Transcript 14. DSL,cable modems

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-tonetwork 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
14 - 1
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
14 - 2
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
14 - 3
ADSL
• Digital Multitone Technique (DMT)
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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
14 - 4
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
14 - 5
ADSL Equipment
• Customer Side – ADSL Modem
• Telephone Company Side - DSLAM
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols
14 - 6
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
14 - 7
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
14 - 8
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
14 - 9
Cable Modem
• The “last mile” is still coaxial cable which is
a bandwidth bottleneck
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols
14 - 10
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
14 - 11
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
14 - 12
Cable Modem
• Cable Modem
• Cable Modem Transmission System
Winter 2006
ECE
ECE 766
Computer Interfacing and Protocols
14 - 13
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
14 - 14