CONVERGENCE 
of Voice, Data, Image and Video, 
Services and Networks

Definition

The New York Chapter of the IEEE Communications Society held a seminar on the “Convergence of Networks and Services” in November of 1996. Its Planning Committee spent more time trying to agree on a definition for “convergence” among its members, than on planning the seminar itself.

For the purpose of this article, CONVERGENCE refers to the move toward the union of data and telecommunications facilities and services, and the move toward the transmission, switching and routing of voice, data, image and video using one type of facilities.

This article will start with the convergence of voice and data. Images, video, etc. will be added later.

Demand

The demand for convergence has two roots. (1) The user demand for transmission and handling of all the different aspects of communications: voice, data, image, video, etc., is growing. Data are just overtaking voice in volume. The trend is for all forms of communications to be handled in digital form. (2) Service providers expect to save money by using the same facilities for all kinds of communications.

History

In the early 1980s, the International Telegraph and Telephone Consultative Committee (CCITT), a United Nations agency, now called the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), formed a group to study the feasibility to integrate the transmission and switching of data over existing user access lines. The initiative came mainly from the German telecommunications agency. In 1984 these efforts resulted in a series of CCITT Recommendations (CCITT did not issue standards, nor does ITU-T) concerning the Integrated Services Digital Network (ISDN), Series I. Access lines are highlighted because the core telephone network, i.e., the switches and communications between switches, started to become digitized in the 1960s and is practically completely digitized today. The implementation of ISDN has been fairly rapid in Europe, but very slow in North America. One reason is that the two major manufacturers in North America, (now called) Lucent Technologies and Nortel, decided to implement the CCITT recommendations differently. Thus a user had to know to what type of switch their telephone company connected them in order to acquire the right kind of ISDN terminal adapter. Another, and bigger problem is that the original ISDN, now called narrowband ISDN or N-ISDN, is just that and limited to a bandwidth of 56 or 64 kilobits per second (kb/s). When modems were operating at 300 bit/second (b/s) in the early 1980s, 64 kb/s seemed very high.

Realizing that user demand was advancing faster than the Recommendations of the CCITT/ITU-T, the agency added broadband-ISDN (B-ISDN) to Its Recommendations in 1990. The agency also recommended the use of asynchronous transfer mode (ATM) for the implementation of B-ISDN. Unfortunately, the telecommunications world was not ready for ATM in 1990. However, the data communications industry in the U.S.A. accepted it as a medium for local area networks (LANs). The U.S. industry formed the ATM Forum. This caused a new problem. The ATM Forum (mainly consisting of data communications manufacturers and users) made specifications that are not compatible with the international recommendations, nor did they initially address voice over ATM.

The situation improved somewhat when the U.S. ATM Forum started to cooperate with the European Telecommunications Standards Institute, an agency of the European Union. Still, there is no international standard for the details of ATM and B-ISDN.

In the meantime, other “broadband” transmission, switching and routing facilities were introduced. These include X.25, Frame Relay, SMDS, etc. Some of these are applicable to the core network only, while others also are applicable to access lines. The CCITT/ITU-T Recommendation X.25 refers to a packet switching standard that caught on in Europe but hardly in the U.S. It was designed when the quality of transmission links was bad and thus includes error detection on each link. When the quality of transmission links improved, frame relay, which essentially abandoned the error-detection feature, was substituted for X.25. Frame relay is also considered a derivative of ISDN.

Switched megabit data service (SMDS), an access standard, was introduced by Bell Communications Research, Inc. (Bellcore), the research company originally owned by the regional Bell operating companies, and created when AT&T was split up in 1984. SMDS provides access to telecommunications networks at bitrates from 1.2 to 34 Mbit/s. SMDS is based on the user-network interface (UNI) distributed-queue dual-bus (DQDB) described in the IEEE 802.6 standard. SMDS has not caught on in the U.S., mainly due to the pricing policies of the Bell Operating Companies. In Europe, ESTI introduced a similar service that is called connectionless broadband data service (CBDS).

In the late 1960s, the U.S. Department of Defense organized a packet switching network to interconnect its own facilities and those of leading universities and research institutions, called ARPANET. That network has today developed into the Internet. It uses the Transmission Control Protocol/Internet Protocol (TCP/IP).

Basic Transport Media — Telephony

Telephony generates analog signals at the microphone and receives analog signals at the receiver. Originally, the electrical signals between two telephone instruments were analog from end-to-end. The transmission facilities and switches in the telephone network limit the usable bandwidth to between 300 Hz and 3,400 Hz. However, the analog lines between the users telephone set and the first local telephone switch can handle bandwidths up to millions of Hz depending on distance and type of wire.

