Signaling The Connectionless PSTN

The preceding section described the digitization of the PSTN central office control and switching matrix. This process was more or less completed by 1976, although even today there are plenty of old analog switches around. But what about signaling, the third switch function Before a computer can be said to be able to do everything a PSTN switch does, signaling must be digital as well. As it turns out, digital signaling followed a development parallel to the development of the digital switch....

X25 Packet Switching

The first international standard for packet switching was the ITU-T's Recommendation X.25. This effort grew out of the popularity in the 1960s for computer time-sharing systems. Just as packet networks are designed for bursty applications, time-sharing computers are designed for the same purpose applications that only occasionally need computing resources. Time-sharing computers and their applications did not need dedicated circuits to function but could share a network as long as the messages...

Basic System 64kbs PCM Voice

The three waveform digitization steps of sample, quantize, and code that produce 64-kb s PCM voice have been embodied in the United States in the T-carrier digital hierarchy and outside of North America in the E-carrier digital hierarchy (Japan uses a hierarchy, often called J-carrier, that is slightly different). Both T-carrier and E-carrier establish a multiplexing scheme for combining the basic 64-kb s DS-0 digital voice channels for transmission over a single physical medium. Despite the...

VoIP Router Concerns

When it comes to VoIP, there is more that needs to be addressed in terms of QoS than the six QoS parameters established earlier in this chapter. There are issues that emerge simply because the Internet and all IP networks are essentially collections of connectionless routers that route each packet independently based on local network information only. Thus a router can send a packet to an adjacent router that can only forward the packet onto severely congested links, much to the dismay of users...

Gatekeepers

These control functions may be communicated between the H.323 terminals or gateways directly, or they may be passed through another device whose sole responsibility is the administration of call control services in a VoIP system, called a gatekeeper. An H.323 gatekeeper is not required for H.323 compatibility, but in systems of any significant size, network designers and vendors agree that it makes sense to centralize the administration of these functions in a single unit. Gatekeeper functions...

Following an IP Telephony Call

Before getting into the protocols themselves, it is perhaps useful to follow an example of IP telephony from beginning to end. This example will be used to illustrate some of the various tasks that must be addressed by different protocols along the way. No single example can realistically encompass all the possible functions of an IP telephony system, especially as development and refinement of existing methods continue. However, the example below outlines a call that not only is possible in...

The X25 Network Service

Some of the aspects of X.25 were discussed in Chapters 3 and 4. Now is the time to add some detail in the operation of the X.25 protocol, the better to understand how the IP functions. The X.25 protocol is a specification for connection-oriented access to a public switched packet data network (PSPDN). As illustrated in Figure 5-5, it is comprised of three parts, or layers, each of which maps more or less to one of the bottom three layers of the Open Systems Interconnect (OSI) reference model...

Circuit Switching with Guaranteed Bandwidth and Stable Delays

Circuit-switched networks provide all the bandwidth all the time. Circuit-switched networks guarantee bandwidth from source to destination for the duration of the call. Call is just a word that is used to mean a connection on a voice network. It is a label for the circuit. For this reason, connections are often called virtual circuits in various data networks including the Internet. In the PSTN, however, circuits are not virtual. In the PSTN, circuits are real embodiments of dedicated...

Lines Trunks and Switches

All telephone networks consist of a few basic hardware elements, just like the global Internet. The basic hardware elements of the Internet are hosts (user devices or computers running TCP IP), local-area networks (LANs, usually some form of Ethernet), access routers (needed to connect the LAN to the Internet service provider, or ISP), backbone routers (to shuttle IP packets between access routers and between ISPs), and WAN links connecting all the routers. The basic hardware elements of the...

Electronic Switching and Routing

The original PSTN switches were human beings sitting in front of electromechanical switchboards. The two wires of the local loop were carried by the tip of a metal plug with a ring around it, and the two wires are called tip and ring to this day. Operators were slowly replaced by electromechanical switches after a crippling operator strike during World War I drove home the utter dependence of the PSTN on human beings. The step-by-step switch (also called the Strowger switch after its inventor)...

Loops and Trunks Two Wire and Four Wire

One other point about loops and trunks should be made before moving on to a discussion of the use of the PSTN rather than its physical components. While it is true that there is no physical difference between the media used for local loops (access lines) and trunks (multiplexed interswitch voice circuits), this does not mean that there are no differences in the way that loops and trunks operate. This can be seen in the fact that it has already been pointed out that local loops generally carry...

The TelePC and the Ethernet Phone

Just because advances in miniaturization and electronics now mean that PCs can be used for IP telephony and telephones can be attached to local-area networks (LANs) and have some data capabilities does not mean that these ways of using IP telephony will become common. However, this does not mean that such products will not appear and be useful either. Both the telePC and Ethernet phone will have a place in the VoIP future. The telePC will not be the size of a current desktop PC or even a small...

