Notes On Chapter Twelve -- Access and Interconnection Technologies

• 12.1 Introduction
  
  
  • Dialup, DSL, Cable Modems
  • High-Capacity Digital Circuits
  • Telephone Multiplexing Hierarchy
  • Circuits that Common Carriers Offer
  • Multiplexing
  • Data Rates
  
  
• 12.2 Internet Access Technology: Upstream and Downstream
  
  
  • Internet Access Technology: "a data communications system that connects an Internet subscriber (typically a private residence or business) to an Internet Service Provider (ISP), such as a telephone company or cable company."
  • Since most subscribers either do more receiving than sending or vice-versa, access technologies are often designed to transfer more data in one direction than the other.
  • Upstream: data traveling from a subscriber to the service provider.
  • Downstream: data traveling from the service provider to a subscriber.
  
  
• 12.3 Narrowband and Broadband Access Technologies
  
  
  • There are Narrowband and Broadband access technologies.
  • Despite the fact that bandwidth and data rates are not really the same thing, purveyors of network services tend to use the terms interchangeably.
  • Thus narrowband access technologies are the ones with low data rates and broadband access technologies are the ones with high data rates.
  • 12.3.1 Narrowband Technologies
    • Dialup telephone connections (<= 56 Kbps)
    • Leased circuit using modems
    • Fractional T1 data circuits
    • ISDN and other telco data services
    • Most anything with <= 128 Kbps data rates
    
    
  • 12.3.3 Broadband Technologies
    • Somewhere above 128 Kbps data rates
    • DSL technologies
    • Cable modem technologies
    • Wireless access technologies
    • Data circuits at T1 speed or higher
  
  
• 12.4 The Local Loop and ISDN
  
  
  • The local subscriber line or local loop is the connection between a subscriber's location and the phone company central office (CO)
  • Although designed for supporting voice at low data rates, the local loop may be able to support broadband data rates.
  • ISDN is Integrated Services Digital Network
  • ISDN is one of the earliest broadband offerings of telephone companies.
  • ISDN offers 2B+D service: two virtual B channels at 64 Kbps each and a virtual 16KBps D channel for control. The B channels can be combined to function as one 128 Kbps channel. The idea is to use the D channel to request services that would be provided on the B channels.
  • ISDN is of historical interest and has some niche markets.
  • However, ISDN never quite caught on very well because its value-to-cost ratio was always pretty low.
  
  
• 12.5 Digital Subscriber Line (DSL) Technologies
  
  
  • DSL is a popular technology for broadband service on local loops
  • Most residential DSL customers have ADSL - Asymmetric Digital Subscriber Line.
  • ADSL uses FDM to divide the bandwidth of the phone line into three regions:
    • 0 to 4 KHz for plain old telephone service (POTS)
    • 26 to 138 KHz for upstream data, and
    • 138 to 1100 KHz for downstream data
    
    
  
  
• 12.6 Local Loop Characteristics and Adaptation
  
  
  • Local subscriber lines were not designed for high-speed data applications.
  • Operating at any particular frequency might work well for one subscriber, but would surely work very badly for many subscribers.
  • ADSL was designed to adapt to the unique characteristics of the local loops over which it must transmit.
  • ADSL uses a combination of frequency division multiplexing and inverse multiplexing technologies (Discrete Tone Modulation: DMT).
  • Communicating ADSL modems cooperate to find frequencies that work well.
  • ADSL uses measures of signal quality to select different modulation schemes to use on different sub-channels.
  • If the signal-to-noise ratio is high on a sub-channel the modulation scheme may use more bits per baud.
  
  
• 12.7 The Data Rate of ADSL
  
  
  • There is no guaranteed data rate with ADSL. ADSL does as well as the line conditions allow. Downstream rates vary tremendously, from 32 Kbps to 8.4 Mbps, and upstream varies from 32 to 576/640 Kbps.
  • ADSL2 can download at up to 20 Mbps
  • These rates only apply to traffic on the local loop(s).
  
  
• 12.8 The ADSL Installation and Splitters
  
  
  • The 'original' ADSL requires that a technician install a Network Interface Device (NID) and splitter - filters to separate the frequency bands used by voice and data. There is another standard (ITU G.992.2 or 'G.lite') that does not require a splitter on the incoming phone line, but uses microfilters inline with the ADSL modem, phones and fax machines. G.lite is cheaper but may run slower.
  
  
• 12.9 Cable Modem Technologies
  
  
  • Cable TV uses coaxial cable - there are inherent advantages over the twisted pair used by telephone companies: shielding from interference and high bandwidth.
  • Cable modem technology uses FDM and statistical multiplexing.
  • Typically all subscribers in a neighborhood share a single channel
  • Cable modems look at all incoming packets and forward to the subscribers equipment only the packets that are addressed to the subscriber.
  
  
• 12.10 The Data Rate of Cable Modems
  
  
  • Cable systems can support up to 52 Mbps downstream and 512 Kbps upstream between the subscriber and the local cable office.
  • Actual rates can be much lower for various reasons.
  • For one thing, the available bandwidth must be shared with other subscribers.
  
  
• 12.11 Cable Modem Installation
  
  
  • There's no need for anything like a splitter. The FDM assures that all the different channels will operate without interfering with each other.
  
  
• 12.12 Hybrid Fiber Coax
  
  
  • HFC connects the cable company office via optical fiber trunk lines to neighborhood. Ordinary coax runs from neighborhood stations to subscribers' homes.
  • HFC would improve service, but it requires the cable company to purchase and install a lot of expensive equipment.
  