In 1962 the Bell System in the U.S. started to digitize the analog signals within the core network and sending voice in digital form at a bitrate of 56 or 64 kb/s per channel. Today's telephone core network is practically digitized to 100 percent. However, the “last mile,” i.e., the access line between the local telephone switch and the users is still mainly analog.

For each telephone call a fixed connection is set up between the two (or more) users through circuit switching. This is uneconomical, because the lines are used less than 50 percent of the time (usually only one person speaks at a time).

Basic Transport Media — Data

Rather than through fixed connections between terminals, data is typically transmitted in the form of packets (also called frames, cells, etc.) and the packets are switched (so-called packet switching) to the addressee based on an address incorporated in the head of the packet. The packets from different users are dumped onto a common transmission channel and picked up by the addressee based on the address in the packet head. Packet switching utilizes the available transmission media much more economical than circuit switching.

Basic Transport Media — Convergence

The best economy would be achieved if all traffic, including voice and video, could be sent as packets. This is the aim of convergence.

The problems with sending voice and video over packet switched networks are (1) packets often are delayed, (2) the delay can vary with each packet, and (3) they do not always arrive in the same order that they were sent. One way of improving this situation is to send the packets at a high speed, so-called fast packet switching. As the transmission speed increases the delays become less noticeable.

Users' increasing demand for high speed Internet access means that conventional, analog telephone lines are insufficient. Different service providers are now offering high speed connections. Cable television companies convert their systems from one-way (towards the home) to two-way systems. Some offer two-way wireless connections, including satellite connections. Electric power companies are looking into the distribution of digital signals over their power lines. The conventional telephone companies are installing high speed access lines, both in the form of so-called digital subscriber lines described below, as well as in the form of coaxial cables and fiber.

As an alternative to circuit or packet switched services, we can have dedicated point-to-point connections. This makes economic sense, if the traffic is high. For low traffic volume it does not make sense. Note also that the access lines to your telephone company are dedicated; no-one else can use them (unless you have a PBX or we go back to multi-party lines)! Similarly, your connection to your cable television company is dedicated (at least close to your location and then shared with other users between a drop point and the cable system's head).

From an over-all economic view, it makes sense to have switching nodes as close to the users as possible. So far the trend has been the reverse. Telephone companies operate less and less local switches and so far cable television companies have had no switches at all. However, telcos do install remote switches, switches that are extensions of main switches. Traffic is sent on a broadband connection from a local switch to a remote switch and is then distributed on individual links to each customer. The traffic between the network and the remote switches is transmitted in digital form.

Digital Telephony

As stated earlier, traffic within the core telecommunications network is mainly digital. Most traffic over “the last mile” is analog, but the trend is towards digital links. Today, cable TV networks are mainly analog, waiting for digital high-definition television. Cable networks have large amount of bandwidth available, and by using cable-modems, that bandwidth can be used for digital traffic to and from individual users.

T1, E1 and AMI

In the telephone industry, digital voice channels are often multiplexed into 24 channels to form what is called T1. Outside of North America and Japan, 30 channels are bundled together to form E1. These systems use a protocol called alternate mark inversion (AMI). T1 and E1 systems are often used to send digital signals and in particular data at 1.544 Mb/s and 2,048 Mb/s respectively, in many applications besides voice transmissions.

There are several problems associated with AMI. Transmission systems using AMI require repeaters at 3,000 feet from the central telephone office and then at each 6,000 feet. Secondly, it uses up 1.5 (or 2) MHz of bandwidth, and thirdly, several T1 lines cannot be put on the same cable unless the pairs are shielded. Normally, no more than a single T1 line can be put on a 50-pair cable and two cables with T1 cannot be next to each other. This rules out the possibility of using T1 (or E1) links out to users.

Frame Relay

Today most broadband access and dedicated lines are of the frame relay type, relaying frames. Frame relay is a packet based protocol that was originally developed as part of ISDN, and later for interconnecting local area networks, LAN-to-LAN, at bitrates of up to 44 Mb/s. Computers and other terminals connected to LANs generate bursty and random traffic, which is suitable for transmission as packets.

