Signal Scrambling

Signal Scrambling
Video signal scrambling is a deliberate act of changing an electrical signal (often distortion of video, digital voice, or data) to prevent interpretation of the signals by unauthorized users that are able to receive the signal. Because the scrambling process is performed according to a known procedure or algorithm, the received signal can be descrambled to recover the original digital stream through the use of a known code or filtering technique.

In 1971, the first system to using scrambling on a cable system was demonstrated. The first scrambling suppressed the synchronization signal so the video of the television picture was distorted. To decode the scrambled video, the synchronization signal was recreated in the setup box by decoding the correct synchronization signal from another portion of the transmitted signal. Another form of signal scrambling that was used was the insertion a signal that was slightly offset from the channel’s frequency to interfere with the picture.

These early video signal scrambling systems were relatively simple in design. As a result, accessory devices soon became available that allowed consumers to decode the scrambled signals without paying subscription fees. To prevent unauthorized viewing, more sophisticated signal scrambling technologies have been developed.

For digital television signals, video signals can be easily encrypted with a key code. To successfully decode the video signal, the set-top box must contain the decryption key code. For two-way cable systems, this code can be dynamically changed and unauthorized viewing has been greatly reduced.

Cable Modems

A cable modem is a device that MOdulates/DEModulates data signals on a coaxial cable and divides the high data rate signals into digital signals designated for a specific user. Cable modems are often asymetrical modems as the data transfer rate in the downstream direction is typically much higher than the data transfer in the upstream direction. The typical gross (system) downstream data rates range between 30-40 Mbps and gross upstream data rates typically range up to 2 Mbps.

Usually 500 to 2000 users share the gross data transfer rate on a cable system. Cable modems also have the requirement to divide the high-speed digital signals into low-speed connections for each user. In 2001, the average data rates for a cable modem users was approximately 720 kbps.

Cable modems contain a tuner, a demodulator, a modulator, media access control (MAC) section, and a control section. The tuner converts a selected RF channel (high frequency) to the modem baseband (low frequency) signal. The tuner makes adjustments to a frequency (usually between 42 and 850 MHz) for downstream traffic and may convert the upstream traffic to a different RF channel (usually between 5 and 42 MHz).

Early cable modems used a hybrid system that used the cable system for downlink channels and a telephone line for upstream traffic. This was desirable as many of the amplifiers in the cable television system only provided for one-way amplification.

The cable modem receiver contains a demodulator that converts the low frequency received signal into its original baseband digital form and performs error detection and correction. The cable modem may contain a decoder to convert compressed video into a form that can be displayed on the computer monitor. The modulator converts the digital information from the computer into a format suitable for transfer back to the Internet. For hybrid systems, this may be a telephone line audio modem and for two-way cable systems, the modulator converts the data into radio-frequency signals for transmission on the cable system. A control section coordinates the upstream and downstream access operations (called media access control - MAC) of the cable modem. The control section also coordinates the overall operation of the cable modem including how it interfaces to communication devices. For example, the data may be converted to Ethernet format for communication with a personal computer.

Figure 1 shows a block diagram of a typical cable modem system. This diagram shows that the Internet is connected to the head-end of the cable system by a gateway. The gateway adapts the data to and from the Internet into a form that can be transmitted through the cable modem system. The cable modems at the head-end convert the digital signals into RF signals that can be transmitted through the cable network. A single 6 MHz RF television channel is converted to a high-speed data channel (30-40 Mbps) that is transmitted to all the users in the cable modem network. To access data on this channel, each cable modem is assigned a portion of the data channel from the CMTS at the head-end. This diagram shows that multiple RF channels may be used to provide more data transfer capabilities to each customer. When the cable modem at the customer’s location wants to send data, it randomly accesses the system through a return RF channel.

Figure 1: Cable Modem System

Digital Video

Digital broadcasting is the sending of a digital signal through a common channel to a group of users that may be capable of decoding some or all of the broadcast information.

Digital Television (DTV) is a method of transferring video images and their audio components through digital transmission. There are several formats used for DTV including high quality digital MPEG and 28.8 video.

