High Speed Digital Access

1. High Speed Digital Access • Modems were initially intended to connect two computers • Its current use is primarily to connect computers to ISP servers  Shift of paradigm from host-to-host to host-to-network connections • Demand and Internet infrastructure is in place • Biggest problem is the connections from home to ISPs • Solution: Use existing telephone and cable TV infrastructure ECE 766 Computer Interfacing and Protocols

2. Digital Subscriber Line (DSL) • DSL provides high speed internet access over local loops • xDSL = {A,S,H,V}DSL • TP cables used for local loops has a bandwidth of 1.1MHz • Telephone companies limit the bandwidth to 4KHz using filters (sufficient for voice communication) • To allow multiplexing of large number of voice channels ECE 766 Computer Interfacing and Protocols

3. ADSL • Asymmetrical DSL • Deliberate asymmetry in upstream and downstream connection • Higher bandwidth downstream (to subscriber), lower bandwidth upstream (to Internet) • Adaptive data rates • Factors like the distance between the residence and switching office, line quality, and signaling used affect the data rate • ADSL tests the condition and the bandwidth availability before deciding on a data rate • Similar to modems with fallback rates ECE 766 Computer Interfacing and Protocols

4. ADSL • Digital Multitone Technique (DMT) • Modulation technique used by DSL • Combines QAM and FDM • No specific way of bandwidth partitioning • Generally, 1.1MHz is divided into 250 channels of 4.312KHz ECE 766 Computer Interfacing and Protocols

5. ADSL • Possible bandwidth division with DMT • Channel 0: Voice • Channels 1-5: Idle to separate voice and data • Channels 6-30: 1 control and 24 data • 24 channels using 4KHz each with QAM24 x 4000 x 15 = 1.44Mbps max upstream • Channels 31-255: 1 control and 224 data • 224 channels using 4KHz each with QAM24 x 4000 x 15 = 13.4Mbps max downstream • Actual rates: • Upstream: 64Kbps-1Mbps • Downstream: 500Kbps-8Mbps ECE 766 Computer Interfacing and Protocols

6. ADSL Equipment • Customer Side – ADSL Modem • Telephone Company Side - DSLAM ECE 766 Computer Interfacing and Protocols

7. xDSL • SDSL – Symmetrical DSL • Unlike ADSL, the bandwidth partitioning is symmetrical • More suitable for businesses • HDSL – High-bit-rate DSL • Alternative to T-1 (1.544Mbps) line, which uses AMI encoding • AMI is susceptible to attenuation at high frequencies  Repeaters needed every 1km • HDSL uses 2B1Q encoding, reaches data rates up to 2Mbps without repeaters over 3.6km distance • 2 TP wires for full-duplex communication • VDSL – Very-high-bit-rate DSL • Similar to ADSL, but uses coaxial, fiber-optic, or TP for higher rates over short distances (300-1800m) • 50-55Mbps downstream, 1.5-2.5 Mbps downstream ECE 766 Computer Interfacing and Protocols

8. Cable Modem • DSL is very susceptible to noise because of the use of UTP cables • Cable TV network provides an alternative • Traditional cable networks ECE 766 Computer Interfacing and Protocols

9. Cable Modem • Hybrid Fiber-Coaxial (HFC) Network • Traditional cable network uses coaxial cables throughout and uses amplifiers • The communication in traditional networks is unidirectional • HFC networks use high-bandwidth fiber for long distance delivery, only last connections go over coaxial cables • Less number of amplifiers is needed and communication can be done in two directions ECE 766 Computer Interfacing and Protocols

10. Cable Modem • The “last mile” is still coaxial cable which is a bandwidth bottleneck ECE 766 Computer Interfacing and Protocols

11. Cable Modem • Cable companies divide coaxial cable bandwidth (5-750MHz) into three bands • Video: 54-55-MHz, carrying ca. 80 channels of 6MHz each • Downstream Data: 550-750MHz, contains 6MHz channels • Modulation: 64 or 256-QAM • Data Rate: 64-QAM, 1 bit for error correction5 x 6MHz = 30Mbps, but connection to computer may be limited with the 10Base-T connection to 10Mbps • Upstream Data: 5-42MHz, contains 6MHz channels • Modulation: QPSK because of the noise susceptibility • Data Rate: 2 x 6MHz = 12 Mbps, usually less than that ECE 766 Computer Interfacing and Protocols

12. Cable Modem • Bandwidth is limited and must be shared among subscribers • Upstream: 6 channels in 37MHz bandwidth • A group of subscribers is assigned a single channel • They contend for channel access • Downstream: 33 channels in 200MHz bandwidth • Data is broadcast in a channel where the subscriber is assigned to • Subscribers discard information not destined to them ECE 766 Computer Interfacing and Protocols

13. Cable Modem • Cable Modem • Cable Modem Transmission System ECE 766 Computer Interfacing and Protocols

14. Cable Modem • Data over Cable System Interface Specification (DOCSIS) • Upstream Communication: • CM checks downstream channels to receive a packet sent by CMTS periodically • A new CM receiving this packet announces its presence • CMTS replies with a packet that defines CM’s allocated downstream and upstream channels • CM “ranges” the distance to CMTS for synchronization • CM sends a packet to ISP to obtain a dynamic IP address • CM and CMTS exchange packets for security transactions • CM sends its unique identifier to CMTS • Upstream communication occurs in the allocated channel • CM contends in minislots to send data ECE 766 Computer Interfacing and Protocols