2 9-2 DIAL-UP MODEMS Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility. Modem Standards Topics discussed in this section:
3 Figure 9.6 Telephone line bandwidth The signal bandwidth must be smaller than the cable bandwidth.
4 Modem stands for modulator/demodulator. Note A modulator creates a bandpass analog signal from binary data. A Demodulator recovers the binary data from the modulated signal.
5 Figure 9.7 Modulation/demodulation
6 Modem standard V.32 Uses a combined modulation and encoding technique called trellis- coded modulation. Data stream is divided into 4-bit stream. A pentabit (5-bit pattern) is transmitted The extra bit is used for error detection. V32 calls with a baud rate of 2400 = 4 X 2400 = 9600 bps. V.32bis Uses 7 bits/baud 1 bit for error control. V.32 calls with a baud rate of 2400 = 6 X 2400 = bps> V.32bis enables the modem to adjust its speed upward or downward depending on the quality of the line or signal (fall-back and fall- forward).
7 Figure 9.8 The V.32 and V.32bis constellation and bandwidth
8 Modem standard V.34bis Bit rate of 28,800 bps with 960-point constellation to 1664-point constellation for a bit rate of 33,600 bps. V-90 Traditionally modems have a limitation on data rate (max Kbps) V-90 modems (56k modems) can be used (up to 56Kbps) if using digital signaling. For example, Through an Internet Service Provider (ISP). V-90 are asymmetric Downloading rate (from ISP to PC) has a 56 Kbps limitation. Uploading rate (from PC to IST) has a 33.6 Kbps limitation.
9 Figure 9.9 Uploading and downloading in 56K modems
10 Modem standard V.92 Modems can adjust their speed, and if the noise allows, they can upload data at the rate of 48 Kbps. The downloading rate is still 56 Kbps. Modem can interrupt the Internet connection when there is an incoming call if the line has call-waiting service.
11 Using Telephone and Cable Networks for Data Transmission Telephone networks were created to provide voice communication. Need to communicate digital data resulted in invention of the dial-up modem. With the appearance of the internet and the need for high-speed downloading and uploading (modem too slow), the telephone companies added a new technology (DSL: digital subscriber line)
DIGITAL SUBSCRIBER LINE After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops. DSL technology is a set of technology ADSL VDSL HDSL SDSL The set is often referd to as xDSL, where x can be replaced by A,V,H, or S.
13 ADSL Asymmetric DSL (ADSL) Is like a 56k modem provides higher speed (bit rate) in the downstream direction than in the upstream direction. ( Why called Asymmetric) Designers of ADSL divided available bandwidth of the local loop unevenly for the residential customer. It is not suitable for businesses who need a large bandwidth in both direction. Using Existing Local Loops How with ADSL we reach a data rate that was never achieved with modems? The existing local loops (twisted-pair) can handle bandwidths up to 1.1 MHz. Filter installed at the end office of the telephone company where each local loop terminates limits the bandwidth to 4 KHz (voice communication). If filter is removed, the entire 1.1 MHz is available for voice and data communication.
14 ADSL Adaptive Technology 1.1 MHz is theoretical bandwidth of the local loop. Numbers of factors affect the bandwidth: Distance between the residence and the switching office The size of the cable, The signaling used. Designers of ADSL were aware of this problem and used an adaptive technology. The system uses a data rate based on the condition of the local loop line. The data rate of ADSL is not fixed; it changes based on the condition and type of the local loop cable.
15 Discrete Multitone Technique (DMT) The modulation technique that has become standard for ADSL is called the discrete multitone technique which combines QAM and FDM. The DMT divides a MHz bandwidth into 256 channels about kHz each. Figure 9.10 Discrete multitone technique (DMT)
16 Figure 9.11 Bandwidth division in ADSL
17 Discrete Multitone Technique (DMT) Voice. Channel 0 is reserved for voice communication. Idle. 1 to 5 are not used and provide a gap between voice and data communication. Upstream data and control: Channels 6-30 (25 channels) are used for upstream data transfer and control: 1 channel for control. 24 channels are for data transfer. If there 24 channels, each using 4 kHz with QAM modulation: Bandwidth = 24 X 4000 X 15 = 1.44 Mbps. However, the data rate is normally below 500 kbps because some of the carriers (channels) are deleted at frequencies where the noise level is large (some of channels may be unused).