Digital video is the sending of a sequence of picture signals (frames) that are represented by binary data (bits) that describe a finite set of color and luminance levels. Sending a digital video picture involves the conversion of a scanned image to digital information that is transferred to a digital video receiver. The digital information contains characteristics of the video signal and the position of the image (bit location) that will be displayed. Digital television continues to send information in the form of frames and pixels. The major difference is the frames and pixels are represented by digital information instead of a continuously varying analog signal.

The first digital television broadcast license for the United States was issued to a Hawaiian television station in September 1997. Digital television sends the video signal in digital modulated form. Ironically, many television signals have been captured and stored in digital form for over 10 years. To transmit these digital video signals, they must first be converted to standard analog television (NTSC or PAL) to be transmitted through analog transmission systems and to reach analog televisions.

When digital transmission is used, most digital video systems use some form of data compression. Data compression involves the characterization of a single picture into its components. For example, if the picture was a view of the blue sky, this could be characterized by a small number of data bits that indicate the color (blue) and the starting corner and ending corner. This may require under 10 bytes of information. When this digital information is received, it will create a blue box that may contain over 7,200 pixels. With a color picture, this would have required several thousand bytes of information for only 1 picture.

In addition to the data compression used on one picture (one frame), digital compression allows the comparison between frames. This allows the repeating of sections of a previous frame. For example, a single frame may be a picture of city with many buildings. This is a very complex picture and data compression will not be able to be as efficient as the blue sky example above. However, the next frame will be another picture of the city with only a few changes. The data compression can send only the data that has changed between frames.

Digital television broadcasting that uses video compression technology allows for “multicasting” (simultaneously sending) several “standard definition” television channels (normally up to five channels) in the same bandwidth as a standard analog television channel. Unfortunately, high definition digital television channels require a much higher data transmission rate and it is likely that only a single HDTV channel can be sent on a digital television channel.

Figure 1 demonstrates the operation of the basic digital video compression system. Each video frame is digitized and then sent for digital compression. The digital compression process creates a sequence frames (images) that start with a key frame. The key frame is digitized and used as reference points for the compression process. Between the key frames, only the differences in images are transmitted. This dramatically reduces the data transmission rate to represent a digital video signal as an uncompressed digital video signal requires over 50 Mbps compared to less than 4 Mbps for a typical digital video disk (DVD) digital video signal.

Figure 1: Digital Video

Technologies - Analog Video

Some of the key technologies used in CATV systems include analog video.

Analog Video
Analog video contains a rapidly changing signal (analog) that represents cthe luminance and color information of a video picture. Sending a video picture involves the creation and transfer of a sequence of individual still pictures called frames. Each frame is divided into horizontal and vertical lines. To create a single frame picture on a television set, the frame is drawn line by line. The process of drawing these lines on the screen is called scanning. The frames are drawn to the screen in two separate scans. The first scan draws half of the picture and the second scan draws between the lines of the first scan. This scanning method is called interlacing. Each line is divided into pixels that are the smallest possible parts of the picture. The number of pixels that can be displayed determines the resolution (quality) of the video signal. The video signal breaks down the television picture into three parts: the picture brightness (luminance), the color (chrominance), and the audio.

There are three primary systems used for analog television broadcasting: NTSC, PAL, and SECAM. The National Television System Committee (NTSC) is used for the Americas, while PAL and SECAM are primarily used in the UK and other countries. The major difference between the analog television systems is the number of lines of resolution and the methods used for color transmission.

There have been enhancements made to analog video systems over the past 50 years. These include color video, stereo audio, separate audio programming channels, slow data rate digital transfer (for closed captioning), and ghost canceling. The next major change to television technology will be its conversion to HDTV.

Figure 1 demonstrates the operation of the basic NTSC analog television system. The video source is broken into 30 frames per second and converted into multiple lines per frame. Each video line transmission begins with a burst pulse (called a sync pulse) that is followed by a signal that represents color and intensity. The time relative to the starting sync is the position on the line from left to right. Each line is sent until a frame is complete and the next frame can begin. The television receiver decodes the video signal to position and control the intensity of an electronic beam that scans the phosphorus tube (“picture tube”) to recreate the display.

