Data Terminals : Protocols

Protocols are a precise set of rules, timing, and a syntax that govern the accurate transfer of information between devices or software applications. Key protocols in data transmission networks include access protocols, handshaking, line discipline, and session protocols.

Access protocols are the set of rules that workstations use to avoid collisions when sending information over shared network media. Access protocols are also known as the media access control (MAC) protocols. Handshaking protocols involve the sequence of events that occur between communication devices that negotiate the data transmission rules and ensure reliable data transmission. When data devices begin to communicate, they discover the capabilities and agree on a common set of protocols to use during data communications session. Line discipline is the sequence of events that must occur to control the reception of data, perform error detection and correction, and multiplexing of control information, if necessary. Session protocols control the end-to-end connectivity of a data communication session. Session protocols ensure all the data is received and in the correct order.

Different protocols may be used in systems that provide similar functions. An example of this is token ring and Ethernet. Although these networks may actually use the same signaling system, they use incompatible protocols. To allow data to transfer between these networks, protocol converters are used. Protocol converts receive data and control messages, reformat data and convert control messages, and retransmit the data using the new protocol rules.

Network Management

Network management is set of procedures, equipment, and operations that keep a telecommunications network operating near maximum efficiency despite unusual loads or equipment failures. Network managers should be able to monitor, configure, and operate their network equipment from distant communication locations using a set of network management protocols.

A key network management protocol is simple network management protocol (SNMP). SNMP is an industry standard communication protocol that is used to manage multiple types of network equipment (most vendors comply at some level). By conforming to this protocol, equipment assemblies that are produced by different manufacturers can be managed by a single network management program. While many vendors supply proprietary configuration and administration software for their products, many support diagnostic and maintenance features through the use of SNMP.

Data networks can be characterized as premises distribution networks (PDNs), Local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), and wireless data networks (WDNs).

Premises Distribution Network (PDN)
A premises distribution network (PDN) is a short-range network that is located at a customer’s facility or even within their personal area. A PDN is used to connect terminals (computers) to other networks and each other. The most common types of PDN are EtherNet, asynchronous transfer mode 25 (ATM 25), universal serial bus (USB), home packet data network (HomePDN), and FireWire (IEEE-1394).

Figure 1 shows several popular forms of PDN. This diagram shows that the data transfer rate varies with the length and type of interconnection cable. This diagram also shows that some PDN technologies are better suited for multimedia applications than others. For example, ATM25 can transfer (multiplex) multiple communication channels with different levels of quality of service (QoS). Other PDN systems are capable of very high-speed data transfer rates (up to 400 Mbps) for very short distances.


Figure 2: Premises Distribution Networks (PDNs)

Local Area Networks (LANs)
Local area networks (LANs) are private data communication networks that used high-speed digital communications channels for the interconnection of computers and related equipment in a limited geographic area. LANs can use fiber optic, coaxial, twisted-pair cables, or radio transceivers to transmit and receive data signals. LAN’s are networks of computers, normally personal computers, connected together in close proximity (office setting) to each other in order to share information and resources. The two predominant LAN architectures are token ring and Ethernet. Other LAN technologies are ArcNet, AppleTalk, and fiber distributed data interface (FDDI).

Token ring traditionally operates at either 4 or 16 Mbps. Token ring operates by passing tokens from computer to computer in the LAN. Ethernet is a packet data network that allows computers to randomly transmit data and each computer in an Ethernet system resolves the potential for packet data collisions.

Figure 2 shows several of the most popular LAN topologies and their configurations. Some data networks are setup as bus networks (all computers share the same bus), as start networks (computers connect to a central data distribution node), or as a ring (data circles around the ring). This diagram shows for popular types of LAN networks: Thinnet, Thicknet, token ring networks, and Ethernet star network.


Figure 2: Local Area Networks (LANs)

Metropolitan Area Networks (MAN’s)
A MAN is a data communications network or interconnected groups of data networks that have geographic boundaries of a metropolitan area. The network is totally or partially segregated from other networks, and typically links local area networks (LANs) together.

MAN’s offer the ability to connect networks across a metropolitan area as if they were co-located in the same building or on the same campus. To create a MAN, businesses install or lease communications links between the LANs. The backbone interconnection for a MAN is routinely fiber-based. This provides a fairly high data transfer rate and provides a high degree of fault tolerance. Fiber networks often are self-healing in case the fiber line is cut or damaged.

