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

3. 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

4. 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

5. 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 : 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

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

7. 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 ( m)
50-55Mbps downstream, Mbps downstream
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols

8. 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

9. 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

10. 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

11. Computer Interfacing and Protocols
Cable Modem
Cable companies divide coaxial cable bandwidth (5-750MHz) into three bands
Video: MHz, carrying ca. 80 channels of 6MHz each
Downstream Data: MHz, 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

12. 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

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

14. 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