Lecture 9

1. Lecture 9 Modems and Access Devices

2. 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

3. Data-Circuit Terminating Devices • Modify computer signals to make them compatible with digital telephone lines • DCE’s are required for digital lines

4. Type of DCE’s • NT-1 • used with ISDN • CSU • T-1, T-3

5. Transferring Data • From computers to phone lines: • analog and digital lines require different conversion devices

6. 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

7. Functions of DCE’s (p.222) • On digital lines only: • ensure the correct number of 0’s and 1’s • shaping the digital signal

8. Functions of DCE’s (p.222) • On analog signals only: • convert digital computer signals to analog signals

9. 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

10. 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

11. 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

12. 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

13. PCMCIA Modems • Used with: • analog POTS lines • analog cellular services • digital cellular services • fixed wireless services

14. 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

15. CSUs/DSUs M U L P L E X E R incoming line CSU DSU

16. 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

17. 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

18. 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

19. 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

20. Cable Modems • Currently working on developing two-way capability • called reverse channels from subscriber to headend

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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