Figure 3 shows a five node MAN connecting that connects several LAN systems via a FDDI system. This diagram shows that each LAN may be connected within the MAN using different technology such as T1/E1 copper access lines, coax, or fiber connections. In each case, a router provides a connection from each LAN to connect to the MAN.


Figure 3: Metropolitan Area Network (MAN)

Wide Area Networks (WAN’s)
WANs are communication networks that provide data transmission services through large geographically separate areas. A WAN can be established by linking together two or more metropolitan area networks, which enables data terminals in one city to access data resources in another city or country.

Figure 4 shows that a WAN is usually composed of several different data networks. Different types of communication lines such as leased lines, packet data systems, or fiber transmission lines can interconnect these networks.


Figure 4: Wide Area Networks (WAN’s)

Wireless Local Area Network (WLAN)
WLANs allow computers and workstations to communicate with each other using radio propagation as the transmission medium. The wireless LAN can be connected to an existing wired LAN as an extension, or can form the basis of a new network. While adaptable to both indoor and outdoor environments, wireless LANs are especially suited to indoor locations such as office buildings, manufacturing floors, hospitals and universities.

Wireless data networks exist in three types: LAN’s, campus interconnect, and wide area wireless (e.g., cellular or PCS). Wireless LAN’s generally use either infrared or radio frequency (RF) as their transmission media. Infrared is line-of-sight only, and poses problems in many office environments when viewed as a single solution. When coupled with twisted pair wire (the basic LAN media) and used to bring in isolated workstations across a factory floor, it has proven to be a sound technology. RF is not line-of-sight and thus is not subject to the problems of infrared. It does, however, encounter interference from many devices found in the office and factory.

Wireless LANs often used radio channels in an unlicensed frequency band. These wireless data systems can transmit data up to 50 Mbps (2-11 Mbps is more typical). Point-to-point wireless data systems may be used to interconnect data networks between buildings within a campus. Providing this wireless data link only requires the installation of 2 antennas with a clear line of site communication. Point-to-point microwave data transmission rates can exceed 45 Mbps. Wide area wireless systems, such as cellular and PCS, can provide wireless coverage over large geographic areas. However, WANs have data transmission rates that are usually below 28 kbps and the usage cost is relatively high.

Wireless LAN systems typically use the unlicensed radio frequency bands instrument, scientific and medial (ISM) frequency bands. These bands include 902-928 MHz, 2.4 - 2.485 GHz, and 5.7 GHz ranges. Each of these frequency bands has usage limitations in different parts of the world. The only unlicensed frequency band that has common authorization to use throughout the world is the 2.4 GHz frequency band. WLANs typically operate up to a distance of 300 feet (100 meters). WLAN systems provide much larger coverage by interconnected radio access nodes. Wireless LAN standards include multiple versions of IEEE 802.11 and Bluetooth.

Figure 5 shows the three key types of wireless data networks. This diagram shows a wireless LAN system that has multiple access nodes. These access nodes operate as gateways between the data communication devices (e.g., mobile computer) and the data network hub. Building 1 uses an older 801.11 wireless LAN system that operates from 902-928 MHz at 2 Mbps. Building 2 uses a newer 802.11 wireless LAN system that operates at 2.4 GHz providing up to 11 Mbps data transfer rate. This diagram also shows a microwave data link that provides a 45 Mbps interconnection between campus buildings. Finally, a user who is operating in a remote area outside the core campus is using the wide area mobile system to transfer data files (at a data transfer rate below 28 kbps).


Figure 5: Wireless Data Networks

Data Terminals : Network Interface Card (NIC), LAN Wiring,

Overview
Data networks are telecommunications networks installed and operated exclusively for information exchange between data communication devices (such as computers). Data network types include premises distribution network (PDN), local area networks (LAN’s), metropolitan area networks (MAN’s), and wide area networks (WAN’s). These are hierarchical with the LAN being the base and the WAN being the umbrella architecture.

PANs are short-range data communications systems that are primarily used to interconnect peripheral equipment with a local computer or computing system. LANs are designed to reliably transfer large amounts of data quickly and error-free over a very small area such as an office. MAN’s facilitate LAN-to-LAN information exchange in a local exchange area. WAN’s allow for information exchange between LAN’s in different exchanges normally across LATA boundaries. For example a LAN in Chicago sharing information with a LAN Seattle would do so across a WAN.

