1. Basic Communications Systems Class 9

2. Today’s Class Topics WAN Data Services DSL Cable Modems T1 Access Lines Frame Relay Voice Processing Voice Call Control Business Services Key Systems, PBX and Centrex

3. What can you do with a copper subscriber loop? Modems Can provide up to 52 Kbps downstream over subscriber loop. Integrated Services Digital Networks (ISDN) Provides 144 Kbps over copper telephone line. Digital Subscriber Line (DSL) Technologies Can provide up to 52 Mbps over short copper loop.

4. Name Service Data RateDistance V.32 V.34 Voice Band Modems 9600 - 28,800 bpsAny Voice Line V.9056K Modem24 - 52 Kbps down 33.6 Kbps up Any Voice Line IDSLISDN DSL144 Kbps18,000-24,000 feet SDSLSingle-Pair DSLUp to 1.544 Mbps12,000 feet HDSLHigh Speed DSL1.544 Mbps 12,000-18,000 feet ADSLAsymmetric DSL Up to 7 Mbps down Up to 640 Kbps up 12,000 – 18,000 feet ADSL Lite Up to 1.5 Mbps down Up to 512 Kbps up 12,000 – 18,000 feet VDSLVery High-speed DSL 13 Mbps down4,500 feet 25.82 Mbps down3,000 feet 52 Mbps down1,000 feet Data over Copper Loop

5. Digital Subscriber Lines Problems Loop Qualification The condition of many copper loops is unknown. Testing and removal of load coils takes time. Distance DSL does not work over 18,000 feet More than 20% of subscriber loops in U.S. are longer than 18,000 feet. Crosstalk Some types of ADSL and VDSL can interfere with data signals on adjacent pairs (crosstalk)

6. ADSL Provides 1.5 Mbps – 7 Mbps downstream, depending on equipment and distance 16 Kbps – 640 Kbps upstream, depending on equipment and distance 4 KHz analog voice channel Currently being aggressively deployed by CLECs and LECs for Internet access and other applications

7. ADSL Lite A simpler version of ADSL (G.lite) designed to be installed without any Telco visit to the home. G.Lite modems will be sold directly to consumers Usually provides 1.5 Mbps downstream 384 Kbps upstream No analog channel (splitterless) May allow faster deployment due to simplified installation

8. Voice over ADSL “Voice over ADSL” products are now being deployed. Allow customer to dynamically allocate multiple voice channels on ADSL data channel Voice allocation choices: static 24 x DS0 allocation as with T1 (i.e. 4 voice channels and 20x64K = 1.28 Mbps of data) packetized voice over DSL data channel Voice/data over ATM over DSL

9. VDSL Very High Speed data channel over short copper loops Provides access to Fiber-to-the Curb installations Carrier provides fiber to building basement or to neighborhood Optical Networking Unit (ONU) VDSL provides data access to ONU Can provide high-speed LAN-to-LAN interconnection at up to 52 Mbps

10. CATV Telephony CATV advantages: Coaxial cable offers much greater bandwidth than copper pair Cable is already installed to ~70% of U.S. homes CATV problems: Coax systems may need to be upgraded Hybrid Fiber Coax (HFC) 2-way amplifiers Cost of telephony equipment is large Return signals are very noisy Not a great perception of reliability

11. Cable TV Plant From: Videon Cable Modem Technology Primer

12. Hybrid Fiber / Coax From: Videon Cable Modem Technology Primer

13. Cable Modem Standards Data Over Cable Service Interface Specification (DOCSIS): Components: Cable Modem Termination System Hybrid Fiber Coax Network Cable Modem Connection is achieved through 10 / 100 Mbps Ethernet connector Downstream speed: 27 Mbps or 40 Mbps.

14. T-Carrier Systems

15. The T1 System T1 was the first T-carrier system deployed by the Bell System (in 1962) Bit Rate: 1.544 Mbps: Digital Information: 1.536 Mbps Framing Bits: 8 Kbps Originally run over 4-wire (2 pair) copper wire with regenerators every 6000 feet. Can also be sent over fiber.

16. T1 System Uses T1 Carrier Trunk Telecommunications companies use T1 trunks between switching offices T1 Access Circuit Business customers use T1s as: PBX - CO trunks (24 digital trunks on 1 cable) Data access lines (1 data channel running at 1.536 Mbps)

17. Channelized? Channelized T-1 Circuit T-1 is utilized as 24 DS0 channels of 64 Kbps each. Each DS0 can be allocated to carry any single service, such as CO trunk, DID trunk, WATS, FX, 56K data, switched 56K, etc. Unchannelized T-1 T-1 is utilized as a single 1.536 Mbps data circuit.

