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CECS 474 Computer Network Interoperability. CHAPTE R 12 Access & Interconnection Technologies. Tracy Bradley Maples, Ph.D. Computer Engineering & Computer Science Cal ifornia State University, Long Beach. Notes for Douglas E. Comer, Computer Networks and Internets (5 th Edition) .

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CHAPTE R 12 Access & Interconnection Technologies

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CECS 474 Computer Network Interoperability

CHAPTER12

Access & Interconnection

Technologies

Tracy Bradley Maples, Ph.D.

Computer Engineering & Computer Science

Cal ifornia State University, Long Beach

Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)


Access Technologies

Internet access technology refers to a data communications system that connects an Internet subscriber to an ISP (such as a telephone company or cable company).

Defn: Downstream refers to data traveling from an ISP in the Internet to a subscriber.

Defn: Upstream refers to data traveling from a subscriber to an ISP.

Note: Most Internet users follow an asymmetric pattern where a subscriber receives more data from the Internet than sending.


  • Narrowband and Broadband Access Technologies

  • Two broad categories of technologies are used for Internetaccess:

    • Narrowband& Broadband

Defn:Narrowband refers to technologies that deliver data at up to 128 Kbps.

For example:The maximum data rate for dial-up phone lines is 56 Kbps and is classified as a narrowband technology.

Defn:Broadbandgenerally refers to technologies that offer high data rates, but the exact boundary between broadband and narrowband is blurry.

• Many people suggest that broadband technologies must deliver more than 1 Mbps.

• Unfortunately, this is not always the case. Thus, broadband may sometimes mean any speed higher than a dial-up line.

+ Fiber to the House (FTTH)


  • The Local Subscriber Loop

  • The terms local loop or local subscriber line are used to refer to the connection between the phone company Central Office (CO) and an individual subscriber’s residence or business.

  • Subscribers that use dial-up modems or DSL obtain access to networks by using analog signals on a conventional analog telephone service.

    • This conventional service consists of twisted pair and dialup call with 4 KHz of bandwidth.

    • It may be able to handle much higher bandwidth

    • a subscriber close to a CO may be able to handle frequencies above 1 Mbps.


  • ISDN (Integrated Services Digital Network)

  • One of the first attempts to provide subscribers with high-speed digital services

  • Provides digitized voice and data over conventional (twisted-pair copper) wiring

  • The ISDN Basic Rate Interface (BRI) is (2B + D) channels

    • B channels: operate at 64 Kbps, intended for digitized voice, data or compressed video

    • D channel: operate at 16 Kbps, intended as a control channel

  • Note: Both B channels can be bonded to form a single 128 Kbps channel.

  • Note: ISDN sounded promising when it was proposed, but never was widely used and is now obsolete.


Digital Subscriber Line (DSL) Technologies

DSL (Digital Subscriber Line) is also a technology for providing digital services across the local loop.

There are several variants of of DSL which differ by the first word in their title, so they are collectively, xDSL.


  • ADSL (Asymmetric Digital Subscriber Line)

  • Most popular xDSL technology

  • Asymmetric service (downstream service higher bit rate than upstream service)

  • Maximum downstream rate is ~8 Mbps

  • Maximum upstream rate is 576 Kbps (640 Kbps – 64 Kbps control channel)*

  • Does not require any changes in local loop wiring

  • Can run simultaneously with standard phone service


  • Digital Subscriber Line (DSL) Technologies (cont’d)

  • Researchers noticed that many local loops can support frequencies higher than those used by the telephone system.

  • ADSL technology is complex

    • No two local loops have identical electrical characteristics.

    • Designers were nor able to pick only on set of carrier frequencies (or modulation techniques) that would work in all cases.

  • ADSL is adaptive

    • When a pair of ADSL modems are powered on, they probe the line to find its characteristics and agree to communicate using optimal techniques.

  • ADSL uses Discrete Multi Tone modulation (DMT)

    • it combines frequency division multiplexing and inverse multiplexing techniques.

  • FDM in DMT is implemented by dividing the bandwidth into 286 separate frequencies called sub-channels

    • 255 sub-channels allocated for downstream data transmission

    • 31 allocated for upstream data transmission

  • Note:Other DSL Technologies also exist. For example, Symmetric Digital Subscriber Line (SDSL) provides symmetric bit rates in both directions.