A data network is composed of several key parts such as data terminals (e.g., personal computers), network adapters, access wiring, and data distribution nodes (e.g., routers, brouters, and switches). In some data networks, network management/control systems are used to configure, monitor, coordinate, and control the network elements.

Data Terminals
Data terminals are data input and output devices that are used to communicate with a remotely located computer or other data communication device. Data terminals frequently consist of a keyboard, video display monitor, and communication circuitry that can connect the data terminal with the remotely located computer.

The term “data terminal” is often used to describe multiple types of devices including personal computers (PCs), dedicated “dumb” terminals, scientific workstations, and other types of computers that can communicate with other computers or a host computer.

Data terminal equipment (DTE) are devices that capture and serialize information for communication to other communication devices. Data communication equipment (DCE) circuits are assemblies that convert data information into a format that can be transferred through a communication network.

Figure 1 shows data terminals that are connected through a modem to interconnect the data terminals with a remote computer. In this diagram, the data terminals are the DTE and the modems are the DCE.



Figure 1: Data Terminals

Network Interface Card (NIC)
A NIC is a device that adapts the data communication network protocol to a data bus or data interface in a computer. The NIC is installed between a computer network (such as the Ethernet) and a computer data bus (such as a PCI socket). The NIC is usually a PC expansion board connector and operating system. Software in the computer is installed and setup to recognize the NIC card.

LAN Wiring

There are typically three types of wiring used for LAN’s: twisted pair, coax, and fiber. Of these, twisted pair is dominant for several reasons: ease of installation, availability, cost, and speed as a function of relative cost.

Twisted pair comes in a variety of “categories” and is either shielded twisted pair (STP) or unshielded twisted pair (UTP). UTP is the less expensive and the most widely used. STP has an outer copper or foil conductor located just beneath the out sheath of the wire. In areas where there is a significant incidence of electromagnetic interference (EMI), such as around factory floor machinery or hospital radiological/MRI equipment, STP is used.

Twisted pair wire is classified by categories that relate to the data transmission speed at which the wire is capable of passing data. For each category there are manufacturing specifications such as wire quality, insulation characteristics, and number of twists per inch. Generally, the higher the number of twists, the higher the data transmission rate can be.

Routinely LAN cable is four-pair (eight conductors) even though most data communication systems (such as Ethernet) only require 2 pairs (transmit and receive pairs). It is installed with all conductors terminated on each end into patch fields, hub equipment, or office wall plates (jack fields). From the office wall jack the typical PC or peripheral device is connected to the LAN via a wall cord that is also four-pair terminated in RJ-45 modular connectors. Most offices are wired for multiple network connections and in many cases the voice and data wiring is installed together and to the same cable specification (e.g., category 3 and above).

Network Distribution and Routing
Network distribution and routing equipment provides communication paths between the end-user and the services they desire to use (e.g., Internet). There are three basic methods used to distribute in data networks: broadcast (distribution hubs), dedicated paths (switching nodes), and packet-switching (routers).

Hubs broadcast information to all the communication devices that are connected to it. Switches create a physical or logical connection between data communication devices. Routers are intelligent switches that can dynamically route (switch to other routers) packets of data toward their ultimate destination.

Network Access Control
Network access control is a process of coordinating access of data communication devices to a shared communications media (transmission medium). Network access control is a combination of media access control (MAC) and service authorization.

There are two key ways data communication devices can access communication systems: non-contention based and contention based. Non-contention based regularly poll or schedule data transmission access attempts. An example of a non-contention based data communication system is token ring. In the token ring system, only the data communication device that has the token is allowed to transmit. This ensures that other data devices will not interfere with the data transmission. Contention based access control systems allow data communication devices to randomly access the system through the sensing and coordination of busy status and detected collisions. Carrier sense multiple access (CSMA) with collision detection (CSMA/CD) or collision avoidance (CSMA/CA) listen to the data activity first to determine if the systems is not busy (carrier sense) before they begin a transmit request. After the device transmits its required, it waits to hear if the system has acknowledged its required (usually an echo of its original signal). If the CSMA/CD device does not hear an acknowledgement, it will wait a random amount of time before transmitting another data transmission service request.

The CSMA/CA system differs from the CSMA/CD system by the assignment of different access wait periods to different priority groups of devices. This allows high priority devices (such as a system management data terminal) to attempt access before a lower priority device (e.g., web browsing terminal).