Figure 1: NTSC (Analog) Video

Distribution Network & Head end

Distribution Network
The distribution network is the part of a cable television system that connects the head-end of the system (video and media sources) to the customer’s equipment. Traditionally, the local connection has been composed of a coaxial cable that allows for the one-way transmission of with a maximum of one hundred and twenty 6 MHz analog television signals.

The hybrid fiber coax (HFC) system is an advanced CATV transmission system that uses fiber optic cable for the head-end and feeder distribution system and coax for the customers end connection. HFC are the 2nd generation of CATV systems. They offer high-speed backbone data interconnection lines (the fiber portion) to interconnect end user video and data equipment. Many cable system operators anticipating deregulation and in preparation for competition began to upgrade their systems to Hybrid Fiber Coax (HFC) systems in the early 1990’s. As of 2002, over 35% of the total cable lines in the United States had already been converted to HFC technology.

Figure 1 shows a typical cable distribution system that uses a combination of fiberoptic cable and coaxial cable for the local connection. This diagram shows that the multiple video signals from the head-end of the cable television system is converted into digital form to allow distribution through high-speed fiber cable. The fiber cable is connected in a loop around the cable television service area so that if a break in the cable occurs, the signal will automatically be available from the other part of the loop. The loop is connected (tapped) at regular points by a fiber node. The fiber node converts the fiber signals into RF television signals that are distributed on the local coaxial cable network. The coax network distributes the RF signals to homes in the cable television network.

Figure 15: Hybrid Cable Television Distribution Network

The head-end is the master distribution center of a CATV system where incoming television signals from video sources (e.g., DBS satellites, local studios, video players) are received, amplified, and re-modulated onto TV channels for transmission down the CATV system.

Figure 2 shows a diagram of a simple head-end system. This diagram shows that the head-end allows the selection of multiple video sources. Some of these video sources are scrambled to prevent unauthorized viewing before being sent to the cable distribution system. The video signals are supplied to video modulators the convert the low frequency video signals into their radio frequency television channel. The output of each modulator is combined and connected to the distribution trunk.

Figure 2: Head-end System

Community Access Television (CATV)

Community access television (CATV) is a television distribution system that uses a network of cables to deliver multiple video and audio channels. Since1941, television broadcast services have been able to bring news and entertainment to listeners without wires. In 1948, television signals began to be delivered by interconnection cables. These early analog cable television systems simply retransmitted existing television channels.

Video broadcasting is the process of transmitting video images to a plurality of receivers. The broadcasting medium may be via radio waves, through wired systems (such as CATV), or through packet data systems (such as the Internet). Television involves the transmission and reception of visual images via electrical signals. Video is an electrical signal that carries TV picture information.

For many years, video (television) broadcasters had monopolized the distribution of some forms of information to the general public. This had resulted in strict regulations on the ownership, operation, and types of services broadcast companies could offer. Due to the recent competition of wide area information distribution, governments throughout the world have eased their regulation of the broadcast industry. In 1996, the United States released the Telecommunications Act of 1996 that dramatically deregulated the telecommunications industry. This allowed broadcasters to provide many new services with their existing networks.

Also in the mid 1990’s, a major shift occurred in the broadcast industry. The conversion from analog systems to digital systems provided broadcasters with the tools they needed to bundle multiple types of services onto a television channel signals. This included cable modems, digital television, and even telephone service. The ability to integrate several services into one transmission signal allows the cable television operator to offer many new services without significant investment in new cable systems. Analog CATV systems typically provide 50-100 video channels while digital CATV systems to provide hundreds of video channels, high-speed Internet access, and telephone service.

Broadcast Television
The technology that is used for television broadcast was developed in the 1940s. The success of the television marketplace is due to standardized, reliable, and relatively inexpensive television receivers and a large selection of media sources. The first television transmission standards used analog radio transmission to provide black and white video service. These initial television technologies have evolved to allow for both black and white and color television signals, along with advanced services such as stereo audio and closed caption text. This was a very important evolution as new television services (such as color television) can be on the same radio channel as black and white television services.