  • Cable Modem Technologies

  • A variety of wireless and wired technologies have been developed for use in the local loop.

  • An alternative access technology that uses the wiring already in place for cable television.

  • It is also known as Community Antenna TeleVision (CATV).

  • Coaxial cable has higher bandwidth and is less susceptible to electromagnetic interference than twisted pair.

  • How fast can a cable modem operate?

    • In theory, a cable system can support data rates of 52 Mbps downstream and 512 Kbps upstream.*

    • In practice, the rate can be much less.

  • The bandwidth is shared among a set of N subscribers (the size of the set is controlled by the cable provider).


  • Hybrid Fiber Coax (HFC)

  • HFC can provide high-speed data communications

    • Uses a combination of optical fibers and coaxial cables

    • Fiber is used for central facilities and coax is used for connections to individual subscribers

  • An HFC system is hierarchical.

    • It uses fiber optics for the portions that require the highest bandwidth

    • It uses coax for parts that can tolerate lower data rates

  • A trunk to refers to the high-capacity connections between the cable office and each neighborhood area

  • Feeder circuits to refer to the connection to an individual subscriber.

  • Trunk connections can be

  • up to 15 miles long.

  • Feeder circuits are

  • usually less than a mile.


  • Access Technologies that use Optical Fiber

  • Fiber To The Curb (FTTC)

  • Uses optical fiber for high capacity trunks

  • Runs optical fiber close to the end subscriber

  • Uses copper for the feeder circuits

  • Utilizes additional media in each feeder circuit to allow the cable system to provide an additional service (for example, voice)

  • Fiber To The Building (FTTB)

  • Use optical fiber to allow high upstream data rates for businesses

  • Fiber To The Home (FTTH)

  • Uses optical fiber to deliver higher downstream data rates to residential subscribers

  • Many channels for entertainment and video

  • Fiber To The Premises (FTTP)

  • A generic term, FTTP, encompasses both FTTB and FTTH


Wireless Access Technologies


  • The Internet Core

  • Access technologies handle the “last mile problem”.

    • Where the last mile is defined as the connection to a typical residential subscriber or a small business.

  • An access technology provides sufficient capacity for a residential subscriber or a small business.

    • the term Small Office Home Office (SOHO) is used

  • The “core” refers to the backbone of Internet.

    • Core technologies are high-speed technologies.


  • High Capacity at the Internet Core

  • What technology can a provider use to move data a long distance at a high data rate (~10 Gbps)?

    • Point-to-point digital circuits leased from a telephone company.

    • High-capacity digital circuits for transferring data are available for a monthly fee.

  • Telephone companies have the authority to install wiring that crosses municipal streets.

    • A circuit can extend between two buildings, across a city, or from a location in one city to a location in another.

    • The fees charged depend on the data rate of the circuit and the distance spanned.


Telephone Standards for Digital Circuits


Highest Capacity Circuits (STS Standards)

Defn: Telephone companies use the term trunk to denote a high-capacity circuit, and have created a series of standards for digital trunk circuits

Synchronous Transport Signal (STS) standards specify the details of high-speed connections:

  • Optical Carrier Standards

  • TELCOs define an equivalent set of Optical Carrier (OC)standards.

  • One should observe a distinction between the STS and OC terminology:

  • STSstandards refer to the electrical signals used in the digital circuit interface (i.e., over copper)

  • OCstandards refer to the optical signals that propagate across the fiber


  • The C Suffix

  • The STC and OC terminology described above has one additional feature:

    • an optional suffix of the letter C, which stands for concatenated

  • The suffix denotes whether inverse multiplexing is used on the circuit:

    • OC-3 consists of three OC-1 circuits operating at 51.840 Mbps each

    • OC-3C (STS-3C) consists of a single circuit that operates at 155.520 Mbps

  • Q: Is a single circuit operating at full speed better than multiple circuits operating at lower rates?

  • A: It depends on how the circuit is being used.

  • In general, having a single circuit operating at full capacity provides more flexibility and eliminates the need for inverse multiplexing equipment.


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