18 Discrete Multitone Technique (DMT) Downstream data and control Channels (225 channels) are used for downstream data and control: 1 channel for control. 224 channels for data transfer. If there are 224 channels, we can achieve up to 13.4 Mbps: Bandwidth = 224 X 4000 X 15 = 13.4 Mbps. However, normally the data rate is below 8 Mbps because some of carriers are deleted at frequency where the noise level is large (some of channels may be unused).
19 ADSL modem installed at a customer’s site. The local loop connects to a splitter which separates voice and data communications. ADSL modem modulates and demodulates the data, using DMT, and creates downstream and upstream channels. Figure 9.12 ADSL modem
20 Figure 9.13 Digital Subscriber Line Access Multiplexer (DSLAM) Digital Subscriber Line Access Multiplexer (DSLAM) At the telephone company site, instead of an ADSL modem, A device called a DSLAM is installed that functions similarly. It packetizes the data to be sent to the Internet (ISP server)
21 ADSL Lite ADSL can be expensive and impractical enough to discourage most subscribers: The installation of splitters at the border premises New wiring for the data line. ADSL Lite is new technology (universal ADSL or splitterless ADSL ): Plug ASDL lite modem directly into a telephone jack and connected to the computer. Splitting is done at the telephone company. ADSL lite uses 256 DMT carriers with 8-bit modulation (instead of 15-bit) However, some of the carriers may not be available. Maximum downstream data rate 1.5 Mbps. Upstream data rate of 512 kbps.
22 HDSL HDSL (high-bit-rate digital subscriber line) Alternative for T-1 line (1.544 Mbps) The T-1 uses alternate mark inversion (AMI) encoding which is very susceptible to attenuation at high frequencies. This limits the length of T-1 to 1 km (3200 ft) (repeater is needed for longer distance) Increase the cost. HDSL uses 2B1Q encoding which is less susceptible to attenuation. A data rate of Mbps (up to 2 Mbps) without repeater up to a distance of 3.86 km (12000 ft). HDSL uses two twisted pairs (One pair for each direction) to achieve full-duplex transmission.
23 SDSL The symmetric digital subscriber line (SDSL) is a one twisted-pair version of HDSL. It provides full-duplex symmetric communication up to 768 kbps in each direction. Could be considered an alternative to ADSL because !!!! Although this feature meets the need of most residential subscribers, is not suitable for business because send and receive data in large volumes in both direction.
24 VDSL The very high-bit-rate digital subscriber line (VDSL) Similar to ADSL, uses coaxial, fiber-optic, or twisted-pair cable for short distances. The modulating technique is DMT: 25 to 55 Mbps for upstream at distance of 3000 to 10,000 3.2 Mbps for downstream
25 Table 9.2 Summary of DSL technologies
CABLE TV NETWORKS The cable TV network started as a video service provider,started as a video service provider, but it has moved to the business of Internet access.but it has moved to the business of Internet access. Traditional Cable Networks Hybrid Fiber-Coaxial (HFC) Network Topics discussed in this section:
27 Figure 9.14 Traditional cable TV network Cable TV started to distribute video signals to locations with poor or no reception in the late 1940s. Called community antenna TV (CATV) because” Antenna on the top of a tall hill or building received the signal Distributed via coaxial cable to the community
28 Traditional cable TV network The cable TV office called the head end Receives video signals from broadcasting stations Feeds the signals into coaxial cables Signals became weaker with distance, so amplifiers were installed through the network. There could be up 35 amplifiers between the head end and the subscriber premises. At the end splitter split the cable, and taps and drop cables make the connection to the subscriber premises. The traditional cable TV system used coaxial cable end to end: Because of: Attenuation of the signals The use of large number of amplifiers, Communication in the traditional cable TV network is unidirectional.