Voice and video can be transmitted over frame relay. Suitable compression algorithms help improve the economy. One example for voice compression is the code excited linear prediction (CELP) algorithm The ITU-T has issued several recommendations in this respect: G.723.1 ACELP, G.728 LD-CELP and G.729 CS-ACELP. Unfortunately, these are not optimized for use on frame relay links. Some proprietary compression algorithms are offered as improvement.

Special video codecs are available for use over frame relay. It is recommended that such codecs are compatible with ITU-T H.320 (Narrow-band visual telephone system and terminal equipment) and possibly H.323 (Visual telephone systems and equipment for local area networks that provide a non-guaranteed quality of service).

The Internet

The Internet has caught on over the last couple of years and it continues to grow. There are approximately 60 million users of Internet worldwide, of which about 40 million in North America. The growth rate is about 30% annually. The number of Internet hosts was 6.6 million in 1995 and grew to 16 million in 1997 at an AAGR of 57%. Among factors contributing to the growth are the relatively low cost, the open system and the generally license-free availability of the technology.

TCP/IP

The TCP/IP protocol suite is a packet switching protocol and is used around the world for Internet traffic. The suite is in the public domain and there are no license fees. It is also used in private networks. Several manufacturers supply equipment that provides for voice traffic over the Internet using the TCP/IP protocol suite. The quality of this type of voice traffic is lower than that of long distance telephone traffic, but the technique(s) used and the quality is improving. Some telephone administrations are investigating the feasibility of using TCP/IP for their regular telephone traffic.

Cable-TV

Traditionally, cable television distributors used a tree formed high-speed network that distributes television programs from a “head” to the individual users. Up to 500 different television channels are being considered. The problem with using this network for other types of traffic, is that it is one-way, going from the “head” to the users. Some providers offer feed-back channels (i.e., for ordering programs, answering surveys, etc.), but mainly over the existing telephone network.

The next step is to update the cable network for two-way traffic. This could be done in the form of a packet switched network. However, as many users share the return channels near the “head,” a traffic jam may occur. Also, the cable companies receive their feed of television programs from television companies. They have no established connections to the Internet, the telephone network, and other networks. Such connections have to be established. This turns the “head” into a switching node for many types of traffic going in different directions.

High-speed Access to the Telephone Network

As mentioned above, frame relay technique, T1, E1 and others can be used to obtain high-speed access lines between users and public networks. Another set of access lines is called xDSL, and is described next.

XDSL

As stated earlier, twisted copper pairs of the type traditionally used between telephone company local switches and the users, can carry much more bandwidth than the standard 300 to 3,400 Hz. Actually that limitation has to do with the core network, not the access lines. Figure 1 shows possible utilizations of the available bandwidth on such access lines using frequency division multiplexing (FDM). In the figure available bands are marked L, M and H for low, medium and high frequency or bandwidth. Typically, the L-band is used for analog telephony (POTS), the M-band for upstream traffic or two-way traffic, and the H-band for downstream traffic requiring a lot of bandwidth, such as TV programs, etc.

Among attempts to increase the bitrate throughput of the “last mile” are a set of protocols referred to as xDSL. DSL stands for digital subscriber line, and the x to a series of versions listed below.

ADSL appears in two versions, discrete multitone (DMT) and carrierless amplitude phase modulation (CAP).

Actually, DSL is a pair of modems, not a line. The technique (actually a protocol) was originally introduced by Bell Laboratories to handle digital traffic over T1 lines.

The only version that has developed beyond a demonstration and test phase, is the asymmetric digital subscriber line (ADSL). With reference to Figure 1, ADSL VDSL use band L for plain old telephony (POT), M for upstream (relatively low bitrate) and H for downstream (relatively high bitrate) digital traffic. Through the use of echo-cancellation bands M and H can be combined. In the case of HDSL the bandwidth of the M and H bands is the same.

ATM

Asynchronous Transfer Mode (ATM) is defined in, and part of CCITT Recommendation I.121 from 1990. It is practically a worldwide standard whether the United States likes it or not. In the U.S. it was accepted for LAN traffic and the ATM Forum was created as mentioned earlier. The ATM Forum's contributions do not advance the use of ATM for voice communications, one of the original features.

High-Speed Data Networks

Data networks can consist of privately own local area networks, other privately own networks, as well as data networks supplied by service providers.

Local Area Network

A local area network (LAN) is a private network of computer equipment in a local area. There are several different standards covering such networks. They differ, depending on the network architecture, as well as the type of computers and peripherals in the LAN: supercomputers, main frame computers, personal computers (PCs), servers, printers, etc.

