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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 QAM 24 x 4000 x 15 = 1.44Mbps max upstream Channels 31-255: 1 control and 224 data 224 channels using 4KHz each with QAM 24 x 4000 x 15 = 13.4Mbps max downstream Actual rates: Upstream: 64Kbps-1Mbps Downstream: 500Kbps-8Mbps Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols ADSL Equipment Customer Side – ADSL Modem Telephone Company Side - DSLAM Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Cable Modem DSL is very susceptible to noise because of the use of UTP cables Cable TV network provides an alternative Traditional cable networks Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Cable Modem The “last mile” is still coaxial cable which is a bandwidth bottleneck Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 correction 5 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Cable Modem Cable Modem Cable Modem Transmission System Winter 2006 ECE ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols 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 Winter 2006 ECE ECE 766 Computer Interfacing and Protocols