CELL-BASED SWITCHING

Cell switching in communications networks can be one of two entirely different things. One use of the term applies to mobile communications backbones where the switching takes place as the mobile telephone passes from one coverage area to an adjacent area or cell. The other use of the term is more commonly known as ATM. One of the more astute and practical observations that can be made about ATM is that it’s easy to get confused upfront by the name. The asynchronous part has to do with the interface between the network, which is truly synchronous and the nature of the disparate traffic, which is asynchronous, even though it maybe from a single, highly accurate digital clock source. Asynchronous also means the traffic, or data, is handled in a start-stop mode, similar to asynchronous serial interface with its start-and-stop bits.

ATM functional elements include a fixed length 53-byte cell, transmission links, and a switching machine. Unlike circuit switching, where the intelligence is resident in an underlying common channel signaling system, ATM intelligence is embedded in each cell and distributed throughout the network in edge and core switches.

Like many other technologies along the way, ATM evolved after such things as time division multiplexing (TDM), plesiochronous network hierarchy, X.25 packet networks, maybe in the same timeframe as SONET/SDH, but before wide acceptance and initial growth of Ethernet and IP networking. Early on, ATM was supposed to be the Holy Grail of all-purpose communications networks. However, that was not to be, as IEEE 802 Ethernet is now testimony to that fact, to say nothing of the past few years rampage to put IP directly on SONET/SDH transport, making IP over ATM obsolete.

ATM was crafted out of a desire to accommodate as much offered traffic as possible from the maximum possible number of users, while at the same time ensuring safe, effective (profitable) traffic movement. A casual look at the network during the time ATM was created would show a pattern much like hotels and airplanes. Equating time slots in digital transmission facilities with rooms in hotels and seats in airplanes, it was easy to see there was significant space available except during peak demand times. It could be seen that if a way to use the idle capacity could be found, it meant incremental revenue. If not, and the facilities in use could be reduced, reducing overall operating cost. Either or both of the two favorably impact the financial bottom line. So it is with Internet and Telecom facilities, not just airplanes and hotels.

From the beginning, computer communications were facilitated with either dial-up modems using PSTN voice grade services and facilities, or private leased lines. Generally the rule of thumb was to use dial-up if it was a local call. If the connection required a long distance connection, the cost was tolerated to the extent necessary to justify a full-time, private, or leased connection at a fixed cost. Even though the private line was a fixed cost and available 24/7, actual traffic passing over the facility was usually far less than full-time at whatever data rate the line was capable of.

In situations where multiple terminals connected to the same central computer, or an enterprise operated several computers in separate locations, data communications networks evolved through various forms of simple aggregate and, later, statistical multiplexing techniques. Even when a terminal is connected to a computer or system hosting an application, traffic between the terminal and host is very asymmetric. Keystroke generation is rarely more than 75 bps, compared to information and screens generated by the host application and sent back to the terminal that can require hundreds of kilobits per second, or even megabits per second. Statistical multiplexing techniques found their way into data communications equipment and networks. As more and more traffic was aggregated and transported by data networks, the same techniques were used to gain efficiencies and utilization in those networks.

Essentially, ATM was conceived and designed to cope with the bandwidth limitations in POTS and ISDN for the data communications user, while at the same time improving use of lower layer transmission facilities providing leased or private line services. The basic characteristics of ATM are built around the virtual circuit concept, including virtual paths and virtual circuits. Because source and destination information is included in each cell, switching and routing of the traffic can be accomplished by network switching equipment examining each cell as it arrives in a network, and then determining where to route it on the outbound side. This basic capability allows configuration of a virtual path through the network between any two or more points connected to the network, and to set up connections between any two or more of those locations. Thus, a virtual circuit exists inside a virtual path.

ATM also provides a capability for customer or user control of the network, enabling the user to configure switched virtual circuits within a path, and paths accommodating two or more circuits or connections. Switched virtual circuit (SVC) means the user pays a fee to gain access to the network, usually a fixed monthly charge based on port capacity and any local loop or access line cost. The user also pays an additional fee each time the network is used, similar to the long distance phone call model. Sometimes called bandwidth on demand, the service is billed according to amount of bandwidth, class of service, and time used. It can be very cost effective within a range of practical, day-to-day content transport needs.

A permanent virtual circuit (PVC) means that the carrier or service provider configures the network to provide service between two or more locations on a permanent basis. Depending on the way the service is ordered and configured, it can be flexible and complex to use, or rigid and easy to use. If the service is configured as a permanent virtual path (PVP), this means the user can configure multiple circuits aggregating up to the maximum amount of bandwidth available on the path. If that’s one circuit on one path, so be it. If that’s x number of circuits with equal or unequal amounts of bandwidth, that works too.

