1. Some classes of connections
   1. Access: Connecting homes and business to the wider Internet.
   2. Interconnection: High-capacity links in the Internet core.
2. Access technology
   1. The last mile problem.
   2. Directions.
      1. Upstream: from home or business to the larger net.
      2. Downstream: from the net to home or business.
   3. Capacities
      1. Narrow-band
         1. Dial-up telephone
         2. Leased circuit using modems
         3. Fractional T1 (T1 is a digital service from the phone co).
         4. ISDN and other phone company digital services.
      2. Broadband
         1. DSL technologies (phone company service again).
         2. Cable modem service.
         3. Wireless access technologies.
         4. Full T1 and higher.
   4. The local loop
      1. Refers to the connection between the subscriber and central office.
         1. Traditionally, just a pair of copper wires.
         2. Officially, designed to carry 4 KHz to carry the analog voice signal.
         3. In practice, may support up to 1 MHz.
         4. Depends on distance from the central office, and conditions of the line.
      2. Dial-up modems
         1. Modulate the data into the 4KHz channel intended for voice.
         2. Modulates two carriers, one for upstream and one for down.
         3. Not very fast, but works without replacing existing equipment.
      3. Integrated Service Digital Network
         1. Engineers decided the voice line could carry 160 Kbps reliably.
         2. The physical line is repurposed to three channels, two 64 Kbps “B channels” and one 16 Kbsp “D channel”.
         3. Each B channel could carry digitized voice call, or other digital data.
         4. Multiplexed with TDM.
         5. Never a big commercial success; no longer much used.
      4. Digital Subscriber Line (DSL).
         1. Primary means for providing net connections to homes.
         2. Many versions.
            1. ADSL/ADSL2: A = Asymmetric. Greater speed down.
            2. SDSL: Symmetric. Intended for businesses with servers or other data sending requirements.
            3. HDSL/VDSL: Higher speeds.
         3. Uses FDM, occupying frequencies above the voice channel. (POTS = Plain Old Telephone Service.)
         4. Uses adaptive techniques, since each line is different.
            1. Discrete Multi Tone (DMT) Modulation.
            2. Creates 286 “subchannels” at different carrier frequencies, at 4.135 KHz intervals. 255 down, and 31 up.
            3. Start at 26KHz, well above the voice range, which continues to operate as before.
            4. Each carrier is modulated separately.
            5. Endpoints test each frequency to see how well it works.
               1. To much interference, don't use it.
               2. Moderate interference, modulate at a low bit rate.
               3. Little interference, modulate at a high bit rate.
         5. Speeds
            1. Rate to user depends on line length (distance to phone central office) and line conditions.
            2. ADSL 8.448 Mbps down, 640Kbps up, optimal conditions. (ADSL2 up to 20.)
            3. Speed may be as low as 32Kbps either way.
            4. Of course, an actual Internet connection speed may be limited by other parts of the path.
         6. Usual to add a frequency splitter (a pair of band-pass filters) at the structure entrance to separate the POTS and ADSL frequencies.
   5. Cable Modems
      1. Community Antenna TeleVision (CATV) or Cable TV.
         1. A system to deliver broadcast television, usually in small towns.
         2. Receiving a TV signal would need a high antenna.
         3. Makes more sense to build one than have everyone build their own.
         4. Providing new programming over the system was a later innovation.
      2. Re-purpose for home digital service.
         1. Homes are connected using coaxial cable.
         2. Cable shares TV channels using FDM.
         3. This is a much higher-bandwith medium.
         4. Need to add a return channel, since CATV is one-way.
      3. Application
         1. Too many subscribers to use a frequency for each.
         2. Subscribers in a neighborhood share a channel using a form of statistical TDM.
            1. Packets are sent on the shared channel labeled with their destination address.
            2. Only the destination retains the packet.
            3. Another channel used for upstream, which is (I think) shared by more subscribers. The system asymmetric.
         3. In current systems, a neighborhood may share several channels, a form of inverse multiplexing, sometimes called bonding.
         4. Cable modems
            1. Subscriber's is the tail-end modem.
            2. Provider's is the head-end. Usually many modems built as a single device.
         5. Architecture.
            1. The trunk refers to higher-capacity links to a neighborhood.
            2. From there, feeder circuits connect businesses or residences.
            3. Originally all connected with coax.
            4. Portions near the provider may be replaced with fiber.
            5. Above is Fiber to the Curb (FTTC).
            6. Possible advances are Fiber to the Premises (FTTP), which comes as Building or Home (FTTB or FTTH).
   6. Wireless Access Technologies.
      1. Cell phones.
      2. Satellite
         1. Geostationary. Requires a dish.
         2. LEO. More portable.
      3. WiMAX
         1. Once was the Next Big Thing until it wasn't.
         2. Signals would be sent by a provider over line-of-sight links to towers around town.
         3. Towers may forward the signal on.
         4. Towers close to subscribers provide wireless connection to home or business.
         5. Line-of-sight links allow high-frequency, high-bandwidth connections.
         6. Allows building out without laying cable.
3. Core Connections
   1. Suppose a provider provides 5000 customers 2Mbps.
   2. It must connect at 10Gbps.
   3. And these ISPs must exchange this data between them.
   4. Mostly leased from phone company.
      1. Telephone circuits are digital, but developed with very different standards.
      2. A Data Service Unit / Channel Service Unit (CSU/CSU), does the conversion. (DSU is the computer side).
      3. Telephone calls.
         1. A telephone call is digitized using 8-bit samples take at 8000 Hz.
         2. 8bits/sample×8000samples/sec=64000bits/sec=0.064Mbps
         3. 24 calls make 1.536 Mbps; T1 is 1.544 Mbps, etc.
      4. Phone company leases connections which are essentially multiples of a phone call, plus some control overhead.
      5. The C postfix means no inverse multiplexing on the line. May have some advantages for data transmission.
   5. Phone circuit protocols
      1. Most use Synchronous Optical Network (SONET).
      2. An isochronous protocol: data must arrive and leave at a constant rate.
      3. Isochronous frames.
         1. A phone call is 8000 samples/second; one sample takes 125μ second.
         2. Each SONET frame takes 125μs to transmit.
         3. The size of a frame changes with the capacity of the channel. For instance
            1. STS-1 transmits 51.840 Mbps = 6480 bits / 125μs.
            2. STS-1 frame size is 6480 bits.
            3. STS-3 transmits 155.52 Mbps = 19440 bits / 125μs.
            4. STS-3 frame size is 199440 bits.
         4. This helps build isochronous lines:
            1. Three STS-1 lines are combined to feed one STS-3.
            2. Each STS-1 provides a 6480-byte frame each 125μs.
            3. Combine into one STS-3 frame and emit, each 125μs.