Download Modems & access devices

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:
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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:
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analog POTS lines
analog cellular services
digital cellular services
fixed wireless services
CSUs/DSUs
• Connecting devices to a digital line, nonISND services
– T1, T3, switched 56k, dedicated 56k
– supplied as one integrated piece of equipment
• CSU: channel service unit
• DSU: data service unit
CSUs/DSUs
incoming line
CSU
DSU
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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
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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
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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