Review of some homework questions

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Cable modems

Review of some homework questions

  • Give your first, last and middle initials. (If you don't have a middle initial, make one up). Put your initials into an 8-bit ASCII format. For ASCII representation of each initial, determine a parity bit assuming even parity.

Start with initials, look up ASCII code

ASCII for T  84 (in decimal)

Decimal ASCII codes

Convert 84 to binary

  • Start/Programs/Accessories/Calculator

  • View Scientific

84  0101 0100. Zero added on left to make it 8 bits.

Binary ASCII codes

Add in even parity bit

Checksum question

  • Add up your binary numbers (without the parity bits) to generate an eight-bit checksum. If all possible errors were equally likely, what percentage of errors would be caught by such a checksum? (The calculator found under Accessories on the computer can be put into Scientific View and then Binary mode. A binary number can be entered and converted to decimal by clicking on the Dec radio button.)

Checksum: Add column two or three

Percentage of random errors caught

  • The Checksum in our example uses 8 bits.

  • Thus the result can be in any of 28 = 256 states.

  • Only one of those 256 possibilities “checks out.”

  • If an error occurs and an error is equally likely to lead to any of the 256 states, then 255/256 (or 99.6%) of those errors will not check out – that is those errors will be detected.

Byte stuffing question

  • Give the binary (ASCII) code for the three special characters mentioned in connection with byte stuffing.

    • The special characters are soh, eot and esc
    • Soh is a special ASCII character called the start of header character; it is not the character “s” followed by the character “o” followed by the character “h”.

soh in ASCII table

Byte stuffing characters

Manchester encoding

Overhead Question

  • Comer 9.4 (overhead) In most technologies, a sending station can choose the amount of data in a frame, but the frame header is a fixed size. Calculate the percentage of bits in a frame devoted to the header for the largest and smallest Ethernet frames. (Take "header" to mean everything that is not actual data. To find the necessary numbers, refer to Fig. 9.3 in Comer)

Ethernet Frame

Ethernet Overhead

MAC Info Question

  • For a computer connected to a LAN, determine the NIC's MAC address, slot type, manufacturer and procotol. How did you arrive at this information?

  • Do an ipconfig /all.

ipconfig /all

Cable Modem Technology

  • Another system of transmission lines, nearly as ubiquitous as that for telephones, is that for cable TV.

    • Cable TV uses coaxial cable which provides better shielding than the twisted pair used by the phone company because it was designed as a broadband technology.
  • A cable modem connects the cable and the computer.

Before Cable TV

  • Non-cable television pretty much required one’s television antenna (receiver) to be in the “line of sight” of the television station’s transmitter.

    • The signals could pass through or bend around some smaller scale objects, but if there was a mountain between the station and the television, the signal did not reach the person’s home.

Cable TV

  • In the Pennsylvania mountains, the problem was solved by placing an antenna on the mountain top and running a cable down to the homes below.

  • The community shared an antenna – hence Community Antenna television (CATV).

    • The signal would have to be amplified many times along the way (as many as 30-40 times). Amplification schemes were an important design issue in cable television.

Bandwidth distribution

  • The Federal Communications Commission (FCC) allocates a 6 MHz range of the “radio frequency” spectrum to be used as a television channel.

  • First they used the Very High Frequency range (VHF) and then used the Ultra High Frequency range (UHF) to allow for more channels.

VHS channels

HBO goes national

  • A cable station in Wilkes-Barre, PA started offering “pay-per-view” channel known as Home Box Office (HBO).

  • When HBO started using a geosynchronous satellite, it could transmit its signal as far as Florida and Mississippi.

Going Digital

  • It was determined that one could encode a digital signal instead of an analog signal and stay within the 6MHz bandwidth assigned to a channel.

  • Digital signals tend to take up more “room” but the MPEG compression scheme made it possible.

Cable modem

  • If the cable station connects to the Internet, then the connection for cable TV can be used for connecting to the Internet.

