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									      Lecture 9
Modems and Access Devices
• 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
 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
  – 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
          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
  – change two wire circuits coming in from
    building into four wires needed for ISDN
  – 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
• 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

incoming line               P
                CSU   DSU   L
– 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
•   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
  – called reverse channels from subscriber to
          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

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