Chapter 9 Using Telephone and Cable
Networks for Data Transmission
1. Telephone Network 2. Dial-up Modems
3. Digital Subscriber Line 4. Cable TV Networks
Telephone Companies: Brief History
• Before 1984
– Almost all services are provided by AT&T Bell System – In 1984, AT&T was broken into
• AT&T long Lines
• 23 BOCs(Bell Operating Companies)
• BOCs are grouped together to several RBOCs
• Between 1984 and 1996
– Divided into more than 200 LATAs (Local Access Transport Area s)
LATAs
• A LATA can be a small or large metropolitan area
• Intra-LATA services are provided by local exchan
ge carriers
– ILEC (Incumbent Local Exchange Carriers) – CLEC (Competitive Local Exchange Carriers)
• Inter-LATA services are handled by IXCs (Inter-E
xchange Carriers)
Signaling
• In-band signaling : the same circuit used for both signaling and voice communication
• Out-of-band signaling
• The task of data transfer and signaling are separated in modern telephone network
• Signaling system are required to
– Providing dial tone, ring tone, and busy tone – Transferring telephone numbers between offices – Maintaining and monitoring the call
– Keeping billing information
– Maintaining and monitoring the status of the telephone network equipment
Telephone Network Services
• Analog services
– Analog switched services • Dial-up service
• 800 service, 900 service
• WATS (wide-area telephone service)
– Analog leased service: called a dedicated line
• Digital service
– Switched/56 service
Dial-Up Modems
Modem
Modem Standards
• V-series standards by ITU-T
– V.32 : Trellis-coded modulation: 32-QAM (4 data bits and a redundant bit for error detection) 9,600 bps – V.32bis : 128-QAM (7 bits/baud with 1 bit for error
control) 14,400 bps
– V.34bis : 28,800 bps with a 960-point constellation and 33,600bps with a 1664-point constellation
V.90: 56K Modem
• Traditional modems have a data rate
limitation of 33.6 kbps by Shannon capacity • Uploading : 33.6 kbps • Downloading: 56 kbps
ADSL
• Asymmetrical Digital Subscriber Line
• ADSL is an asymmetric communication
technology designed for residential users; it is not
suitable for businesses
• The existing local loop can handle bandwidths up
to 1.1 MHz
Bandwidth Division in ADSL
• There is no set way that the bandwidth is divided • Upstream
– 24 channels x 4 kHz x 15 bits/Hz = 1.44 Mbps • Downstream
DSL: Actual Bit Rate
• Because of the high signal/noise ratio
• Upstream
– Normally below 500 kbps
• Downstream
Telco Site: DSLAM
Other DSL Technologies: xDSL
• HDSL (High-bit-rate DSL)
– Designed as an alternative to the T-1 line (AMI encoding) – 2B1Q encoding used for less susceptible to attenuation – Up to a distance 12,000 ft without repeaters
– Two twisted pairs for full-duplex transmission • SDSL (Symmetric DSL)
– One twisted pair version of HDSL
– 768 kbps in each direction, symmetric communication
– Send and receive data in large volumes in both directions • VDSL (Very-high-bit-rate DSL): Alternative approach to ADSL
– Coaxial, fiber-optic, or twisted-pair cable for short distances
Traditional Cable TV Network
• Started to distribute video signals to locations with poor or no reception in the late 1940s: Unidirectional
HFC (Hybrid Fiber Coaxial) Network
• RCH (Regional Cable Head) feeds the distribution hubs • Fiber node splits the signal to each coaxial cable
Coaxial Cable Bands
• Video band: 6 MHz per TV channel
• Downstream data band: 64-QAM (or possibly 256-QAM) – Theoretical rate is 30 Mbps (6 MHz/sec x 5 bits/Hz) • Upstream data band: QPSK
– Theoretical rate is 12 Mbps (6 MHz/sec x 2 bits/Hz)
Data Transmission Schemes: DOCSIS
• Standard for data transmission over a HFC
network by Multimedia Cable Network System
(MCNS)
– Data Over Cable System Interface Specification