COMT 220

1

Digital Subscriber Loops

COMT 220

COMT 220

2

Switching

• The network moves information between two subscribers

• Normally, subscribers do not have direct connections to each other

• The network switches create appropriate connections on demand to allow flow of the information

COMT 220

3

Switching Types

• Circuit Switching• Packet/Message Switching or

Routing• Fast Packet Switching

– Can emulate Constant Information Rate (CIR) operation

• Access Contention (LANs or Polled Multipoint Lines)

COMT 220

4

Circuit Switching

• Call Setup Phase– Subscriber informs the network about

the type and destination of the connection needed

– Network completes the request• Transmission

– Information of the type indicated in the setup phase flows without network interaction (“Transparent”)

COMT 220

5

Message Switching

• Subscriber collects the information to transmit

• The “Message” is transfered to the network access points

• The network moves the message through various points until delivery

COMT 220

6

Packet Switching

• Based on message switching• Messages can be very large, and

storage in the network may not be sufficient

• Message it cut into “Packets”; each packet travels through the network independently

COMT 220

7

The Digital Access Issue

• The PSTN network is optimized for voice

• The voice network is based on circuit switching, expecting call durations of 3-5 minutes

• Data network access calls last much longer, and use circuit switching to provide packet transport service

COMT 220

8

Access Alternatives

• Circuit Switched– Modems– ISDN

• Packet-Friendly– Cable Modems– LMDS– xDSL

COMT 220

9

Cable Modems

• Cable plant has high bandwidth available (6MHz per cable channel, 45MHz “upstream”)

• Cable TV acts like a LAN, the capacity must be shared

• There is no circuit switching, the connection is “always on”.

COMT 220

10

LMDS

• Designed as “Wireless Cable”, based on the same idea as cellular telephony

• Large capacity available (1.15GHz in Block A, 150MHz in Block B)

COMT 220

11

xDSL

• Started due to a desire to run T1/E1 without repeaters at every mile.

• Uses sophisticated coding -- like modern dial-up modems -- on a much larger frequency range

COMT 220

12

ADSL

• 16 to 640kbps subscriber to CO• Up to 8Mbps CO to subscriber• Limit 18000 feet under good

conditions• Impaired by

– Wire gauge changes– Bridge Taps– Loading Coils

COMT 220

13

ADSL Structure

CustomerLAN

CustomerVoice Devices

ADSLModem

Splitter Local Loop

CO

Internet

Voice Switch

COMT 220

14

Some Structural Questions

• Where does ISDN fit into the xDSL strategy?

• How is the Internet connection accomplished– For ISDN– For ADSL