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.