LANs have a basic transport media, a ring or a bus. In a bus architecture the computers and other components are connected to a linear bus. A ring has the computers and other components connected to a ring. Traffic goes both ways on the ring. If a link is unavailable, traffic goes the other way and is maintained.

Originally, LANs were designed to operate in a closed area, without any connection(s) to the rest of the world. This meant that one of several standards could be used. Eventually, it was realized that the LANs need to internetwork with other LAN networks and with digital networks in general. The fact that the LANs are operating according to different standards caused problems.

The major LAN standards are initiated and documented by the Institute of Electrical and Electronic Engineers (IEEE). Among them is IEEE 802.3 Carrier-sense multiple access with collision detection (CSMA/CD), also called Ethernet. Token rings were promoted by IBM and became IEEE standards 802.4 and 802.5. These standards differ in coding and transmission speed. The earliest versions were designed for bitrates of 4 to 10 Mbit/s. It should be noted, however, that that bitrate is shared by all the users of the LAN. Thus, let's say that there are twenty users on a particular LAN. This means that each user can use 200 kb/s to 500 kb/s on the average. The distance between nodes is also limited.

With more users connected to a LAN, and each user sending and/or receiving more traffic, the original types of LANs became inadequate. In response, IEEE approved standards for faster LANs, such as IEEE 802.3u, Fast Ethernet, IEEE 802.8 FDDI and others with speeds of 100 Mb/s.

The protocols used by these standards are all different, and they are different to protocols used outside of the LANs. In order to interface between them, protocol converters will be required in most cases.

In February of 1986, IEEE formed a group to study the integrated voice/ data (IVD) LAN solutions. This group became the IEEE 802.9 working group, which mainly dealt with the interface between LANs and ISDN. In the early 1990s, the group was renamed Integrated Services LAN (ISLAN) and the IEEE 802.9 ISLAN standard was approved late in 1993.

The new IEEE 802.9 standard describes not only the interface with ISDN but also with B-ISDN, ATM, FDDI, etc. Specifically, it describes the interfaces between an integrated services terminal equipment (ISTE), and an access unit (AU). IEEE 802.9 equipment supports interface between telephones, personal computers (PCs), fax machines, etc., as well as Ethernet, token ring, FDDI and any other IEEE 802-type LAN.

IEEE 802.9 offers four different types of channels between the ISTEs and AUs, each with its distinct protocol matching the traffic. A B-channel handles 64-kb/s traffic such as voice, switched digital data and Group 4 facsimile, like an ISDN B-channel. A C-channel provides service for circuit switched channels with bitrates in increments of 64 kb/s up to a total of 1.920 Mb/s per channel, corresponding to ISDN's H-channels. The D-channel is a signaling channel, same as the ISDN D-channel. The P-channel is intended for packet switched data with rates corresponding to those of the attached data links.

Efficient Utilization of Available Bandwidth

As stated earlier: (1) The core telephone network is limited to a bandwidth of 300 to 3,400 Hz. (2) The digital core network can handle up to Giga bits. (3) Local Area Networks (LANs) can handle hundreds of Megabits in the forms of Fast Ethernet and FDDI. (4) Cable TV networks can handle Megabits downstream but upstream transmission is limited because solutions for upstream transmission conflict with simultaneous use of available bandwidth.

We also stated that access lines can handle megabits of traffic, which can be divided between POTS, downstream and upstream.

All the systems discussed use different protocols. This means that the procedures for setting up a connection, transmitting information and closing the transmission, as well as the detailed protocols regarding framing, synchronization, coding, etc., are different.

In an attempt to put order in this area and to unify the description of protocols, the International Standards Organization (ISO) put together the open systems interconnection (ISO-OSI). It groups the protocols of the systems into seven levels: (1) physical, (2) data link, (3) network, (4) transport, (5) session, (6) presentation, and (7) applications. Figure 2 shows this presentation, as well as that of TCP/IP and ATM.

Exploring Convergence

In order to arrive at convergent networks, those designed for data traffic will have to be changed to handle voice traffic as well, and those designed for voice will have to accommodate data, image and video traffic, too. As the different types of traffic use different types of protocols, convergence also means introducing protocol converters or changing protocols that can handle all types of traffic.

Future Developments

Frame Relay will remain the major solution for dedicated lines, and TCP/IP will remain the major solution for Internet and maybe voice transmissions for the foreseeable future. xDSL will be around for some time. Because it is an international recommendation, ATM will take over eventually.

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Last revised July 16, 2003
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