Computer Networks
Digital Access Technologies
Spring 2006 Computer Networks 2
How Computer Networks are Built?
LANs (Local Area Networks) are relatively cheap and easy to built.
WANs are expensive and difficult to built PSTN (Public Switched Telephone
Network) is ”an older brother” to the computer networks.
Computer still need to use infrastructure built by PSTN, esspecially when it comes to WAN
Spring 2006 Computer Networks 3
Core, Distribution and Access Network
Core Network Combination of switching centers and
transmission systems connecting switching centers.
Distribution Network Network in between the access and core
network
Access Network The portion of public network that connects
individuals or companies to some access node through which they can reach the core network (directly or via some distribution network)
Spring 2006 Computer Networks 4
Example
Fiber-optic OC12 DPT
HFCHFC
OC48SONET
ISP1
Core Distribution Access
HFCHFC
HFCHFC
RegionalCore
ISP2Switch
Spring 2006 Computer Networks 5
Various Access Options
Access at the customer premises Narrow band
PSTN based access ISDN based access
Broad band xDSL Cable modem Fiber to the curb
Distribution network E1, E2, E3, SDH (T1, T2, T3, SONET)
Spring 2006 Computer Networks 6
PSTN Based Access
PSTN (Public Switched Telephone Network) is acronym for the telephone network that uses circuit switching When using PSTN access to a computer network, the
modems are needed at both ends of the connection Circuit switching is used although inconvenient for
data traffic Data are transmitted in bursts and therefore the
bandwidth is not used all the time during the connection The bit rate is limited with the bandwidth (usually 4KHz
for voice) and the Shannon’s theorem Different modulation techniques (QAM, multiple PSK)
can improve the bit rate Due to the known S/N for voice channel these
techniques cannot do much more then 33600bps
Spring 2006 Computer Networks 7
Local Loop Based Access
Local loop describes connection from
telephone office to home
Also known as local subscriber line
Most local loops use analog signals
Sometimes called POTS (Plain Old
Telephone Service)
Legacy infrastructure is copper
Spring 2006 Computer Networks 8
Access through Dial-up or Leased Line
Dial-up The connection goes through the switch (telephone
exchange (the line is released after disconnection) The customer is charged by the time it uses the
connection Leased line (Dedicated line)
The connection does not go through the switch (the line is dedicated all the time to the connection)
The customer is charged with fixed monthly rate
PSTN
Dial-up or leased-line modem
Spring 2006 Computer Networks 9
Connecting to ISP through Local Loop
Up to 56 Kbps from ISP to the subscriber
Up to 33.6 Kbps
Spring 2006 Computer Networks 10
ISDN Based Access
ISDN stands for Integrated Services Digital Network
Provides digital service on existing local loop copper Establishes a digital pipe between the customer and the
telephone company Allows access to multiple services through a single access
(digital telephone, digital terminal, digital facsimile machine)
Attempt to replace the analog telephone system with digital one (for voice and data traffic)
Obsolete for many reasons Too expansive
Charged by time
Almost equivalent to analog modems (64 Kbps)
Spring 2006 Computer Networks 11
ISDN Architecture
ISDNPBX
PRA BRA
NT1 NT1ISDN Exchange
LAN Gateway
Carrier’s officeCustomer’s office Customer’s office
Spring 2006 Computer Networks 12
ISDN Standardized Channels
BRA/BRI (Basic Rate Access/Basic Rate Interface) 2 B + D 2 x 64 Kbps + 16 Kbps = 144 Kbps (not including
overhead) designed to operate using the average local copper pair
PRA/PRI (Primary Rate Access/Primary Rate Interface) 23 B + D 23 x 64 Kbps + 64 Kbps = 1.536 Mbps (not including
overhead) Designed to operate using DS-1/E1 In Europe 30 B + D Optional backup D channel.
Spring 2006 Computer Networks 13
Access with xDSL
xDSL stands for a family of DSL (Digital Subscriber Loop) technologies
ADSL (Asymmetric DSL) is most popular for residential access Higher speed into home than out of home More bits flow in ("downstream") than out
("upstream")
The maximum speed depends on the length and quality of the copper in the subscriber loop
Spring 2006 Computer Networks 14
ADSL - Configuration
Takes advantage of higher frequencies on most local loops
Can be used simultaneously for POTS (Plain Old Telephone Service)
Network Interface
Device
AccessMultiplexer
Spring 2006 Computer Networks 15
ADSL Frequency Spectrum
Divides the bandwidth into 256 x 4.3K channels 1 (ch 0) POTS, 5 (ch 1-5) not used, 1 upstream control, 1
downstream control Typical 6-30 for upstream, rest for downstream Each 4.3K channel 4K baud sample, V.34 QAM
modulation, up to 15 bits per baud4K * 15 = 60 Kbps per channel
Spring 2006 Computer Networks 16
Other DSL Technologies
SDSL (Symmetric DSL) divides frequencies evenly
HDSL (High-rate DSL) provides DS1 bit rate in both directions
Short distances
Four wires
VDSL (Very high bit rate DSL) provides up to 52 Mbps
Very short distance
Requires Optical Network Unit (ONU) as a relay
Spring 2006 Computer Networks 17
Access with Cable Modem
Cable TV already brings high bandwidth coaxial cable into the houses
Cable modems encode and decode data from cable TV coaxial cable Instead of a traditional cable box, the splitter is
installed in the home (directs the TV bands to the TV set and the Internet access bands to the PC)
Bandwidth dedicated to the Internet is multiplexed among all users Usually the rate is asymmetrical (500 Kbps to 1
Mbps from PC to Internet and 3 to 10 MHz in the oposite direction
Spring 2006 Computer Networks 18
Cable Network Configuration
Traditional cable TV network
Hybrid fiber-coax (HFC) cable TV network
Spring 2006 Computer Networks 19
Cable Frequency Spectrum
TV band: 54 – 550 MHz, 6MHz per channel Downstream : 550 – 750 MHz 6MHz QAM-64 (6bit) == 36 Mbps (gross), 27 Mbps
(net) Total effective downstream bandwidth 200 / 6 * 27 =
891 Mbps Upstream : 5 – 42 MHz 6MHz QPSK (2bit) == 12 Mbps (gross), 9 Mbps (net) Total effective upstream bandwidth 37 / 6 * 9 = 54
Mbps
Spring 2006 Computer Networks 20
Access through Fiber to the Curb
Infrastructure with fiber can be build especially for Internet access
Spring 2006 Computer Networks 21
Distribution with Optical Hierarchies
SONET (Synhronious Optical NETwork) A standard for TDM used in United States
SDH (Synhronous Digital Hierarchy) A standard for TDM in Europe
Both use synhronous communication Digital telephony systems use clocking for
synchronous data delivery Synchronous network moves data at a
precise rate
Spring 2006 Computer Networks 22
Optical Hierarchies - Characteristics
SONET and SDH are based on the principal of direct synchronous multiplexing.
Provide advanced network management and maintenance features.
Both SONET and SDH can transport signals for all the networks in existence today and it has the flexibility to accommodate any networks defined in the future.
Can be used in the three traditional telecommunications areas: long-haul networks, local networks and loop carriers. It can also be used to carry CATV video traffic.
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