18. Fractional T1 (Channelized) A customer may request a leased Fractional T1, which means: Customer only sends data on an agreed subset of the DS0s (example: DS0s 1-6) Carrier only forwards these particular DS0s to the far-end Customer pays less than full T1 fee Example: 256 Kbps fractional T1 (4 x DS0) from Chicago-NY could be ~$2500/month

19. UnChannelized T1: 1.536 Mbps Leased Data Service

20. T1 Details Bipolar Representation T1 uses Bipolar Coding to represent 1 and 0 bits ‘1’ bit represented by alternating +3 volt, - 3 volt pulses ‘0’ bits represented by no voltage Framed Format T1 transmits 8000 frames per second, 193 bits per frame (8000 * 193 = 1,544,000).

21. Bipolar Representation

22. T1 Frame Format n Each DS0 called a time slot n 8000 frames/sec * 8 bits/slot = 64 Kbps n 24 * 8 + 1 = 193 bits/frame n 8000 frames/sec * 193 bits/frame = 1.544 Mbps n 8000 Framing bits sent per second

23. T1 Framing Bits D4 T1 lines (1972): Allow receiver to find the start-of-frame (frame synchronization). Group sets of 12 frames into superframes Indicate that frames 1 and 6 contain signaling bits (to specify if channel is in use or not) D5 (ESF) T1 lines (1983): Provide error checking (CRC) (ESF T1) Provide Facilities Data Link channel to transmit network management messages (ESF T1)

24. D4 Frame Format n Frames 6, 12: n Frames 1-5, 7-11:

25. T1 Framing Bits (ESF Frame) D5 Framing - Extended Superframe T1 (1983) F-bit pattern marks 24-frame extended superframes F-bit pattern: Odd frames: Facilities Data Link Every 4th frame: 001011 (Framing pattern) Every 4th frame: CRC for previous ESF

26. ESF Frame Advantages Facilities Data Link Network diagnostics and management messages sent between carrier equipment Cyclic Redundancy Check (CRC) Allows error detection on T1 lines Carrier can offer Automatic Protection Switching service to customer (switches to another T1 line if errors detected)

27. Data over T1 A T1 carries 24 DS0 channels A DS0 may carry a maximum of 56 Kbps or 64 Kbps of data A restricted T1 can carry up to 24 x 56 Kbps = 1.344 Mbps. A clear-channel T1 can carry up to 24 x 64 Kbps = 1.536 Mbps.

28. T1 Data – 56K or 64K ? T1 capacity depends on: Line Coding An AMI T1 carries 56 Kbps per DS0 A B8ZS T1 carries 64 Kbps per DS0 Signaling A T1 using robbed bit signaling is limited to 56 Kbps per DS0 for data (to avoid signal bits) A leased-line T1 (no signaling) or a T1 on a Signaling System 7 (SS7) network does not need robbed bit signaling.

29. Frame Relay Telecommunications carriers maintain networks of Frame Relay switches Customer get access line to nearest switch to get Frame Relay service Higher data rates than X.25 Lower delays, higher throughputs and better security than the Internet

30. Frame Relay Basics Data is sent over pre-established Virtual Circuits, like X.25. Frame Assembler/Disassemblers (FRADs) can be used to connect internal devices to the frame relay network FRADs generate Frame Relay headers/trailers and send data frames into the network

31. Frame Relay Basics Data only sent over Permanent Virtual Circuits (PVCs), which are set up by the carrier, not customer - always available Customer can access network at data rates from 56 Kbps to 45 Mbps No error control done by network switches (error control is responsibility of customer)

32. Frame Relay Basics Fixed monthly cost based on “Line charge” for access line between user site and frame relay carrier location “Port charge” for each connection into carrier equipment “PVC charge” for each PVC defined between ports Frame Relay is a layer 2 protocol, so any layer 3 protocol (like IP, for example) can be carried over a Frame Relay network

33. Replacing Leased Lines Typically used to replace leased lines: Customer gets one Frame Relay PVC to replace each leased line Customer still gets guaranteed delay and throughput (CIR) similar to leased line Customer uses one access line at each business location for all frame relay data Customer saves money - PVCs cost less than leased lines

34. Leased Line Problem: Number of leased circuits (and cost!) grows very large as number of sites increases!!

35. The Frame Relay Solution: 1 Access Line for each site!!

36. Frame Relay Addressing The carrier assigns each PVC a 10-bit Data Link Connection Identifier (DLCI). Customer sets up a table in each access router that maps each possible destination to its DLCI. Router puts correct DLCI into each frame header before sending frame into network.