Figure 2 shows the key ways networks can control data transmission access: non-contention based and contention based. This diagram shows that non-contention based regularly poll or schedule data transmission access attempts before computers can begin to transmit data. This diagram shows that a token is passed between each computer in the network and computers can only transmit when they have the token.


Figure 2: Data Network Access Control

Because there is no potential for collisions, computers do not need to confirm the data was successfully transmitted through the network. This diagram also shows contention based access control systems allow data communication devices to randomly access the system through the sensing and coordination of busy status and detected collisions. These devices first listen to see if the system is not busy and then randomly transmit their data. Computers in the contention-based systems must confirm that data was successfully transmitted through the network, because there is the potential for collisions.

Future Enhancements: Wireless Cable, HDTV, Cable Telephony, Interactive Television

Some of the future enhancements for CATV include wireless cable, cable telephony, video on demand (VOD), and interactive television.

Wireless Cable
“Wireless Cable” is a term given to land based (terrestrial) wireless distribution systems that utilize microwave frequencies to deliver video, data, and/or voice signals to end-users. There are two basic types of wireless cable systems, multichannel multipoint distribution service (MMDS) and local multichannel distribution service (LMDS).

Multichannel video and data services are being offered over microwave frequencies. The data-over-cable service interface specification (DOCSIS) with a few modifications can also be used in 2.6 GHz wireless multipoint, multichannel distribution service (MMDS), and 28 GHz local multipoint distribution service (LMDS) systems [14]. The DOCSIS specification is being adapted for the wireless cable marketplace. A consortium called “Wireless DSL” is working to produce an adapted version of DOCSIS called DOCSIS+ that is suitable for offering cable modem technology via microwave transmission. The DOCSIS+ standard has been proposed to the IEEE 802.16 for conversion into an official standard.

Figure 1 shows a LMDS system. This diagram shows that the major component of a wireless cable system is the head-end equipment. The head-end equipment is equivalent to a telephone central office. The head-end building has a satellite connection for cable channels and video players for video on demand. The head-end is linked to base stations (BS) which transmits radio frequency signals for reception. An antenna and receiver in the home converts the microwave radio signals into the standard television channels for use in the home. As in traditional cable systems, a set-top box decodes the signal for input to the television. Low frequency wireless cable systems such as MMDS wireless cable systems (approx 2.5 GHz) can reach up to approximately 70 miles. High frequency LMDS systems (approx 28 GHz) can only reach approximately 5 miles.


Figure 1: Local Multipoint Distribution System (LMDS)

High Definition Television (HDTV)
High definition television (HDTV) is a TV broadcast system that proves higher picture resolution (detail and fidelity) than is provided by conventional NTSC and PAL television signals. HDTV signals can be in analog or digital form.

The specifications for HDTV digital systems allow for many types of data services in addition to digital video service. Digital HDTV channels carry high-speed digital services that can be addressed to a specific customer or group of customers that are capable of decoding and using those services. Examples of these services include: special programming information, software delivery, video or audio delivery (like pay-per-view programming), and instructional materials.

The data rate available for additional services is dynamic and ranges from a few kbps to several Mbps, depending on the video and audio program content. The gross data rate of the HDTV system is 19 Mbps. The amount of this data rate that is used by the HDTV video signal depends on the compression technology. Video data compression produces a data rate that changes dependent on the original video signal. When the video program contains rapidly changing scenes, most of the 19 Mbps signal is required for transmission. If the video signal is not changing rapidly, much of the 19 Mbps can be used for other types of services.

Transmission of the additional services has a lower priority than transmission of the primary program. If the primary service (HDTV) consumes a large part of the data (such as a rapidly changing video action scene), the customer may have to wait for some time prior to receiving large blocks of data.

Cable Telephony
Cable telephony is the providing of telephone services that use CATV systems to initiate, process, and receive voice communications. Cable telephony systems can either integrate telephony systems with cable modem networks (a teleservice) or the cable modem system can simply act as a transfer method for Internet telephony (bearer service). Because of government regulations (restrictions or high operational level requirements) in many countries, some cable operators are delaying the integration of telephone services with cable network. In either case, cable telephony systems are data telephony systems that include a voice gateway, gatekeeper, and a media interface.