  • The signal must be prepared to be placed on the cable – the cable modem does this.

  • Then the signal must be placed in another form by the provider so that it can be placed on the Internet – the cable modem termination system does this.

Cable modem/cable modem termination system


  • One drawback of the cable TV system is that it was designed for downstream only.

  • One of the first solutions was to use a conventional (phone) modem for upstream and the cable for downstream, this is known as “telco-return” cable modem.

Cable modem


  • A tuner selects out a small set of frequencies from a whole spectrum of frequencies.

  • In a cable modem a tuner will separate out the channel used for Internet traffic from the television channels – or more specifically it will separate out the downstream sub-channel.


  • With a cable modem, the equivalent of a television channel (6MHz) is used for Internet access. As with ADSL, the sub-channels are distributed asymmetrically between upstream and downstream.

Sharing Problem

  • Another problem with cable is its (physical) topology.

  • Again it was designed as a broadcast transmission system.

  • The phone system has a star topology, every end user has its own wire to the central office.

  • The cable system has a bus topology, subscribers share the “bus” (connection to the central office).

Star versus Bus

  • Phone

Sharing Problem

  • With cable the connection to the central office is shared.

  • So the bandwidth of the connection is shared.

  • So the more active subscribers, the less bandwidth per subscriber .


Have some standards

  • Data Over Cable Service Interface Specification DOCSIS.

    • Developed by CableLabs and approved by the ITU.
    • Defines interface standards for cable modems and supporting equipment.


  • There is an increasing movement to use fiber optic cable for more of the connections.

  • Hybrid Fiber Coax

  • Keep the coaxial cable connections to computers and TVs in the homes.

  • But replace more central connections with fiber optic cable which supports higher bandwidth and is less susceptible to interference.

HFC (Cont.)

  • The head-end office receives information.

  • Forwards it through a SONET ring to distribution centers.

  • The distribution centers forward it to neighborhood fiber nodes.

  • It is converted from an optical signal to an electrical signal.

  • It is forwarded on coaxial cables to a home or office.

HFC (Cont.)

  • Provides enough bandwidth for home broadband applications.

  • It uses

    • 5 MHz to 450 MHz for conventional downstream analog information.
    • 450 MHz to 750 MHz for digital broadcast services such as voice and video telephony, video-on-demand, and interactive television.
  • It has ample bandwidth to allow for upstream transmission.

To what extent fiber?

  • Most systems use fiber for their “trunk”

    • A trunk is a line that carries multiple voice or data channel between two telephone exchange switching systems.
    • In digital communications, a trunk is often a T-carrier system (T1 or T3).
  • This is an example of “Fiber to the Neighborhood”, a more ambitious goal is “Fiber to the Curb” (FTTC)

    • Progress is slow because of expense.


  • More and more, the “trunks” and “backbones” use fiber optic cable.

  • One must have standards and specifications for the physical layer.

  • The broad set of standards goes by the name SONET, and specific line capacities go by the name STS (synchronous transport signal) and/or OC (optical carrier).

Highest Capacity Circuits

High Capacity Standards

  • STS standard (Synchronous Transport Signals) actually refers to the electrical signals used in digital circuit interfaces (over copper.)

  • OC (Optical Carrier) refers to the the same set of transmission capacities but using optic fiber.

High Capacity Standards

  • Most of high capacity standards use inverse multiplexing.

    • Recall multiplexing is putting several signals onto one line or channel; inverse multiplexing is putting one signal on several channels.
  • A standard with a suffix “C” (meaning concatenated) means that the circuit inverse multiplexing is not being used.

    • Some designers prefer concatenated circuits for data networks.

Inverse Multiplexing


  • Synchronous Optical Network (used in North America)

  • Synchronous Digital Hierarchy (SDH) (used in Europe)

  • Protocol, standards, specifications for use on high capacity optical lines, how they interface with lower capacity lines.

Other References



  • Computer Dictionary, Mitchell Shnier


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