37. Frame Relay Frame Format Flag - Fixed bit pattern to start and end frame - set to 01111110 Data Link Connection ID – PVC Address Discard Eligible – Determines whether this frame can be discarded at network switches CRC - Allows error detection Some unused header bits not shown

39. CIR For each PVC, customer specifies a Committed Information Rate (CIR): CIR represents a guaranteed throughput for this PVC Carrier also guarantees limited data delivery time if customer does not exceed CIR Price of PVC is directly related to CIR High CIR = high monthly price Low CIR = lower monthly price

40. CIR Example: I have a T1 (1.536 Mbps) access line into the frame relay network in Chicago I ask my carrier to create a PVC from Chicago to Dallas with CIR = 512 Kbps. If I stay within my CIR (i.e., send less than 512,000 bps Chicago-Dallas, on average): Carrier guarantees 99.99% traffic gets through Carrier guarantees <= 20 ms. Network delay

41. Frame Relay PVCs with CIRs

42. Frame Relay Pricing Example: 4 sites, each with T1 access line 6 PVCs providing connectivity between sites, with 56 Kbps CIR on each PVC Monthly Costs: 4 x (T1 access port cost) plus 6 x (56 Kbps PVC cost)

43. What if I exceed my CIR? Example: I have a T1 (1.536 Mbps) access line into the frame relay network in Chicago I ask my carrier to create a PVC from Chicago to Dallas with CIR = 512 Kbps. Isn’t it possible for me to exceed my CIR (send more than 512 Kbps Chicago-Dallas)? YES!!!!!

44. What if I exceed my CIR? Most carriers will allow customers to exceed CIR up to a fixed Burst Rate (Br) for up to 2 seconds with no penalty. If customer continues to exceed CIR beyond 2 seconds, carrier sets Discard Eligible (DE) bit in frame headers If network congestion occurs, DE marked frames are discarded by network switches

45. The Tradeoff Low CIR ==> Low cost, but your data may be discarded by the network High CIR ==> High cost, but data throughput is guaranteed by carrier Note: Many customer still choose to pay lowest cost by selecting a Zero CIR option that provides no delivery guarantees

46. Frame Relay vs. The Internet Frame Relay advantages: Guaranteed throughput and delay (ISPs generally give no guarantees) Security (hackers cannot break into PVCs between corporate sites) Frame Relay disadvantages Price (more expensive than Internet service) Inflexibility (can’t send data to another site unless PVC is already in place)

47. Network-to-Network Interfaces Do you ever want to set up a PVC between sites connected to 2 different Frame Relay providers?? YES!! To connect between LEC networks To set up an extranet with trading partner To interconnect sites after company merger

48. Network-to-Network Interfaces How do you set up an inter-carrier PVC? The carriers need to set up a Frame Relay Network-to-Network Interface (NNI) which controls traffic between the networks

49. Network-to-Network Interfaces Carriers don’t like NNIs!! How do they split the fees ($$)?? If the PVC goes down, who is to blame? What are the end-to-end performance guarantees? Coordinating PVC addresses (DLCIs) for both networks is a hassle

50. Network-to-Network Interfaces NNIs: The bottom line: Some carriers refuse to set up NNIs If you have a big enough contract with them, perhaps you can convince them otherwise. Alternatives You can move all your FR sites to one carrier You can set up your own router with connections to PVCs on both carrier networks But this may cause traffic bottleneck at that router. You can switch to a routed IP network service

51. Mini-Case Study GREGCO has a main office in Chicago and branch offices in New York, Miami and Los Angeles. Branch offices and main office exchange data files in both directions Average file size is 150 Kbytes Average file transfer time must be less than 30 seconds

52. Mini-Case Study Can GREGCO satisfy its requirements using dial-up modems? If GREGCO uses leased lines, what are the monthly service costs? If GREGCO uses Frame Relay service How many PVCs? What are the CIRs? What are the monthly service costs?

