Cable modems

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Cable modems

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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.

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Start with initials, look up ASCII code

InitialASCII

(Decimal)

ASCII

(Binary)

Parity

bit

T

E

B

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ASCII for T 84 (in decimal)

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Decimal ASCII codes

InitialASCII

(Decimal)

ASCII

(Binary)

Parity

bit

T 84

E 69

B 66

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Convert 84 to binary

Start/Programs/Accessories/Calculator View Scientific

Start/Run Alternative

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

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Binary ASCII codes

InitialASCII

(Decimal)

ASCII

(Binary)(Even)

Parity Bit

T 84 0101 0100

E 69 0100 0101

B 66 0100 0010

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Add in even parity bit

InitialASCII

(Decimal)

ASCII

(Binary)(Even)

Parity Bit

T 84 0101 0100(odd number of 1’s)

1

E 69 0100 0101(odd number of 1’s)

1

B 66 0100 0010(even number of 1’s)

0

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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.)

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Checksum: Add column two or three

InitialASCII

(Decimal)

ASCII

(Binary)

T 84 0101 0100

E 69 0100 0101

B 66 0100 0010

219 1101 1011

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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.

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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”.

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soh in ASCII table

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Byte stuffing characters

CharacterASCII

(Decimal)

ASCII

(Binary)

soh 1 0000 0001

eot 4 0000 0100

esc 27 0001 1011

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Manchester encoding

If the following graph uses Manchester coding, what is the binary sequence it encodes?

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Down 0 Up 1 Down 0 Up 1Down 0Up 1 0 0 0 1

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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)

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Ethernet Frame

Non-payload: 8 + 6 + 6 + 2 + 4 = 26

There are 26 bytes of non-payload data

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Ethernet Overhead

Overhead =

Non-Payload

Non-payload + Payload

Overhead =

26= 36.1 %26 + 46

Overhead =

26= 1.7 %26 + 1500

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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.

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ipconfig /all

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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.

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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.

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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.

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Network of cable, amplifiers and subscribers. It has a “tree” structure.

The source of the signal on this network is known as the “head end.”

Cables from head end to neighborhoods are known as “trunk cables.” Today trunk cables are often fiber optic cable.

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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.

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VHS channels

To understand the gap here, just look at your FM radio dial.

This range can be used in a cable signal since it is shielded from the environment – the difference between wired and wireless.

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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.

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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.

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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.

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Cable modem/cable modem termination system

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Telco-return

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.

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Cable modem

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Tuner

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.

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Upstream/Downstream

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.

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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).

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Star versus Bus

Phone Cable

Central Office

Central Office

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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 .

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Star-Bus

Distribution Center

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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.

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HFC

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.

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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.

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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.

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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.

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Optic

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).

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Highest Capacity Circuits

Standard Name

Optical Name Bit Rate (in Mbps)

Voice Circuits

STS-1 OC-1 51.840 810

STS-3 OC-3 155.520 2430

STS-12 OC-13 622.080 9720

STS-24 OC-24 1244.160 19440

STS-48 OC-48 2488.320 38880

Data rates for STS-24 and 48 are over 1Gbps.

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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.

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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.

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Inverse Multiplexing

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SONET

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.

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Other References

http://www.webopedia.com http://www.whatis.com Computer Dictionary, Mitchell Shnier http://entertainment.howstuffworks.com/

cable-tv.htm