Voice gateway is a network device that converts communication signals between data networks and telephone networks. A gatekeeper is a server that translates dialed digits into routing points within the cable network or to identify a forwarding number for the public telephone network. A multimedia transfer adapter converts multiple types of input signals into a common communications format.

Figure 2 shows a CATV system that offers cable telephony services. This diagram shows that a two-way digital CATV system can be enhanced to offer cable telephony services by adding voice gateways to the cable network’s head-end CMTS system and media terminal adapters (MTAs) at the residence or business. The voice gateway connects and converts signals from the public telephone network into data signals that can be transported on the cable modem system. The CMTS system uses a portion of the cable modem signal (data channel) to communicate with the MTA. The MTA converts the telephony data signal to its analog audio component for connection to standard telephones. MTAs are sometimes called integrated access devices (IADs).


Figure 2: Cable Telephony

Because of the high data transmission capability of cable television systems, cable telephony system can provide video telephony service. Video telephony is a telecommunications service that provides customers with both audio and video signals between their communications devices.

Interactive Television
Interactive television is a combination of cable, television, multimedia, PCs, and network programming that allows dynamic control of media display using inputs from the end-user. Interactive television has three basic types: “pay-per-view” involving programs that are independently billed, “near video-on-demand” (NVOD) with groupings of a single film starting at staggered times, and “video-on-demand” (VOD), enabling request for a particular film to start at the exact time of choice. Interactive television offers interactive advertising, home shopping, home banking, e-mail, Internet access, and games.

Video on demand (VOD) is a service that allows customers to request and receive video services. These video services can be from previously stored media (entertainment movies or education videos) or have a live connection (sporting events in real time).

A limited form of VOD is called near video on demand (NVOD). Near video on demand is a video service that allows a customer to select from a limited number of broadcast video channels. These video channels are typically movie channels that have pre-designated schedule times. Unlike full VOD service, the customer is not able to alter the start or play time of these broadcast videos.

Pay per view (PPV) is a process that allows customers to request the viewing of movies through an unscrambling process. PPV movies are usually broadcasted to all customers in a cable television network. To prevent unauthorized viewing, each PPV channel has its own scrambling code. To provide a customer with a reasonable selection of movies, the same movie is broadcasted on different channels with start intervals that range from 15 to 60 minutes. To provider twenty PPV movies, approximately 80 to 160 television channels would be required.

Analog cable systems provide up to 800 MHz of bandwidth. Using 6 MHz wide video channels, this allows up to 120 analog video channels. By digitizing each 6 MHz channel and using compressed digital video (10:1 compression), this increases the capacity of a cable system to over 500 digital television channels.

Cable converter boxes, known as set-top boxes, have different reception and decoding capabilities. Set-top boxes are required to convert distributed signals into a format suitable for viewing. Set-top boxes also can coordinate access to video on demand channels.

Electronic programming guide (EPG) is an interface (portal) that allows a customer to preview and select from possible list of available content media. EPGs can vary from simple program selection to interactive filters that dynamically allow the user to filter through program guides by theme, time period, or other criteria.

Figure 3 shows a video on demand system. This diagram shows that multiple video players are available and these video players can be access by the end customer through the set-top box. When the customer browses through the available selection list, they can select the media to play.


Figure 3: Video on Demand (VOD)

Hypervideo is a video program delivery system that allows the embedding of links (hotspots) inside a streaming video signal. This allows the customer (or receiving device) to dynamically alter the presentation of streaming information. Examples of hypervideo could be pre-selection of preferred advertising types or interactive game shows.

Synchronized television (syncTV) is a video program delivery application that simultaneously transmits hypertext markup language (HTML) data that is synchronized with television programming. Synchronized television allows the simultaneous display of a video program along with additional information or graphics that may be provided by advertisers or other information providers.

Services:Cable Television Distribution, Pay per View, & High Speed Data

Services that are offered by cable system operators include television distribution, pay per view, and high-speed data services. Cable television networks pay distribution fees to content provides for the right to redistribute network broadcasts in their systems. In return, cable television operators charge customers for access to this content on a monthly subscription, per-event fee, or by quantity of information (data) transferred.

Cable Television Distribution
Television distribution involves the receiving and re-sending of television signals to groups of consumers that are connected to the cable television network. Cable network operators typically charge a monthly access fee for connection to cable television systems.

