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Lecture 8

Lecture 8. Specialized Network Services. Overview. This chapter explores: high-speed services digital network services Will discuss: how they work speed and capacity applications in which these services are used. Services to be Discussed. T-1 and T-3 ISDN ATM DSL Sonet.

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Lecture 8

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  1. Lecture 8 Specialized Network Services

  2. Overview • This chapter explores: • high-speed services • digital network services • Will discuss: • how they work • speed and capacity • applications in which these services are used

  3. Services to be Discussed • T-1 and T-3 • ISDN • ATM • DSL • Sonet

  4. What in common? • All technologies to be discussed today are digital services • Meet requirements for high speed and accurate transmission

  5. T-1 -- Voice or Data • Developed in 1960’s to save on cabling • Scheme designed to carry 24 voice channels over one telephone circuit • In order to reduce number of cables wired between telephone company switches • Available to user locations in 1983

  6. Circuit A path for electrical transmissions between two points

  7. Initial Users and Applications • Large organizations, universities, financial institutions • Used to connect locations in support of applications such as: payroll, inventory • Cost was high • Better than hand carrying huge data tapes from location to location

  8. T-1 in 80’s • High rates • Service took months to install • Needed long lead times to plan for installation • T-1 digital, needed to be connected to analog system by using channel banks

  9. Channel Banks • Multiplexing device used to connect T-1 circuits to analog PBX and CO systems • samples signals from PBX systems • digitizes these signals • sends down the T-1 channel • channel bank on other end decodes digital signal back to analog signal • PCM: pulse code modulation (page 183)

  10. Channel Banks • Also used to split up the use of T-1 line into voice and data applications • Controls the communication to the devices to which it connects, PBX or computer • Can be used to break out each of the 24 T-1 channels

  11. Sampling • Converting continuous signals, like voice or video, into discrete values, e.g. digital signals • Sampling rate: • The number of times per second that an analog signal is measured and converted to a binary number, most commonly used algorithm is 8,000 per second

  12. PCM • Pulse Code Modulation • most common method of encoding an analog voice signal into a digital bit stream

  13. Digital Signal Levels • DS-O: 64,000BPS • DS-1: 1,544,000BPS • speed at which T-1 and T-2 lines run

  14. DS-1 refers to entire T-1 line entire bandwidth of the circuit 1,544Mbps DS-O refers to each speed of each channel in T-1 64Kbps DS levels

  15. European, Japanese and US T-standards

  16. Time Division Multiplexing • All T carrier signals are based on division multiplexing • each device which communicates over a T-line is assigned a time slot • 8 devices communicating, 8 time slots assigned • assigned for duration of the communication, even when no information being transmitted • TDM: not efficient in handling a WAN

  17. Idle Time Slots • Pauses in data transmission result in idle time slots • In a network with millions of time slots, empty time slots represent waste or inefficient use of transmission capacity • ATM and IP do not assign specific time slots to each device

  18. T-1 Configurations • T-1 circuit has 24 lines available for voice, data, or video • voice usually connected to a PBX • data usually bypasses the PBX • video can bypass or be routed through the PBX, depending on weather it can transmit at high speeds

  19. Digital Cross Connects • Used to connect multiple sites with dedicated lines • Digital cross connects provided by telco’s • Used to divide a T-1 connection over several users

  20. Digital Cross Connectivity 6 lines T-1 Mux 6 lines 12 lines Digital Cross Connect

  21. T-3 • Equivalent to 28 T-1 lines or 672 channels 28 X 24 = 672 • Total speed 44,736, includes bits for overhead and 28 X 1.544Mbs • Used for businesses with large calling volume • Fractional T-3 service, partial use of a T-3 circuit

  22. ISDN • Integrated Services Digital Network • BRI ISDN: basic rate • PRI ISDN: primary rate • Worldwide public standard for sending voice, video, and data or packets over public switched network

  23. Characteristics ISDN • Digital connectivity: consistent, high quality service • Out of channel signaling: call set up fast, total bandwidth of each channel dedicated to transfer of user data • Switched service: fees based on usage time • Standard interface: all ISDN users can interface with each other

  24. ISDN in the “Real World” • Works on copper wiring • Availability an issue • Takes long time to install • Poor support from providers • 1% of US access lines equipped in US • Must be within 18,000 feet from central office

  25. Basic Rate Interface • Two bearer channels (voice, data, video) • 64Kps • One signaling channel • 16Kps

  26. Uses Basic Rate Interface • Internet access • desktop video conferencing • Centrex ISDN multi-line telephones • use of D signal for credit card verification • downloading software • work-at-home • connecting remote local area networks