On the other hand, if the carrier configures PVC service, it can’t be changed by the user, but only by the carrier after an order for changed or new service is issued. This has economic and operational implications, which may be significant or insignificant. For example, if the circuit is in use 24/7, as might be the case with a studio-to-transmitter link (STL), it may be necessary to go off air to make changes. If that is not acceptable, then establishing a new set of access and transport facilities and then moving the traffic to the new facility, followed by decommission of the other facility in previous use.

Both PVC and SVC have configuration parameters that should be considered carefully when specifying and commencing use of ATM transport. The classes of service include constant bit rate (CBR), variable bit rate, real time (VBR-rt), variable bit rate, non–real time (VBR-nrt), available bit rate, and unspecified bit rate. Each class of service has different performance and cost characteristics.

CIRCUIT SWITCHING | Network Technology And Methodology

Circuit switching and routing is the basis for all domestic and international telephone or voice grade, dial-up traffic. The circuit switching function is distributed between end-office switching systems and network switching systems. End office switching systems may be a private branch exchange (PBX) physically located on subscriber premises, or a partition in the telephone company’s nearest office, commonly referred to as centrex service. Network switching systems include the local serving central office and any other systems facilitating a path for a telephone call. Nowadays, these systems range in size from a few thousand to hundreds of thousands of ports capable of handling millions of calls per hour.

Circuit switching in functional terms is nothing more than connection of one transmit-receive pair on one side of a switch to a transmit-receive pair on another port on the same path or route, or a different path or route, sometimes called the other side of the switch. Tandem switches are nothing more than transit points that link up network or inter-network transmission facilities. For example, each of the 200+ local access and transport areas (LATA) in the United States has a minimum of one tandem switch, which acts as the transit point between the access and transport networks used by long distance carriers to carry calls from one LATA to another.

Voice grade dial-up service is almost all digital in the United States. However, many analog switches remain in other parts of the world. Where digital switches provide the service, integrated services digital network (ISDN) services—really an access method, not a service—is available. In highly populated areas of many countries, digital subscriber line (DSL) access is available and growing.

Transmission bandwidth available in circuit switched facilities varies from below 64 Kbs (rarely more than 49 Kbs) to 1.536 Mbs. The limitation in analog service is a matter of the ability of a modem to talk to another modem over a local telephone loop. Of course, it doesn’t much matter to voice grade service. After all most, if not all, telephone equipment is bandwidth limited to around 3.5 kHz, which fits easily into 8-kHz sampled PCM.

ISDN and DSL access provide higher capabilities though. ISDN Access is either 144 Kbs, called basic rate interface (BRI), or1.544 Mbs, called primary rate interface (PRI). BRI is channelized into three channels, two bearer or B channels at 64 Kbs, and one delta or data or D channel (16 Kbs) used for signaling and control purposes. PRI access is facilitated with T1 transmission facilities and is channelized into 23 to 64 Kbs B channels and 1 to 64 Kbs D channel. It should be emphasized that the previous explanation is purely in terms of technical capability. Leveraging the bandwidth into variable amounts and getting charged for it on a case-by-case, service-by-service basis is an entirely different matter.

For example, ISDN-based Internet access never achieved large usage because the equipment used by ISPs and their users was limited to BRI rates—64 Kbs at best. And because the ISPs are not the telephone company and have no capability, such as a big digital circuit switch, and have no funds available to buy a big digital circuit switch and therefore no interest in competing with the telephone company, they only offer Internet access service. From the telephone company viewpoint, they simply are prevented from being in the data services—Internet access, or Internet service provider (ISP) business—by current FCC rules and legislation. The telephone company can only sell POTS, ISDN, or private line service. It cannot offer any type of switching other than these services. Some of the independent non–regional Bell operating telephone companies have purchased and operate ATM equipment, but basically they are quite limited in the service they can provide using these or other non-voice service, frame relay, and IP-based switching and routing systems.

DSL access varies according to several factors, the main one of which is the distance between the subscriber premises equipment and nearest central office or wire center. Conceptually and technically, DSL access is intended to be capable of multiple service types such as voice and data. However, implementation reality has driven most service providers to offer only Internet access without any voice service initially. It remains to be seen how long this is likely to continue. The classical telephone companies don’t want to cannibalize their bread and butter—lucrative voice services—and they desperately want to tap into new revenue streams of their up and coming competitors—cable modems and DSL-capable ISPs. Therefore, initial DSL service is limited to Internet access. As the Internet matures—achieves a grade and quality of service capable of supporting voice-over IP—this situation will change. Who knows when, but someday in the future it may be possible to call up the telephone company and ask them to discontinue POTS.

Keep in mind that the main purpose of the switching function is to share use of the transmission function. Also, keep in mind the fact that change in the network is more a direct result of economic pressure than technological or regulatory forces.

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
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.