Internet Fax Standards

As with the rationale for voice over IP (VoIP), the main interest in Fax over the Internet Protocol (IP) is related to cost savings. Therefore, the typical IP fax network topology closely mimics the topology of toll bypass VoIP, with standard fax machines substituting for the standard PSTN phones at both ends of the call, which is itself completed by local or corporate gateways (Figure 10-2). Figure 10-2 Typical fax over IP network Figure 10-2 Typical fax over IP network There are a number of...

VoIP and FoIP

Before examining the IP fax protocol architecture in detail, it makes sense to first address what seems an obvious question Why can't Internet fax calls use the same components and protocols as Internet voice calls Since PSTN fax uses the same infrastructure as PSTN voice, it would stand to reason that fax data simply could ride for free over VoIP networks. The trouble with this approach is that G3 fax data are encoded in a way that assumes the presence of the relatively high-bandwidth PSTN,...

SS7 as a Connectionless Packet Switched Network

The SS7 signaling network of the PSTN is a connectionless packet network. SS7 has to be connectionless one of its main jobs is to set up connections in the form of telephone calls. In fact, the leap of genius that led to the creation of the connectionless Internet came with the realization that all connection-oriented networks that supported SVCs had to be connectionless networks as well. This was true if the network was circuit-switched or packet-switched. It was true if the network was for...

Kbs Packet Voice

What's the big deal about packetized voice at 1 kb s Voice has gone from 64 to 8 kb s without too much of a problem. Why should the move from 8 to 1 kb s be any different What should be added here is 1-kb s packet voice that sounds like it should. Early VoIP and Internet telephony products from the mid-1990s sounded, well, pretty horrible. The voice was distorted and clipped, noisy and barely understandable. Improvements in echo cancellation, noise suppression, compression, and so forth will...

Circuit Switching and Packet Switching

The PSTN developed as a circuit-switched network to better serve the needs of voice communications. Data networks like the Internet developed as packet-switched networks to better serve the needs of data communications. Some of the differences between the needs of voice communications and the needs of data communications have been explored already, but this is the place to be more precise about the differences between voice and data networking needs. These needs were so distinct that until VoIP...

Analog Voice and Digital Data

Analog Voice

Most scientists agree that what makes human beings radically different from all other mammals is that we talk with each other. There has not been enough time for evolution to make human brains that much different from apes, and DNA research has reinforced this belief. Human speech, however, learned rapidly after birth, appears to drive many of the differences in brain organization that allow people to carry on a conversation with their parents but not with their dogs. This came as a surprise to...

Quantizing

In reality, the analog signals that produce 64-kb s analog voice are digitized twice. The PAM samples are first converted to a string of 0s and 1s through the quantizing process, and then they are converted once again into a form suited for long-distance transmission over a digital telecommunications link. It is also possible to quantize and then use a modem to send digitized voice over an analog link, but this possibility is not considered here. Not only is this rarely done today, but 64-kb s...

Digital Computer Networks

The growth of the analog voice network in the United States was rapid by the standards of the day. By 1941, even after the hard times of the global Depression in the 1930s, more than 40 percent of homes in the United States had telephones. Almost everyone who wanted a telephone could get a telephone, although even local service could be expensive in some cases. Perhaps surprisingly, use of the telephone for routine business purposes was slow to catch on. Businesses generally had an office...

Postdivestiture PSTN

Since 1984, the architecture of the PSTN is taken on a structure of LEC, LATA, POP, and IXC. Instead of a hierarchy of switches, SS7 has allowed for a system of IXC backbone switches linked in a flatter structure. There is often still a hierarchy of access switches and backbone switches, but nothing as elaborate as the class switching structure that prevailed before 1984. The architecture of the postdivestiture PSTN is shown in Figure 3-11. All of the blocks are voice switches, distinguished...

Network Requirements

While H.323 and SIP both provide common and as yet incompatible languages by which the end points in a VoIP conversation may signal each other, interoperate with gatekeepers, and code and decode audio signals, they are comparatively vague when it comes to the network facilities over which telephony services should run. For example, H.323 states that Novell's IPX protocol is as valid a network layer as IP for voice, and even the IETF mentions this as a possibility. These network layer protocols...

Session Initialization Protocol

The ITU-T is not the only standards organization to weigh in with a proposal for establishing IP telephony connections and packetizing audio for them. The Internet Engineering Task Force IETF , which more than any other body in the loose world of Internet standards manages or at least legitimizes the development of the Internet Protocol suite, has its own proposal for VoIP systems. This is called the Session Initialization Protocol SIP . Proponents of SIP claim that H.323, arising as it does...