29 Figure 9.15 Hybrid fiber-coaxial (HFC) network The network use a combination of fiber-optic and coaxial cable. From the cable TV office to a box, called the fiber node, is optical fiber. From the fiber node through the neighborhood and into the house is coaxial cable
30 Hybrid fiber-coaxial (HFC) network The regional cable head (RCH) serves up to 400,000 subscribers. The RCH feed the distribution hubs each serves up to 40,000 subscribes. The distribution hub does the modulation and distribution of signals. Feed the signals to the fiber nodes through the fiber-optic cables. The fiber node splits the analog signals, so the same signal is sent to each coaxial cable. Each coaxial cable serves up to 1000 subscribers. The fiber-optic cables reduce the need for amplifiers down to eight or less. The reason to move fro traditional to hybrid infrastructure is to make the cable network bidirectional.
CABLE TV FOR DATA TRANSFER 9-5 CABLE TV FOR DATA TRANSFER Cable companies are now competing with telephone companies for the residential customer who wants high-speed data transfer. DSL uses unshielded twisted-pair cable, which is very susceptible to interference. Which impose an upper limit on the data rate. Another solution is the use of the cable TV network. Bandwidth Sharing CM and CMTS Data Transmission Schemes: DOCSIS Topics discussed in this section:
32 Figure 9.16 Division of coaxial cable band by CATV Coaxial cable in HFC system has a bandwidth that ranges MHz. Cable company has divided this bandwidth: Each TV channel occupies 6 MHZ, how many channels we can accommodate? 80. From the Internet to the subscriber premises.. Each channels is 6 MHz. Each channel is divided into 6 MHz
33 Downstream data are modulated using the 64-QAM modulation technique. Note
34 The theoretical downstream data rate is 30 Mbps. Note 5 bits/Hz X 6 MHz Because the cable modem is connected to the computer through a 1oBase-T cable, this limits the data rate to 10 Mbps.
35 Upstream data are modulated using the QPSK modulation technique. Note Uses lower frequencies that are more susceptible to noise and interference, thus the QAM technique is not suitable. The better solution is QPSK.
36 The theoretical upstream data rate is 12 Mbps. Note in QPSK, 2 bits/baud is used. (2 bits/Hz X 6 MHz)
37 Sharing Both upstream and downstream bands are shared by the subscribers. Upstream Sharing Bandwidth is 37 MHz, there is 6 MHz channels. How can six channels shared in an area with 1000, 2000 or 100,000 subscribes. The solution is time sharing by dividing the band into channels using FDM. One channel is allocated for upstream for one subscriber. Downstream sharing Has 33 channels of 6 MHz. Each channel must be shared between a group of subscribers. Here we have a multicasting situation. Subscriber with the matched address receives the data and the other subscribers discard the data.
38 Figure 9.17 Cable modem (CM) CM and CMTS To use a cable network for data transmission, we need two key devices: 1.A cable modem 2. A cable modem transmission system (CMTS)
39 Figure 9.18 Cable modem transmission system (CMTS)
40 Data Transmission Schemes DOCSIS DOCSIS defines all the protocols necessary to transport data from a CMTS to a CM. Upstream Communication 1.The CM checks the downstream channels for a specific packet periodically sent by the CMTS. The packet asks any new CM to announce itself on a specific upstream channel. 2.The CMTS sends a packet to the CM, defining its allocated downstream and upstream channels. 3.The CM then starts a process, called ranging, which determines the distance between the CM and CMTS. This process is required for synchronization between all CMs and CMTSs for minislots used for timesharing of the upstream channels. 4.The CM sends a packet to the ISP, asking for the Internet address. 5.The CM and CMTS the exchange some packets to establish security parameters. 6.The CM sends its unique identifier to the CMTS. 7.Upstream communication can start in the allocated upstream channel: the CM can contend for minislots to send data. Downstream Communication The CMTS sends the packet with the address of the receiving CM, using the allocated downstream channel.