While analog television technology is efficient at distributing good video and audio signals, it does not easily allow the sending and receiving of digital data. Several new television broadcast exist that can deliver high quality video and audio as well as information services using digital signal transmission. New technologies allow transmission of high-definition television (HDTV). HDTV is the term used to describe a high-resolution video and high quality audio signal as compared to standard NTSC or PAL video transmission. HDTV signals can be in analog or digital form. Digital HDTV systems have the added benefits of providing data and other multimedia services.

A television system consists of a television production studio, a high power transmitter, a communications link between the studio and the transmitter, and network feeds for programming. The production studio controls and mixes the sources of information including videotapes, video studio, computer created images (such as captions), and other video sources. A high power transmitter broadcasts a single television channel. The television studio is connected to the transmitter by a high bandwidth communications link that can pass video and control signals. This communications link may be a wired (coax) line or a microwave link. Many television stations receive their video source from a television network. This allows a single video source to be relayed to many television transmitters.

Cable Television
Cable television is a television distribution system that uses a network of cables to deliver multiple video and audio channels to consumers. Cable television systems can be one-way systems (only from the head-end to consumers) or two-way (both to and from the customer).

Figure 1 shows a one-way cable television system. This diagram shows that various video sources are selected in the head-end. Each of the video sources that will be distributed on the cable network are applied to an RF modulator that converts the video signals into RF signals on a specific frequency. The many RF signals are combined (added together), amplified, and sent to the cable television system distribution network.

Figure 1: One-Way Cable Television System

The distribution network supplies part of the signal (signal tap) as the cable passes near each home or business location. As the distribution system progresses away from the head-end, the signal level begins decrease. Periodically, amplifiers are used to increase the composite video signal.

A two-way cable television system allows customers to receive and send information between the cable system and their set-top box. There are two options for two-way cable television systems: a hybrid system and an integrated system. Hybrid two-way systems use different technologies to transfer information in different directions and integrated systems use the cable network for both directions of communication.

Because the design of most cable systems started as a one-way cable system, hybrid systems were first used to add two-way communication capability. For hybrid systems, a different technology is used to transfer information to the user (downstream) and from the user (upstream). Early systems used the cable for the downstream and the telephone network or wireless data devices for the upstream.

As cable systems evolved to include fiber (optical) cable and two-way amplifiers, cable networks evolved to allow data transmission in both directions. On the coaxial (RF) cable, the return path was assigned to frequencies in the range below 50 MHz. This frequency range was unassigned for television operation. Fiber optic cables use separate strands for each direction as each fiber cable often has many (30+) fiber strands.

The two-way cable system requires cable modems at the user end and a coordinating modem at the head-end of the system. The cable modem is a communication device that modulates and demodulates (MoDem) data signals to and from a cable television system. A modem at the head-end coordinates the customer’s modem and interfaces data to other networks (such as the Internet).

Figure 2 shows a two-way cable television system. This diagram shows that the two-way cable television system adds a cable modem termination system (CMTS) at the head-end and a cable modem (CM) at the customer’s location. The CMTS also provides an interface to other networks such as the Internet.

Figure 1: Two-Way Cable Television System

Circuit Switched Data & Teleservices

Circuit Switched Data
Circuit switched data is a data communication method that maintains a dedicated communications path between two communication devices regardless of the amount of data that is sent between the devices. This gives to communications equipment the exclusive use of the circuit that connects them, even when the circuit is momentarily idle.

Circuit switched data can be in the form of permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). A PVC is a virtual circuit is manually created for a continuous communication connection. The path for the customer is setup one time by programming routers or switches in the communications network with the connection addresses for the PVC. A SVC is a circuit that is automatically and temporarily created when the virtual connection is requested.

To establish a circuit switched data connection, the address is sent first and a connection (may be a virtual connection) path is established. After this path is setup, data is continually transferred using this path until the path is disconnected by request from the sender or receiver of data. An example of circuit switched data service is integrated services digital network (ISDN).

Teleservices are information services that process or store user data as it is transported through a communications network. An example of a teleservice is a fax forward and storage service. Because IXC competitors all can provide similar services (e.g., more minutes of voice for less money), IXCs may differentiate themselves by providing value added teleservices. IXC teleservices include prepaid services, information access management, and international call back service.

