Lecture 9

Modems and Access Devices

Overview

• Computers are connected to telephone lines through the use of modems– modems: are connecting devices between the

telecommunication medium, such as copper or fiber, and a communicating device

– also called: DCE or data circuit-terminating devices

Data-Circuit Terminating Devices

• Modify computer signals to make them compatible with digital telephone lines

• DCE’s are required for digital lines

Type of DCE’s

• NT-1– used with ISDN

• CSU– T-1, T-3

Transferring Data

• From computers to phone lines:– analog and digital lines require different conversion devices

Functions of DCE’s (p.222)

• On analog and digital lines:– ensure even data flow, synchronization function– maintain proper voltages– error detection, correction– compression of data– remote diagnostics

Functions of DCE’s (p.222)

• On digital lines only:– ensure the correct number of 0’s and 1’s– shaping the digital signal

Functions of DCE’s (p.222)

• On analog signals only:– convert digital computer signals to analog

signals

Remote Diagnosis

• Network ownership and cost of maintenance distributed over web of devices and communication media

• DCE’s offer a point of reference in order to diagnose repair problems

• Test bits sent to DCE devices, if received assumed problem not in the telephone line or DCE

NT1’s and ISDN

• ISDN supports voice, video, data

• out-of-band signaling

• NT1 needed to connect ISDN to video conference units, PCs, PBXs, key systems and multiplexers

• Network Termination type 1

NT1’s and ISDN

• BRI ISDN– change two wire circuits coming in from

building into four wires needed for ISDN service

– plugs into the ISDN LINE with one cable– another cable plugs into a TA, terminal adapter

PCMCIA Modems

• Personal Computer Memory Card International Association

• 3.37 inches long by 2,126 inches wide

• plug into slots on laptops, palmtops

• Speeds vary, RJ11 jack or Ethernet, cellular, analog cellular and ISDN connections

PCMCIA Modems

• Used with:– analog POTS lines– analog cellular services– digital cellular services– fixed wireless services

CSUs/DSUs

• Connecting devices to a digital line, non-ISND services– T1, T3, switched 56k, dedicated 56k– supplied as one integrated piece of equipment

• CSU: channel service unit

• DSU: data service unit

CSUs/DSUs

CSU DSUincoming line

MULPLEXER

DSU

– Regulates voltage rates present on the signals from DTU to the CSU

– connects to a data terminating device such as a multiplexer or a PBX

CSUs

• Plugs into phone jack

• similar in function to a NT1

• provides clocking and signal reshaping

• must send out bits a even rate

• if clocking off will not transmit– the line is slipping– the timing is off

Cable Modems

• Provide high speed connections

• Same cable used for TV works for Internet

• Agreements as to how to transmit, “the handshake” hardware based– modem standards exist between sender/receiver– modems certified by Cable Labs

Cable Modems

• Connect to a HEADEND– point from which programming is transmitted

to local customers

• Traditionally a one-way medium, where content flows from headend to end-users

Cable Modems

• Currently working on developing two-way capability– called reverse channels from subscriber to

headend

Reversed Channels

• Created by using different frequencies for upstream and downstream transmissions– Upstream: 5-30 MHz or 5-42 MHz– Downstream: 54-350MHz or 54-750MHz

• Both set at different speeds

• Cable used more efficiently when splitting up the use of frequencies

Cable TV Systems

• When used for data communications:– analogous to being on an Ethernet LAN– all messages are broadcast onto the cable

connecting devices to the LAN– shared medium by all users on network

PC Cable Connection

• Cable modem connects home users to cable for Internet access– modem plugs into Ethernet card in PC– Another cable runs from cable modem to sliced

cable outlet

Cable Internet Modem Service

• All messages broadcast over entire network• Modem with specified address takes message of

the network, security issue• Every message first transmitted to the headend

and then to the addressee, congestion• Modem requires local power• Service always on, no extra phone line required• Extremely fast downloading speeds

Cable Modems for Remote

• Accomplished with Virtual Private Network

• Interface and security software installed on laptop to access remotely

• Corporate end: installs Internet connection and switch

• Both cable operator and customer connected to the Internet

Cable TV Set-Top Boxes

• Interface with TV and cable TV network for access to cable services

• Tuners

• Set-top box includes filters and traps– give access to basic and premium channels– some give Internet access from TV

Third Generations Set-Top Boxes

• Advanced digital security

• Advanced programming

• TV viewing while accessing Internet

• Ethernet plug in set top box

• video on demand

• mandated to be available to consumer by FCC