COMT 2201 Digital Subscriber Loops 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 2203 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 2204 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 2205 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 2206 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 2207 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 2208 Access Alternatives Circuit Switched –Modems –ISDN Packet-Friendly –Cable Modems –LMDS –xDSL

COMT 2209 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 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 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 ADSL 16 to 640kbps subscriber to CO Up to 8Mbps CO to subscriber Limit feet under good conditions Impaired by –Wire gauge changes –Bridge Taps –Loading Coils

COMT ADSL Structure Customer LAN Customer Voice Devices ADSL Modem Splitter Local Loop CO Internet Voice Switch

COMT Some Structural Questions Where does ISDN fit into the xDSL strategy? How is the Internet connection accomplished –For ISDN –For ADSL