Pre-paid calling cards provide method of payment for telephone calls that negate the need for cash or credit card. Such cards come in preset increments usually beginning at $10 and can be purchased in many convenience stores, drug stores, and discount department stores. Issued by telecommunications service providers, these cards contain coded identification information that permits the cardholder to initiate a call or request information service. Calling cards contain a number or code on a magnetic stripe that uniquely identify the card and authorized services to the system. When the pre-paid amount is spent the card is no longer usable.

Figure 1 lists the typical cost structure of pre-paid calling card services. This table shows that prepaid calling card services offer higher average revenue through cost-added services. This table shows that a low cost prepaid service can achieve higher than average revenue per minute by adding pay telephone and toll free/freephone access charges, call setup charges, and minimum usage charges.

Figure 7.16: Cost of Pre-Paid Calling Card Services

International callback is a call processing service that reverses the connection of calls. International callback service is popular in countries that have high tariffs (fees) for outgoing (originating) international calls and have low tariffs for incoming (received) international calls. This process is divided into the call setup (dial-in) and callback stages. The international caller dials a number that provides access to the international callback service. This number may be local in the visited country or be an international number. The international callback gateway receives the call and prompts the caller to say or enter (e.g., by touch tone) the international number they desire to be connected to and the number they want the callback service to connect to. The international callback center then originate calls to both numbers and connects the two individuals to each other.

Voice Services & Packet Data

Voice Services
IXC voice service is the providing of audio communication circuits that can pass analog frequencies below 3.3 kHz between switches. IXC voice services must automatically compensate for differences in digital signal formats between countries.

IXCs often use tiered (multi-level) voice service rate plans that depend on regions or countries where the call is connected. In the United States, the average per minute revenue for regional calls has dropped over 80% between 1990 through 2000.

Figure 1 compares the cost of regional and international telephone service in various countries around the world. This table shows that that the costs are based on a recurring charge with unlimited usage. The customer may also pay additional recurring fees for advanced services.

Figure 1: Comparison of Regional and International Telephone Service Cost.
Source: Federal Communications Commission (FCC) and LAN Times.

Figure 2 shows several long distance cost plans. This table shows that the average per minute charge in the United States in 2001 was approximately 7 cents per minute.

Figure 2: Cost of Long Distance Voice Services.

Packet Data
Packet data service is the sending of data through a network in small packets (typically under 100 bytes of information at a time). A packet data system divides large quantities of data into small packets for transmission through a switching network that uses the addresses of the packets to dynamically route these packets through a switching network to their ultimate destination. When a data block is divided, the packets are given sequence numbers so that a packet assembler/disassembler (PAD) device can recombine the packets to the original data block after they have been transmitted through the network. Examples of IXC packet data services include X.25, frame relay, and ATM packet data.

X.25 packet data services are being replaced in developed countries by frame relay and ATM services. However, X.25 systems are expanding in developing countries that have poor (low quality) communication lines.

Frame relay (often known as only “frame”) is a variable bandwidth packet data service. It was designed in the 1980’s primarily for data traffic and resulted from improved digital network transmission quality that reduced the need for error protection. It provides for dynamic bandwidth assignment governed by two transport speeds: committed information rate (CIR) and burst information rate (BIR). This is accomplished by varying frame sizes.

Frame relay access is available from IXC’s and requires each site to be connected to the IXC’s frame network by an access line (e.g., T-1 or fractional T-1) circuits.

Figure 3 shows the typical cost of frame relay service is based on the level of access, the CIR, and the BIR. This diagram shows that the end-user must own or lease an access line to the IXC carrier to enter into the frame relay network. The customer pays a port charge for each entry point into the frame relay network. The IXC service provider charges the customer a monthly fee based on a minimum CIR. The end customer may exceed the CIR if the network is not busy (congested).

Figure 7.14: Cost of Frame Relay Services

ATM packet switching service is a connection based high-speed packet data transmission service. To connect to the ATM network, the customer pays for an access line (leased line) and pays a port charge for each ATM entry and exit point. The cost of ATM service can vary based on the guaranteed data transmission rate and level of quality of service (QoS).

Figure 4 shows the typical cost for connecting to an ATM system. This table shows that the cost per megabyte of data dramatically drops from $500 per megabyte for a DS1 connection ($750/1.5 Mbps) to under $50 per megabyte ($7000/148 Mbps) as higher capacity connections are used.