Figure 1 shows the typical cable television subscription fees in the United States. This diagram shows an initial connection fee of approximately $45 with a monthly connection fee of $30.00. This figure shows that the types of channels may be grouped into categories with varying fees charged for these groups of services.


Figure 1: Cable Television Subscription Fees

Pay per View (PPV)
Pay-per-view is a video signal subscription service that allows customers to pay for individual video selections they desire to view. Pay-per-view service can be limited to viewing pre-scheduled broadcasted channels or video on demand (VOD) videos. The typical charge for pay per view services is approximately $6.00.

High Speed Data (Cable Modems)
High-speed data service via cable modems provides customers with the ability to transfer data at broadband data transmission rates (1 Mbps or above). Customers usually pay a monthly fee for high-speed data connection in addition to an Internet service provider (ISP) account.

Figure 2 shows the average charges for high-speed cable modem data access. This chart shows that cable modem access cost includes an initial connection fee of $100, a monthly subscription fee of approximately $50, and an equipment leasing cost of $10 for the connection equipment.


Figure 2: Sample High Speed Data Cable Modem Access Cost

Systems : NTSC, PAL, SECAM, MPEG, and DOCSIS

There are several systems that are used for video distribution. The system (standards) used in CATV systems include: NTSC, PAL, SECAM, MPEG, and DOCSIS.

National Television Standards Committee (NTSC)
The NTSC system is an analog video system that was developed in the United States and is used in many parts of the world. The NTSC system uses analog modulation where a sync burst precedes the video information. The NTSC system uses 525 lines of resolution (42 are blanking lines) and has a pixel resolution of approximately 148k to 150k pixels.

The NTSC system uses 6 MHz wide radio channels that range from 54 MHz to 88 MHz (for VHF channels 1-6), 174 MHz to 216 MHz (for VHF channels 7-13) and 470 MHz to 806 MHz (for UHF channels 14-69). Initially, the frequency range of 806 MHz to 890 MHz was available for UHF channels 70 to 83. The FCC reallocated these channels for cellular and specialized mobile radio (SMR) use in 1983.

When used in the United States, NTSC systems have a maximum transmitter power level that varies from 100 kWatts for low VHF channels (1-6), 316 kWatts for high VHF channels (7-13) to 5 million Watts for UHF channels (14-69). Television transmission limits are also established based on the class of service (local or wide area) for the authorized television broadcast company.

Phase Alternating Line (PAL)
The PAL system was developed in the 1980’s to provide a common television standard in Europe. PAL is now used in the Middle East and parts of Asia and Africa. The PAL system uses a phase alternation process to enhance the video signal’s resistance to chromatic distortions as compared with the NTSC video signal. Although PAL and NTSC systems are similar in function, they are not compatible. A converter box is required between the two systems.

The system provides 625 lines per frame, 50 frames per second. A modified version of PAL (PAL-M) is used for the Brazilian television system. PAL-M provides 525 lines per frame and 60 frames per second. The PAL system uses 7 or 8 MHz wide radio channels.

Sequential Couleur Avec Memoire (SECAM)
Sequential Couleur Avec Memoire (SECAM) is a video transmission system that was developed by France and the former Union of Soviet Socialist Republics to improve on the NTSC video transmission system. This translates to “sequential color with memory.” SECAM is a color video transmission system that provides 625 lines per frame and 50 frames per second. This system transfers color difference information sequentially on alternate lines as a FM signal. The SECAM system requires 8 MHz of bandwidth.

Motion Picture Experts Group (MPEG) Compression
There are several digital video compression systems. The most common form of digital video compression of video signals conforms to the motion picture experts group (MPEG). There are various levels of MPEG compression; MPEG-1 and MPEG-2. MPEG-1 compresses by approximately 52 to 1. MPEG-2 compresses up to 200 to 1. MPEG-2 ordinarily provides digital video quality that is similar to VHS tapes with a data rate of approximately 3.2 Mbps. MPEG-2 compression can be used for HDTV channels, however this requires higher data rates.

Data Over Cable Service Interface Specifications (DOCSIS)
The data over cable service interface specifications (DOCSIS) is a standard used by cable systems for providing Internet data services to users. The DOCSIS standard was primarily developed by equipment manufacturers and CATV operators. It details most aspects of data over cable networks including physical layer (modulation types and data rates), medium access control (MAC), services, and security. The DOCSIS cable modem specifications are available from CableLabs® at http://www.cablemodem.com/specifications.html.