53. Example: Leased Line Pricing InterLATA T-1$2000 + $4/mile ServiceMonthly Charge IntraLATA 56Kbps $300 IntraLATA T-1$600 InterLATA 56 Kbps $900 + $1.50/mile

54. Example: Frame Relay Port Charges Port SpeedMonthly Charge 56Kbps$220 128 Kbps$400 256 Kbps$495 512 Kbps$920 T-1$1620

55. Example: Frame Relay PVC Charges PVC CIRMonthly Charge 16 Kbps$25 32 Kbps$40 48 Kbps$50 56 Kbps$60 128 Kbps$110 256 Kbps$230 384 Kbps$330 512 Kbps$410 1024 Kbps$1010 1536 Kbps$1410

56. Data Communications and Computer Networks Chapter 12 Basic Telephone Systems A telephone number consists of an area code, an exchange, and a subscriber extension. The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services.

57. Data Communications and Computer Networks Chapter 12

58. Address Signaling þ Rotary Pulse Dial After pulling dial for digit ‘N’, telephone opens and closes subscriber loop ‘N’ times as dial returns to its original position. Dual Tone Multifrequency (DTMF) or Touch Tone One frequency assigned to each row and each column. Pressing any key sends a 2- frequency signal tone.

59. North American Numbering Plan Administration (NANPA) Determines format for dialed numbers. Chooses new area codes, exchange codes Format before 1995: Prefix + 0/1 + N(0/1)X - NNX - XXXX (N digits are 2-9, X digits are 0-9) Format after January, 1995: Prefix + 0/1 + NXX - NXX - XXXX (N digits are 2-9, X digits are 0-9) www.nanpa.com

60. North American Numbering Plan (NANP) Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9) NNX - Numbering Plan Area / Area Code Determines geographic area. NXX - Central Office Prefix / Exchange Code Determines Central Office handling this number. XXXX - Station Number Identifies particular end station on C.O.

61. North American Numbering Plan (NANP) Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9) Prefix - Can be used to select IXC carrier 10-XXX Selects carrier associated with Carrier Identification Code (CIC) XXX. 101-XXXX (as of 1998) Selects carrier associated with Carrier Identification Code (CIC) XXXX. 0/1 - Used to request “operator assistance”: 0 = Collect, calling card, etc.

62. Central Office Switch Technologies Central Office

63. Wire Center Components Central Office Switches Main Distribution Frame (MDF) Cable Vault

64. Wire Center Components Central Office Switch Components Common Control is central processor Data Store contains all customer feature and billing information Service Circuits are shared circuits for ringing, tones, digit collection, etc.

65. Wire Center Components Central Office Switch Components Switch Matrix can interconnect Line/Trunk cards and Service Circuits Line Cards perform per-line processing (BORSCHT functions). Trunk Cards perform per-trunk processing.

66. Wire Center Components Distribution Frame Patch cables connect switch interfaces (Switch Side) to outside cable (Cable Side) Fuses protect against foreign potential Cable Vault Outside Plant Subscriber loops Trunks

67. How is a Voice Call Made?

68. A Voice Call Originating phone goes off-hook. Line Card sees current, notifies Common Control Common Control checks customer records Common Control sets up connections to Service Circuits: Digit Decoder circuit Dial Tone Generator circuit

69. A Voice Call Customer dials number. Dial Tone is disconnected after 1st digit. Digit Decoder de-allocated after all digits dialed. Common Control: Consults Routing Table. Determines features of terminating line. Connects originating line to Ringback Generator Connects terminating line to Ringing Circuit.

70. A Voice Call Call is answered. Ringing Line Card detects off- hook and informs Common Control Common Control disconnects ringing circuit Common Control connects originating line card to terminating line card through Switch Matrix Talking path has been established.

71. A Voice Call þ Phones go on-hook. Tear down connection (after time-out period if one phone still off-hook). Store call record.

72. Inter-Office Calls

73. Business Service Pricing Fixed Price Services Same monthly cost regardless of how much service is used Example: Leased Line Measured Services Monthly cost based on minutes of usage Example: Direct Distance Dialing (DDD or long distance calling)

74. Private Line Services Private Line Service provides a dedicated circuit between two locations Fixed connection is made on the distribution frame Central Office provides no dial tone service Customer pays flat rate each month Also called a “dedicated circuit” or “leased line” service

75. Private Line Services Private Line / Leased Line / Dedicated Line Direct circuit between 2 locations No switching service from Central Office (only transmission service). Flat Rate charge based on distance and bandwidth.

