Computer Networks
Digital Access Technologies
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
2
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
3
Example
Core
Distribution
ISP1
Fiber-optic
Access
OC12 DPT
HFC
Regional
Core
HFC
OC48
SONET
Switch
ISP2
HFC
Spring 2006
Computer Networks
4
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
5
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
6
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
7
Access through Dial-up or Leased Line
PSTN
Dial-up or leased-line modem
 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
Spring 2006
Computer Networks
8
Connecting to ISP through Local Loop
Up to 56 Kbps
from ISP to the
subscriber
Up to 33.6 Kbps
Spring 2006
Computer Networks
9
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
10
ISDN Architecture
Customer’s office
ISDN
PBX
Customer’s office
Carrier’s office
PRA
BRA
NT1
ISDN
Exchange
NT1
LAN Gateway
Spring 2006
Computer Networks
11
ISDN Standardized Channels
 BRA/BRI (Basic Rate Access/Basic Rate Interface)
 2B+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
12
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
13
ADSL - Configuration
Network
Interface
Device
Access
Multiplexer
 Takes advantage of higher frequencies on most local loops
 Can be used simultaneously for POTS (Plain Old Telephone Service)
Spring 2006
Computer Networks
14
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 baud
4K * 15 = 60 Kbps per channel
Spring 2006
Computer Networks
15
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
16
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
17
Cable Network Configuration
Traditional
cable TV
network
Hybrid fibercoax (HFC)
cable TV
network
Spring 2006
Computer Networks
18
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
19
Access through Fiber to the Curb
Infrastructure with fiber can be build especially for Internet access
Spring 2006
Computer Networks
20
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
21
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.
Spring 2006
Computer Networks
22