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CSIT 220 (Blum) 1 Cable modems. CSIT 220 (Blum) 2 Review of some homework questions Give your first, last and middle initials. (If you don't have a middle.

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Presentation on theme: "CSIT 220 (Blum) 1 Cable modems. CSIT 220 (Blum) 2 Review of some homework questions Give your first, last and middle initials. (If you don't have a middle."— Presentation transcript:

1 CSIT 220 (Blum) 1 Cable modems

2 CSIT 220 (Blum) 2 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.

3 CSIT 220 (Blum) 3 Start with initials, look up ASCII code Initial ASCII (Decimal) ASCII (Binary) Parity bit T E B

4 CSIT 220 (Blum) 4 ASCII for T 84 (in decimal)

5 CSIT 220 (Blum) 5 Decimal ASCII codes Initial ASCII (Decimal) ASCII (Binary) Parity bit T84 E69 B66

6 CSIT 220 (Blum) 6 Convert 84 to binary Start/Programs/Accessories/Calculator View Scientific Start/Run Alternative

7 CSIT 220 (Blum) Zero added on left to make it 8 bits.

8 CSIT 220 (Blum) 8 Binary ASCII codes Initial ASCII (Decimal) ASCII (Binary) (Even) Parity Bit T E B

9 CSIT 220 (Blum) 9 Add in even parity bit Initial ASCII (Decimal) ASCII (Binary) (Even) Parity Bit T (odd number of 1s) 1 E (odd number of 1s) 1 B (even number of 1s) 0

10 CSIT 220 (Blum) 10 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.)

11 CSIT 220 (Blum) 11 Checksum: Add column two or three Initial ASCII (Decimal) ASCII (Binary) T E B

12 CSIT 220 (Blum) 12 Percentage of random errors caught The Checksum in our example uses 8 bits. Thus the result can be in any of 2 8 = 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.

13 CSIT 220 (Blum) 13 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.

14 CSIT 220 (Blum) 14 soh in ASCII table

15 CSIT 220 (Blum) 15 Byte stuffing characters Character ASCII (Decimal) ASCII (Binary) soh eot esc

16 CSIT 220 (Blum) 16 Manchester encoding If the following graph uses Manchester coding, what is the binary sequence it encodes?

17 CSIT 220 (Blum) 17 Down 0Up 1Down 0 Up 1 Down 0 Up

18 CSIT 220 (Blum) 18 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)

19 CSIT 220 (Blum) 19 Ethernet Frame Non-payload: = 26 There are 26 bytes of non-payload data

20 CSIT 220 (Blum) 20 Ethernet Overhead Overhead = Non-Payload Non-payload + Payload Overhead = 26 = 36.1 % Overhead = 26 = 1.7 %

21 CSIT 220 (Blum) 21 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.

22 CSIT 220 (Blum) 22 ipconfig /all

23 CSIT 220 (Blum) 23 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.

24 CSIT 220 (Blum) 24 Before Cable TV Non-cable television pretty much required ones television antenna (receiver) to be in the line of sight of the television stations 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 persons home.

25 CSIT 220 (Blum) 25 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 times). Amplification schemes were an important design issue in cable television.

26 CSIT 220 (Blum) 26 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.

27 CSIT 220 (Blum) 27 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.

28 CSIT 220 (Blum) 28 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.

29 CSIT 220 (Blum) 29 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.

30 CSIT 220 (Blum) 30 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.

31 CSIT 220 (Blum) 31 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.

32 CSIT 220 (Blum) 32 Cable modem/cable modem termination system

33 CSIT 220 (Blum) 33 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.

34 CSIT 220 (Blum) 34 Cable modem

35 CSIT 220 (Blum) 35 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.

36 CSIT 220 (Blum) 36 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.

37 CSIT 220 (Blum) 37 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).

38 CSIT 220 (Blum) 38 Star versus Bus Phone Cable Central Office

39 CSIT 220 (Blum) 39 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.

40 CSIT 220 (Blum) 40 Star-Bus Distribution Center

41 CSIT 220 (Blum) 41 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.

42 CSIT 220 (Blum) 42 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.

43 CSIT 220 (Blum) 43 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.

44 CSIT 220 (Blum) 44 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.

45 CSIT 220 (Blum) 45 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.

46 CSIT 220 (Blum) 46 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).

47 CSIT 220 (Blum) 47 Highest Capacity Circuits Standard Name Optical NameBit Rate (in Mbps) Voice Circuits STS-1OC STS-3OC STS-12OC STS-24OC STS-48OC Data rates for STS-24 and 48 are over 1Gbps.

48 CSIT 220 (Blum) 48 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.

49 CSIT 220 (Blum) 49 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.

50 CSIT 220 (Blum) 50 Inverse Multiplexing

51 CSIT 220 (Blum) 51 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.

52 CSIT 220 (Blum) 52 Other References Computer Dictionary, Mitchell Shnier e-tv.htm


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