76. Private Line Services Foreign Exchange (FX) Line Allows a telephone to get dial tone from a C.O. switch other than the closest one Off Premises Extension (OPX) Allows a telephone to connect to a PBX at a different customer location

77. Private Line Services Foreign Exchange (FX) Line Provides local switched telephone service from a Central Office outside of the subscriber’s exchange area.

78. Private Line Services Off Premises Extension (OPX) Private line service that allows a remote business telephone to access local PBX services.

79. FX Line Example GregCo Corporation in Chicago has many customers in New York DDD Chgo-NY costs 15 cents per minute FX line Chgo-NY costs $800 per month GregCo makes about 25 calls to NY each day, each call lasting about 10 minutes SHOULD GREGCO GET AN FX LINE?

80. Private Line Services Tie Line Used to directly connect PBX switches at 2 customer sites Dedicated Access Line to IXC POP Directly connects PBX switch to IXC point of presence. Eliminates LEC access charges for calls made on this line (replaces them with flat monthly fee for private line)

81. Private Line Services Tie Line A dedicated circuit between two PBXes. Tie lines may be used due to cost savings (as compared with switched service) or to allow proprietary signaling between PBXes.

82. Private Line Services Dedicated access to IXC POP A dedicated circuit between customer site and IXC POP Eliminates LEC access charges for long-distance calls over this line.

83. Switched Line Services Direct Distance Dialing (DDD) Plain long-distance telephone service Wide Area Telecommunications Service (WATS) Reduced rates to certain geographic areas IN-WATS (800, 888 services) Business pays for each call received on this line.

84. Switched Line Services 900 Services Allows customer to charge whatever they want for calls to this number International WATS Reduced rates to certain countries Remote Call Forwarding; Ex: Business in Chicago can advertise local phone number in New York. May be cheaper than FX line.

85. RCF Example GregCo Corporation in Chicago has many customers in New York. GregCo wants to offer a local New York number. DDD Chgo-NY costs 15 cents per minute RCF service costs $50 per month FX line Chgo-NY costs $800 per month Customers in NY make about 10 calls to GregCo each day, each call lasting about 10 minutes SHOULD GREGCO GET AN FX LINE?

86. Virtual Private Networks Virtual Private Networks (VPNs) are an alternative service, where The carrier does not dedicate an end-to-end circuit to the customer circuit. Carrier charges fixed rate per month for calls between 2 locations Carrier guarantees a particular service level (i.e., 99.99% of calls get through on first try).

87. Virtual Private Networks So, Virtual Private Networks look and perform a lot like Private Lines, but they cost less.

88. Virtual Private Networks Example: Private line from Chicago - New York Cost: $800 per month Reliability: 99.999% uptime VPN Service from Chicago - New York Cost: $600 per month Reliability: 99.99% calls get through on first try Which would you choose?

89. Private Line Features Fixed cost per month regardless of usage Typically best value when lots of calls go between two fixed locations Can be imitated by Virtual Private Network carrier service

90. Switched Service Features Variable cost per month depending on usage May provide special service (like 800 number) Lower cost than leased lines when there is not much call traffic Price discounts offered for large call volumes predictable call patterns (for example, WATS services to particular geographic regions)

91. Key Telephone Systems Key System Unit (KSU) ties many multi-line sets into a set of C.O. lines. User selects outside line by pressing corresponding line button on key set.

92. Typical Key System Features Call Transfer Call Hold Music on Hold Conference Calling Speed dialing Intercom calling between key sets Paging Call Pickup and Call Barge-In Speakerphones / Hands-free calling Automatic Call Information Recording

93. A Call using a Key System Frank calls Sam in New York Frank picks up phone, hits LINE #2 button Key system connects set to C.O. line #2 Frank hears dial tone from the C.O. Frank dials 1-212-894-6622.