Figure 4: Cost of ATM Services. Source: Pacific Bell, 2001

Asynchronous Transfer Mode (ATM) System

Asynchronous transfer mode (ATM) is a packet data transmission and switching system that transfers information by dividing all types of data into small fixed length packets of data (53 byte cells). The ATM system uses high-speed transmission (usually 155 Mbps or above) and is a connection-based system. When an ATM circuit is established, a patch through multiple switches is setup and remains in place until the connection is completed. ATM service was developed to allow one communication medium (high-speed packet data) to provide for voice, data, and video service.

As of the 1990’s, ATM has become a standard for high-speed digital backbone networks. ATM networks are widely used by large telecommunications service providers to interconnect their network parts (e.g., DSLAMs and Routers). ATM aggregators operate networks that consolidate data traffic from multiple feeders (such as DSL lines and ISP links) to transport different types of media (voice, data, and video).

The ATM switch rapidly transfers and routes packets to the pre-designated destinations. To transfer packets to their destination, each ATM switch maintains a database (called a routing table). The routing table instructs the ATM switch to which channel to transfer the incoming packet to and what priority should be given to the packet. The routing table is updated each time a connection is setup and disconnected. This allows the ATM switch to forward packets to the next ATM switch or destination point without spending much processing time.

The ATM switch also may prioritize or discard packets that it receives based on network availability (congestion). The ATM switch determines the prioritization and discard options by the type of channels and packets within the channels that are being switched by the ATM switch.

Figure 1, shows a functional diagram of an ATM packet switching system. This diagram shows that there are three signal sources going through an ATM network to different destinations. The audio signal source (signal 1) is a 64 kbps voice circuit. The data from the voice circuit is divided into short packets and sent to the ATM switch 1. ATM switch 1 looks in its routing table and determines the packet is destined for ATM switch 4 and ATM switch 4 adapts (slows down the transmission speed) and routes it to it destination voice circuit. The routing from ATM switch 1 to ATM switch 4 is accomplished by assigning the ATM packet a virtual circuit identifier (VCI) that ATM switch can understand (the packet routing address). This VCI code remains for the duration of the communication. The second signal source is a 384 kbps Internet session. ATM switch 1 determines the destination of these packets is ATM switch 4 through ATM switch 3. The third signal source is a 1 Mbps digital video signal from a digital video camera. ATM switch 1 determines this signal is destined for ATM switch 4 for a digital television. In this case, the communication path is through ATM switches 1, 2, and 4.

Figure 1: Asynchronous Transfer Mode (ATM) Systems

Frame Relay System

Frame relay is a packet-switching technology provides dynamic bandwidth assignment. Implementations of frame relay in 2002 allowed for dynamic bandwidth allocation up to 45 Mbps. Frame relay systems offer dynamic data transmission rates through the use of varying frame sizes. The frame relay system is a connection based switching system. Switches are initially programmed to create a logical path (virtual connection) from the entry point to the exit point.

Frame relay systems are a simple bearer (transport only) technology and do not offer advanced error protection or retransmission. This reduces the time for packet switching (reduced transmission delay time). It is up to the sender and receiver of frame relay data to ensure the integrity of the data. When used in systems that have good digital communication systems, frame relay provides reliable data communication service.

The first frame relay standard I.122 was defined in 1988 by the International Telecommunications Union (ITU). The current frame relay specification standards include the ITU I.233 and American National Standards Institute (ANSI) T1.606.

The key components of a frame relay system include frame relay access device (FRAD) frame relay network devices (FRND) and frame relay switches. The FRAD converts end user data into protocol data unit (PDU) variable length packets. The FRAD communicates to the FRND over an access line (e.g., fractional T1/E1 or ISDN line).

The FRND is a packet switch that also operates as a gateway to the frame relay network. The FRND passes frames it receives from the FRAD to other frame relay switches that forward packets toward their destination network. Frame relay switches have buffer memory that allows them to hold and prioritize packets before they are retransmitted. Packet switches can selectively discard packets if network congestion occurs. The FRAD and FRND provide information about the priority of the frames (e.g., non-essential discard eligibility) and status of the system (e.g., network congestion notification).