  
• 12.13 Access Technologies that Employ Optical Fiber
  
  
  • FTTC = Fiber to the Curb
    • Similar to HFC
    • Idea is to run fiber close to subscriber, then copper feeder
    • Uses two media in each feeder for additional service such as voice
    • Being deployed in US and Canada
    
    
  • FTTB = Fiber to the Building
    • Use of fiber to allow high upstream data rates in businesses
    
    
  • FTTH = Fiber to the Home
    • Counterpart to FTTB
    • optical fiber to deliver higher downstream data rates to residential subscribers
    • Emphasis on many channels of entertainment and video
    
    
  • FTTP = Fiber to the Premises
    • Generic term - encompasses FTTB and FTTH
  
  
• 12.14 Head-End And Tail-End Modem Terminology
  
  
  • A modem at the subscriber end is a tail-end modem
  • A modem at the provider central office end is a head-end modem
  • Head-end modems are configured in large sets built as a single unit, configured, monitored and controlled together.
  • The head-end modem sets used by cable providers are called Cable Modem Termination Systems (CMTS)
  • Industry standards "Data Over Cable System Interface Specifications" (DOCSIS) specify data format and messages used to request services like pay-per-view movies.
  
  
• 12.15 Wireless Access Technologies
  
  
  • Remote locations may be too far from a CO to use ADSL
  • Such locations are also unlikely to have cable TV service
  • Even in urban areas ADSL may not be an option because of the existence of loading coils, bridge taps, or repeaters on the phone lines.
  • Possible alternatives:
    • 3g Services: Third generation cellular telephone services for data (e.g. EVDO)
    • WIMAX: Wireless access technology up to 155 Mbps using RF
    • Satellite: Various commercial vendors offer data services over satellite
  
  
• 12.16 High-Capacity Connections at the Internet Core
  
  
  • The data network connections to homes and small businesses are like tiny streams.
  • Even the connections to large businesses may be dwarfed by the data flow occurring in the core of a large wide area network.
  • In the core, channels capable of supporting many billions of bits per second (Gbps) may be required.
  • Telephone companies lease point-to-point digital circuits that can be used for voice, but also for data.
  • Private companies cannot run wires that cross public streets, but public utilities like phone companies have the authority to do that.
  • The monthly fees for leased lines are determined by channel capacity and distance between points connected.
  
  
• 12.17 Circuit Termination, DSU/CSU, and NIU
  
  
  • Digital standards in use by telephone companies and computers differ very significantly.
  • Therefore when using a leased line for data, a special device is needed at both ends to convert between the digital formats.
  • The device is called a Data Service Unit/Channel Service Unit (DSU/CSU) - two devices combined in a single chassis.
  • The CSU handles line termination - diagnostics - loopback service
  • The CSU prevents "too many ones" from being transmitted.
  • The DSU is the device that performs the format translation
  • The computer can use RS-232, RS-449 or V.35 communication standards, depending on the data rate.
  • The phone company also installs a piece of equipment called a Network Interface Unit (NIU), or demarc. This serves as a boundary between equipment owned by the phone company and equipment owned by the subscriber.
  
  
• 12.18 Telephone Standards for Digital Circuits
  
  
  • Single Voice Circuit - 0.064 Mbps
  • T1 - 1.544 Mbps - 24 Voice circuits - used in North America
  • T2 - 6.312 Mbps - 96 Voice circuits - used in North America
  • T3 - 44.736 Mbps - 672 Voice circuits - used in North America
  • E1 - 2.048 Mbps - 30 Voice circuits - used in Europe
  • E2 - 8.448 Mbps - 120 Voice circuits - used in Europe
  • E3 - 34.368 Mbps - 480 Voice circuits - used in Europe
  
  
• 12.19 DS Terminology and Data Rates
  
  
  • Note that a T2 has about four times the data rate of a T1, and a T3 has about 28 times the rate of a T1.
  • E2 is about 4 times faster than E1, and E3 is about four times faster than E2 and (hence) 16 times faster than E1.
  • It is possible to lease "fractions" of a leased line. So subscribers can match what they pay for more closely to their needs.
  • The T-standards actually define "the underlying carrier system" whereas DS-standards "specify how to multiplex multiple phone calls onto a single connection." Since the DS-standards specify the bit rates, it is actually more proper to refer to "DS1 speed" instead of "TS1 speed."
  
  
• 12.20 Highest Capacity Circuits (STS Standards)
  
  
  • There are Synchronous Transport Signal (STS) standards for digital trunk (high-capacity) circuits. The associated speed info is;
  • STS-1 - OC-1 51.840 Mbps - 810 voice circuits
  • STS-3 - OC-3 155.520 Mbps - 2430 voice circuits
  • STS-12 - OC-12 622.080 Mbps - 9720 voice circuits
  • STS-24 - OC-24 1,244.160 Mbps - 19440 voice circuits
  • STS-48 - OC-48 2,488.320 Mbps - 38880 voice circuits
  • STS-192 - OC-192 9,953.280 Mbps - 155520 voice circuits
  
  
• 12.21 Optical Carrier Standards
  
  
  • The STS are the standards for copper, whereas
  • The OC are the standards for optical fiber
  
  
• 12.22 The C Suffix
  
  
  • The C suffix denotes a single (not inverse multiplexed) circuit - e.g. OC-3C
  • Generally data network designers prefer such circuits because they don't require inverse multiplexing equipment and one has some additional flexibility in deciding how to use them.
  
  
• 12.23 Synchronous Optical NETwork (SONET)
  
  
  • The SONET phone company standard specifies, for example, how data is framed and how lower-capacity circuits are multiplexed into a high capacity circuit.
  • The frame size depends on the data rate. If the circuit is equivalent to N voice circuits, each frame is the size required to hold N voice samples.
  • Most data networks use SONET as an encoding scheme on a single point-to-point circuit.