94. A Call using a Key System Frank calls Sam in New York C.O. routes call to New York, rings Sam’s phone. Sam answers phone. Frank and Sam talk for a while

95. A Call using a Key System Frank calls Sam in New York Frank decides to consult with Jane Frank hits HOLD button - Key System puts current call to Sam on hold. Frank hits INTERCOM button and dials Jane’s set number

96. A Call using a Key System Frank calls Sam in New York Jane hears a special intercom ring Jane and Frank talk

97. A Call using a Key System Frank calls Sam in New York Frank hits LINE #2 button again and now talks with Sam again.

98. A Call into a Key System Joe calls Frank Joe picks up phone, dials number associated with LINE #2 of Frank’s Key System

99. A Call into a Key System Joe calls Frank C.O. rings LINE #2 connected to KSU KSU sends ringing signal out to all phones programmed for access to LINE #2

100. A Call into a Key System Joe calls Frank Frank answers phone and talks to Joe

101. Private Branch Exchange (PBX) PBX is a true intelligent switching system. Provides local switching between station sets or access to C.O. Users dial access code (like ‘9’) to get C.O. service. Provides advanced intelligent features to users.

102. PBX Features ( not found on Key Systems) Private Dialing Plans 4-digit, special prefixes for WATS, FX, etc. Automatic Route Selection PBX collects dialed digits and intelligently decides how to route this call for lowest cost Over FX line Over WATS trunk Over DOD trunk

103. PBX Features ( not found on Key Systems) Voice Mail Automatic Call Distribution (ACD) Routes incoming calls (usually INWATS) to the best station set and location Voice Response Unit (VRU) Provides recorded messages and responds to touch-tone requests from callers

104. PBX Features ( not found on Key Systems) Class-of-Service Permission to use each PBX feature can be given or taken away from each user. Authentication Codes PBX users can identify themselves with “passwords” when dialing calls to get special access permissions Data Services ISDN, Dial-Up digital services

105. A Call using a PBX Frank calls Sam in New York Frank picks up the phone and gets dial tone from the PBX. Frank dials 9-1-212-894-6622. PBX consults routing tables, determines that this call should go over the FX trunk to New York

106. A Call using a PBX Frank calls Sam in New York PBX sends dialed number over FX trunk, dropping area code (sends “894-6622”). Call to New York is completed. Frank and Sam talk.

107. A Call into a PBX Joe calls Frank Joe picks up phone. Dials Frank’s public number 1-312-362-6587 C.O. chooses trunk to PBX (could choose ANY trunk).

108. A Call into a PBX Joe calls Frank C.O. sends OFF-HOOK signal to PBX over the trunk PBX sends WINK-BACK signal to C.O. C.O. sends last 4 digits - “6587” (this is a DID trunk).

109. A Call into a PBX Joe calls Frank PBX looks into its station address table and determines that “6587” corresponds to Frank’s station set. PBX rings Frank’s station set.

110. A Call into a PBX Joe calls Frank Frank picks up telephone handset. PBX sends ANSWER signal back to C.O. indicating that the call has been answered Frank and Joe talk

111. PBX Trunk Signaling 1-way trunks: A call can only be initiated from one end (incoming or outgoing). 2-way trunks: A call can be initiated from either end of the trunk (PBX or C.O.)

112. PBX Trunk Signaling Trunk signaling for PBX trunks: Loop start : Like subscriber loop. Ground start : Either side can ground a wire to initiate a call. Better control than loop start. E&M : Separate wire pair used for signaling.

113. PBX Trunk Types CO Trunk (1-way outgoing) Standard switched service. WATS (1-way outgoing) Switched service to limited geographic area. INWATS (1-way incoming) Trunk for incoming 800 number calls.

114. PBX Trunk Types DID (Direct Inward Dial) Trunk 1-way incoming trunk CO sends dialed number after call connect so PBX can automatically ring station. DOD (Direct Outward Dial) Trunk 1-way outgoing trunk PBX repeats dialed number to C.O.

115. Centrex Centrex is a service that imitates all PBX features at the C.O. Private dial plans, special features, etc. are all available at a cost from your local telco.

116. Centrex Advantages (compared with PBX system) Low up-front cost - no need to buy expensive PBX or Key System equipment System Maintenance and backups are all done by the LEC carrier rather than customer. Easy to combine multiple geographic locations into one logical Centrex group Advanced features are available that may not be included on all PBX or Key systems

117. Centrex Disadvantages (compared with PBX system): Customer has less direct control With PBX, customer can modify switch data at any time needed (i.e. add new telephone set, change phone number, etc.) With Centrex, customer must ask LEC to make changes using service order. Centrex may be more expensive if most calls are internal (between phones on PBX) Fixed costs: Centrex is per-line, PBX is per-trunk. Fewer trunks needed if calls mainly internal. Usage costs: Lines see much more usage than trunks if calls are mainly internal. Line-to-line calls are “free” with PBX.