The frame relay system uses a discard eligibility (DE) flag system to indicate the essential nature of the packet’s data. The DE flag(s) allow systems to selectively discard data packets or frames that are non-essential. This process allows some data transmission systems to send more data than is agreed to (dynamic bandwidth). If the network is not congested, it may allow the extra packets of data to reach their destination.

Congestion notification is a control flag signaling system that is used to indicate status of network congestion in a data network. Congestion notification allows data communication devices that are connected to the data network to send or delay the sending of data dependent on the status of the network.

The frame relay system uses both forward and backward congestion notification. Forward explicit congestion notification (FECN) indicates to upstream switching devices that data that is being transmitted through congested switches and it is likely that some of the remaining packets may be discarded. The upstream switch can then change the data discard priority level accordingly. Backward explicit congestion notification (BECN) indicates to the sending (downstream) switching devices that congestion is occurring and packets that are received may be discarded. The sending switch can then change the priority of packet discarding and send and indication to other switches indicating network congestion. This should eventually reduce the amount of data end-users are sending into the network.

The frame service provider usually agrees to provide the frame relay service at certain data transmission rate (service level). The frame relay system may provide a committed information rate (CIR) and a maximum burst information rate (BIR).

Figure 1 shows a frame relay system. This diagram shows a local area network (LAN) in San Francisco is connected to a LAN in New York. A virtual path is created through the frame relay network so data can rapidly pass through each frame relay switch as its path is previously established. When data is to be transferred through the LAN (e.g., a large image file), the data file passes through a FRAD that is the gateway to the frame relay network. The FRAD divides the data file from the LAN into variable length data frames. The FRAD sends and receives control commands to the frame relay network that allows the FRAD to know when and if additional data frames can be sent.

Figure 1: Frame Relay Systems

X.25 Packet

X.25 packet is an international standard for reliable data communications through the use of a packet-data switching network. The X.25 standard specifies the protocol between the data device (such as a computer) and the network such as a public packet data network (PDN).

The X.25 system is a connection based packet switching system. X.25 packet data switches are initially programmed to create a logical path (virtual connection) from the entry point to the exit point before data transmission begins.

X.25 systems are used to ensure reliable data transmission as it uses advanced error protection and retransmission processes. To provide this reliable transmission of packets of data, each link in the packet data network receives, checks, requests retransmission if necessary, and forwards the data onto the next link.

The key components of a X.25 system are packet assembler and disassemblers (PAD) and packet nodes (packet switching points). The PAD divides or converts blocks of data (such as data files) to and from small packets of information. In the disassembly process, a PAD usually assigns sequential numbers to the packets as they are created to allow the reassembly PAD to identify the correct sequence of data packets to reproduce the original data signal. The ITU specification for a X.25 system PAD is X.3.

A packet node is a packet switch in an X.25 network. The packet node receives and forwards packets of data. The packet switch receives the packet of data, reads its address, searches in its database for its forwarding address, and sends the packet toward its next destination.

X.25 systems are public data network (PDN) or private data systems. The X.25 specification only defines the communication with the X.25 network. Communication within the X.25 network is often implementation specific (company proprietary). To interconnect X.25 systems together, the X.75 specification is used.

Because of the error checking and retransmission process used in the X.25 system, packet transmission time is generally longer than in newer packet switching systems such as frame relay. In a packet network, packet switches are networked together over a wide area (normally a country or continent). Packet switches are connected to each other via dedicated high-speed communication lines. Each switch is configured to have at least two leased circuits to at least two different switches. The local switch is in turn connected to local hosts via dedicated, leased lines and to multiple modems (modem banks) to allow local dial up access. The switches are constantly programmed with remote host addresses and the least cost routes to those devices.

Figure 1 shows a X.25 packet data system. This diagram shows bank teller machine in Rome is connected to a bank processing system in London. The X.25 system is setup so a virtual path is created through the X.25 network so data can reliably pass through each packet node to reach its previously established destination. This diagram shows that a virtual connection is made through a packet node in Paris. Each packet that is sent is validated over each link until it reaches its destination.

Figure 1: X.25 Packet Data System

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