  27. ISDN for work at home • To share line for voice and other: • need a terminal adapter with an NT-1 (network terminating device) • used to connect non-ISDN equipment to the ISDN line

  28. ISDN and Analog Lines • ISDN is a digitally based technology • Can not transmit to an analog line • Can only communicate with ISDN-equipped services • Must have matching modems on each end of the connection

  29. Pricing BRI • Ranges considerably • Installation fees $150-300 • Monthly fees $10-40 more than charge for analog line • Per minute, sometimes flat rate for 140-300 monthly hours

  30. Primary Rate Interface ISDN • PRI: has 24 channels • 23 channels are bearer channels • 1 channel used for signaling • Similar to a T-1 both have 24 channels • T-1 in-band signaling • PRI out-of-band signaling on 24th channel • Trunk connection

  31. Trunk Connection A communication line between two switching systems.

  32. Uses PRI ISDN • Video conferencing (128 to 384 Kbps) • Receiving the calling party numbers at large call centers • backing up LAN-to-LAN connections • backing up dedicated lines • Internet service providers for dial in from BRI ISDN customers • Corporate sites for remote access

  33. PBXs with PRI Trunks • Calls and signals sent separately • PBX may send the signal to a database to match the number and then pull the data • call centers • call screening

  34. PRI Bandwidth on Demand • PRI: use of channels dynamic always open for voice or data • setup signals notifies network how to route the call • voice: public network • data: data network • T-1 channels permanently set aside for voice or data purposes

  35. Digital Subscriber Line • Initially used to transmit video and TV signals from CO to end-users over copper • Main application has changed to Internet access • ADSL: asymmetric digital subscriber line • line has different speeds from and to customer • upstream slower than downstream connection

  36. Digital Subscriber Line • Many different flavors • speeds may vary • distance from CO varies • Has potential to relieve network congestion caused by modem traffic on PSTN • DSL traffic routed onto a data network when it hits the CO switch

  37. DSL Flavors • ADSL: asymmetric, upstream slower than downstream • DSL Lite: Universal DSL, ITU-based standard for residential consumers, offered by local exchange providers • HDSL: high-bit-rate, two competing flavors • VDSL: very-high-bit-rate for TV and Internet usage

  38. DSL Applications • Alternative to cable modem for Internet • CLEC: to serve data only communications • IXC: to provide high speed local voice, Internet access, and support telecommuting • Internet service providers

  39. Obstacles DSL • Availability been slow, now popular • Cost high to install for providers • loading coils and bridge caps must be removed by providers • Not easy to install for end users

  40. Technical Explanation DSL • Uses unused frequencies available in the telephone wire for data transmission • Voice carried between 300-3300 Hrz • DSL uses rest of frequencies available for data communications • DMT: discrete multi-tone modulation used to send data in frequencies over 4,000Hz

  41. Technical Explanation DSL • DMT used compression algorithms to encode and compress signals • Different frequencies are used for the upstream and downstream portions of the channel • Routing functionality enables multiple users to share one DSL line

  42. DSL Lite • Developed to lower the cost of providing DSL service • also referred to as: Universal DSL • known as a splitterless DSL • save telco’s from having to dispatch a technician to install splitter • Works consistently on longer telephone company loops, within 3 miles of telco CO • no internal wiring changes required on part of user

  43. Splitter • Used with full rate ADSL • sits between the customer demarcation point and the ADSL line and another at telco • separates the low frequencies voice signals from the high frequency data signals • small in size

  44. Concatenation • Puts streams of data into one “fat” or one high bandwidth contiguous stream • Joining several fibers together end-to-end • To link together in a series or chain

  45. DSLAM’s • Digital subscriber lines access multiplexers • Located at network providers site • Take traffic from multiple DSL lines and combine into higher speeds before sending it out to Internet or data network • Can stream video together rather than separating it with data

  46. DSL for Internet Access • Competes against cable modems, T-1, ISDN and high speed modems • Suitable for: • Internet access • Telecommuters to corporate databases • High bandwidth applications • Telecom Act notable impetus to this service

  47. Frame Relay • Network access method offered by local and long distance providers • Shared by multiple users • Acts like a private, dedicated network • Leasing dedicated lines not necessary • Alternative to those building their network

  48. Frame Relay • First implemented in 1992 • Allows customers to transmit data between multiple locations • Used for: • LAN connectivity • Internet access • connections for remote users

  49. Frame Relay • Advantages: • network managed by provider • less hardware required at each location • capacity on frame relay flexible • has internal backup routes

  50. Frame Relay • Shared network • Fast, due to low error checking • Customer must check data upon arrival • X.25: packet network, oldest most widely used, slower due to extensive error checking into every packet transmitted • Excels at LAN-to-LAN communications

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