curb  Distribution network  E1, E2, E3, SDH (T1, T2, T3, SONET)

6. Spring 2006Computer Networks6 PSTN Based Access  PSTN (Public Switched Telephone Network) is acronym for the telephone network that uses circuit switching  When using PSTN access to a computer network, the modems are needed at both ends of the connection  Circuit switching is used although inconvenient for data traffic  Data are transmitted in bursts and therefore the bandwidth is not used all the time during the connection  The bit rate is limited with the bandwidth (usually 4KHz for voice) and the Shannon’s theorem  Different modulation techniques (QAM, multiple PSK) can improve the bit rate  Due to the known S/N for voice channel these techniques cannot do much more then 33600bps

7. Spring 2006Computer Networks7 Local Loop Based Access  Local loop describes connection from telephone office to home  Also known as local subscriber line  Most local loops use analog signals  Sometimes called POTS (Plain Old Telephone Service)  Legacy infrastructure is copper

8. Spring 2006Computer Networks8 Access through Dial-up or Leased Line  Dial-up  The connection goes through the switch (telephone exchange (the line is released after disconnection)  The customer is charged by the time it uses the connection  Leased line (Dedicated line)  The connection does not go through the switch (the line is dedicated all the time to the connection)  The customer is charged with fixed monthly rate PSTN Dial-up or leased-line modem

9. Spring 2006Computer Networks9 Connecting to ISP through Local Loop Up to 56 Kbps from ISP to the subscriber Up to 33.6 Kbps

10. Spring 2006Computer Networks10 ISDN Based Access  ISDN stands for Integrated Services Digital Network  Provides digital service on existing local loop copper  E stablishes a digital pipe between the customer and the telephone company  Allows access to multiple services through a single access (digital telephone, digital terminal, digital facsimile machine)  Attempt to replace the analog telephone system with digital one (for voice and data traffic)  Obsolete for many reasons  Too expansive  Charged by time  Almost equivalent to analog modems (64 Kbps)

11. Spring 2006Computer Networks11 ISDN Architecture ISDN PBX PRA BRA NT1 ISDN Exchange LAN Gateway Carrier’s office Customer’s office

12. Spring 2006Computer Networks12 ISDN Standardized Channels  BRA/BRI (Basic Rate Access/Basic Rate Interface)  2 B + D  2 x 64 Kbps + 16 Kbps = 144 Kbps (not including overhead)  designed to operate using the average local copper pair  PRA/PRI (Primary Rate Access/Primary Rate Interface)  23 B + D  23 x 64 Kbps + 64 Kbps = 1.536 Mbps (not including overhead)  Designed to operate using DS-1/E1  In Europe 30 B + D  Optional backup D channel.

13. Spring 2006Computer Networks13 Access with xDSL  xDSL stands for a family of DSL (Digital Subscriber Loop) technologies  ADSL (Asymmetric DSL) is most popular for residential access  Higher speed into home than out of home  More bits flow in ("downstream") than out ("upstream")  The maximum speed depends on the length and quality of the copper in the subscriber loop

14. Spring 2006Computer Networks14 ADSL - Configuration  Takes advantage of higher frequencies on most local loops  Can be used simultaneously for POTS (Plain Old Telephone Service) Network Interface Device Access Multiplexer

15. Spring 2006Computer Networks15 ADSL Frequency Spectrum  Divides the bandwidth into 256 x 4.3K channels  1 (ch 0) POTS, 5 (ch 1-5) not used, 1 upstream control, 1 downstream control  Typical 6-30 for upstream, rest for downstream  Each 4.3K channel 4K baud sample, V.34 QAM modulation, up to 15 bits per baud 4K * 15 = 60 Kbps per channel

16. Spring 2006Computer Networks16 Other DSL Technologies  SDSL (Symmetric DSL) divides frequencies evenly  HDSL (High-rate DSL) provides DS1 bit rate in both directions  Short distances  Four wires  VDSL (Very high bit rate DSL) provides up to 52 Mbps  Very short distance  Requires Optical Network Unit (ONU) as a relay

17. Spring 2006Computer Networks17 Access with Cable Modem  Cable TV already brings high bandwidth coaxial cable into the houses  Cable modems encode and decode data from cable TV coaxial cable  Instead of a traditional cable box, the splitter is installed in the home (directs the TV bands to the TV set and the Internet access bands to the PC)  Bandwidth dedicated to the Internet is multiplexed among all users  Usually the rate is asymmetrical (500 Kbps to 1 Mbps from PC to Internet and 3 to 10 MHz in the oposite direction

18. Spring 2006Computer Networks18 Cable Network Configuration Traditional cable TV network Hybrid fiber- coax (HFC) cable TV network

19. Spring 2006Computer Networks19 Cable Frequency Spectrum  TV band: 54 – 550 MHz, 6MHz per channel  Downstream : 550 – 750 MHz  6MHz QAM-64 (6bit) == 36 Mbps (gross), 27 Mbps (net)  Total effective downstream bandwidth 200 / 6 * 27 = 891 Mbps  Upstream : 5 – 42 MHz  6MHz QPSK (2bit) == 12 Mbps (gross), 9 Mbps (net)  Total effective upstream bandwidth 37 / 6 * 9 = 54 Mbps

20. Spring 2006Computer Networks20 Access through Fiber to the Curb Infrastructure with fiber can be build especially for Internet access

21. Spring 2006Computer Networks21 Distribution with Optical Hierarchies  SONET (Synhronious Optical NETwork)  A standard for TDM used in United States  SDH (Synhronous Digital Hierarchy)  A standard for TDM in Europe  Both use synhronous communication  Digital telephony systems use clocking for synchronous data delivery  Synchronous network moves data at a precise rate

22. Spring 2006Computer Networks22 Optical Hierarchies - Characteristics  SONET and SDH are based on the principal of direct synchronous multiplexing.  Provide advanced network management and maintenance features.  Both SONET and SDH can transport signals for all the networks in existence today and it has the flexibility to accommodate any networks defined in the future.  Can be used in the three traditional telecommunications areas: long-haul networks, local networks and loop carriers. It can also be used to carry CATV video traffic.