The downstream information flows to all users that are tuned to a specific RF channel on the cable system. There may be several RF channels used to serve many cable modem users in a system. Each individual cable modem decodes their portion of the data on a specific RF channel. For transmitting on the upstream side, each user is assigned time of a few milliseconds each where the user can transmit short bursts of data. Dividing the channel into small slices of data is well suited for short delays to keyboard commands.

To convert the Internet data into a format suitable for delivery on a cable channel, a CATV upconverter is used at the head-end of the cable system. The CATV upconverter handles both digital and analog television signals. Usually 10-20 upconverters are installed into a single equipment chassis. To allow cable modems to connect to data networks (such as the Internet), a cable modem termination system (CMTS) is used. The CMTS an interface device (gateway) that is located at the head-end of a cable television system to send and adapt data between cable modems and other networks.

A single 6 MHz wide television channel is capable of 30-40 Mbps data transmission capacity. This is because coaxial cable offers a communication medium that is relatively noise free (compared to radio or unshielded twist pair cable) that allows the use of complex modulation technologies (combination of amplitude and phase modulation). These modulation technologies can transfer several bits of data for each Hertz of bandwidth (bits per Hertz). In 2001, cable modems could transmit data using 64 QAM modulation technology. To increase the data rate, even more complex modulation technologies such as 256 QAM or even to 1024 QAM have been demonstrated [13].

The DOCSIS system is focused around packet service such as Internet Protocol (IP) and asynchronous transfer mode (ATM) to provide a variety of services (e.g., variable bit-rate, constant bit-rate) with the ability to offer varied levels of quality of service (QoS). This allows the DOCSIS system to offer multiple channels to a home or business that can provide for various services such as voice (constant bit-rate), data (high reliability), and video (high-speed data).

High Definition Television (HDTV)

High definition television (HDTV) is a TV broadcast system that proves higher picture resolution (detail and fidelity) than is provided by conventional NTSC and PAL television signals. HDTV signals can be in analog or digital form.

HDTV has been offered in several countries since its introduction in Japan in 1988. The first HDTV receivers in the United States were introduced at the 1998 Winter Consumer Electronics Show in Las Vegas.

HDTV radio broadcast channels can use the same 6 MHz channel bandwidth. However, it must replace the existing NTSC signal with a new high resolution analog or high speed digital radio signal. Initial demonstrations of HDTV required 2 standard television channels. The FCC has finally approved the “Grand Alliance” standard for high-definition television for the United States that only requires one standard television channel to send a HDTV digital channel and supplementary services.

The FCC introduced a new table of digital television channel numbers and RF power level assignments for existing full-power television stations in the United States in April of 1997. The new assignments were designed to give each television station coverage comparable to the station’s existing radio coverage area when they convert to digital transmission.

The change in channel numbers is likely to be a significant challenge for television stations; many stations, especially in Southern California, have a reduced coverage area. This has resulted in the contesting of the new assignments by some television broadcasters. The Association of Maximum Service Telecasters (an association of local television stations) has proposed an alternative table of channel assignments to address the issues of established broadcasters.

The digital technology that allows high-definition television broadcasts in the U.S. can also be used for “multicasting,” that is, transmitting up to five channels of “standard-definition” television programming. Many broadcasters are examining multicasting as an alternative to high-definition television. If the ability to provide more video channels is more desirable than providing high-definition broadcast quality video, HDTV broadcast service and products may be delayed for their entry in the US marketplace.

In July 1996, WRAL in Raleigh, North Carolina became the first United States television station to commence broadcast of high-definition television signals. As of early 1998, more than a dozen stations have licenses for digital transmission, and the number of licenses is increasing every month. HDTV is likely only to be available in the largest markets in the United States for at least the first year the service is provided, so viewers in smaller markets may have to wait many years before they have the opportunity to use digital and/or high-definition television receivers.

The data transmission rate of the HDTV system is 19 Mbps and it uses the motion pictures experts group (MPEG-2) video compression format. To allow for a gradual migration to HDTV service, HDTV transmission will also contain regular programming of standard television on HDTV radio channels. The simulcast transmission will continue for up to 15 years as standard NTSC televisions and transmitting facilities are phased out. Initially, HDTV receivers will have the capability to receive and display regular NTSC broadcasts.