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Basic Communications Systems Class 9

Basic Communications Systems Class 9. 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. What can you do with a copper subscriber loop?. Modems

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Basic Communications Systems Class 9

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  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 Data Rate Distance V.32 V.34 Voice Band Modems 9600 - 28,800 bps Any Voice Line Service V.90 56K Modem 24 - 52 Kbps down 33.6 Kbps up Any Voice Line IDSL ISDN DSL 144 Kbps 18,000-24,000 feet SDSL Single-Pair DSL Up to 1.544 Mbps 12,000 feet HDSL High Speed DSL 1.544 Mbps 12,000-18,000 feet ADSL Asymmetric 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 VDSL Very High-speed DSL 13 Mbps down 4,500 feet 25.82 Mbps down 3,000 feet 52 Mbps down 1,000 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 • Each DS0 called a time slot • 8000 frames/sec * 8 bits/slot = 64 Kbps • 24 * 8 + 1 = 193 bits/frame • 8000 frames/sec * 193 bits/frame = 1.544 Mbps • 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 • Frames 1-5, 7-11: • Frames 6, 12:

  25. T1 Framing Bits (ESF Frame) • D5 Framing - Extended Superframe T1 (1983) • F-bit pattern marks 24-frame extendedsuperframes • 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

  38. 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

  39. 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

  40. Frame Relay PVCs with CIRs

  41. 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)

  42. 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!!!!!

  43. 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

  44. 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

  45. 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)

  46. 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

  47. 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

  48. 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

  49. 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

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