The Conflicting Characteristics of Voice and Data

It is attractive to think that when voice is digitised, it then becomes in some way

"the same" as data. Or it "becomes" data. Up to a point this is true, but there are many differences between traditional data traffic and digitised voice which make the integration of the two a challenging technical problem.

Length of Connection (Call)

© Copyright I BM Corp. 1992

Traditionally, the most important difference between voice and data has been that voice calls are (on average) around three minutes and data calls can last for many hours. Telephone exchanges have been designed to have large numbers of external lines but relatively few

"paths" through the exchange for calls. So it is possible, when the exchange is busy, for calls to "block". That is, for the caller to be attempting to make a connection and the called interface to be unused but the call cannot be made because all paths are "blocked" (in use by other calls).

Therefore, when data is passed through a traditional telephone

exchange, all the paths can be used up very quickly and the rest of the exchange will become unavailable because all paths are "blocked".

Modern digital PBXs have solved this problem by providing the capacity to handle more calls than there are interfaces. For example, an

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exchange with 500 telephones can have a maximum of 250 simultaneous calls ... but an internal capacity for perhaps 400 calls. This is because the internal data path equipment in a digital PBX represents only two or three percent of the total cost of the exchange; whereas, in the past, the function accounted for perhaps 300/0 of the cost. Nevertheless, while this difference is solved for the time being, in the future, the problem will appear again as the sharing of voice and data on the tails becomes more common. Hence, the number of connections to be made increases and the internal limitations imposed by bus speed become a factor.

Flow Control

The bandwidth required for voice is dictated by the digitisation technique and the circuit is either in use (using the full bandwidth) or not. Data can minutes of data at the maximum transmission rate. Added together statistically the traffic does NOT "average out". What happens in large data networks is that interactive traffic tends to "peak" at different times of the day, and on particular events (for example, after a computer system has been "down" and has just recovered).23

Voice does exist in bursts (talk spurts) also and in general only one party speaks at one time, but statistically it poses quite a different problem for a switching system than does data. very important control, that of the link speed itself. Most equipment is designed to handle data at whatever speed the link can deliver it (at least at the link connection level). At the "box" (communication controller, packet switch) level, the switch is never designed for every link to operate simultaneously "flat out", but any individual link

attachment must have that capability. Link speed provided an implicit control of the rate at which data could be sent or received.

23 Another peculiarity here is that the difference between the peaks and the troughs in data traffic becomes greater as the iietwork gets iarger. This is not due to network size per se but rather is an effect that follows from the same cause.

Networks get larger because terminal costs decrease. As the cost of terminals and attachments decreases, users are able to afford many for dedicated functions. An example is in the development of banking networks. In the early networks there was

only one (expensive) terminal per branch and work was "queued" for it. It was in use all of the time with a dedicated operator ( (with others taking over during lunch). Thus there was very little variance in the traffic over time (though the mixture of ~

transaction types changed quite radically). Now, with cheaper terminals, most bank branches have many terminals and they are operated by their direct users not by dedicated operators. Thus, in midmorning for example, after the mail arrives, there is a processing peak with every terminal in use. At other times there can be little or no traffic.

But the new technology allows link speeds which are very much faster than the attaching "box". For example, a link connected to a terminal (personal computer) might run at 64 Kbps but the device, while handling instantaneous transmission or reception of blocks at this speed may not allow for aggregate data rates much faster than (say) 500 characters per second. The same device might also be connected to a local area network at four Mbps with the same restriction that only a few hundred characters per second can be handled by the device. The same characteristic at higher speeds applies to the data switching processor itself. split up or amalgamated for transport). Telephone traffic is continuous or effectively so. It can be considered as very long indeterminate length blocks but the "real time" characteristic does not allow the network to receive a burst of speech as a single block and treat it that way.

Acceptable Transit Delay Characteristics

An acceptable network delay for even the most exacting real time data network is about 200 milliseconds (one way). More usual is a data interactive traffic delay of 500 milliseconds or so. Batch data does not have a problem with transit delays. Voice traffic, however, is marginal on a satellite where the transit delay is 250 milliseconds one way. For first quality voice, the transit delay should be no greater than 50 milliseconds.

Further, variable transit delays (variations in response time), while an annoyance in data traffic, make voice traffic impossible. Voice packets must be delivered to the receiver at a steady, uniform rate. They must not "bunch up" and get delivered in bursts (a characteristic of today's data networks).

A short interruption to the circuit (for example, caused by an aeroplane flying between two microwave repeaters) which could result in a one-second outage of the link will have quite different effects for voice than for data. For data, it is nearly always preferable to have a delay of a few seconds rather than losing the data. With voice, a packet that is one half a second old is just garbage. It is much better to discard delayed voice packets quickly, thus allowing the circuit to return to normal, than it is to build up a queue, particularly due to the fixed speed of the receiving (and of the transmitting) device.

Error Control

The most important thing about data traffic is that errors must be controlled, either detected, or (preferably) detected and corrected. This

24 There is an unfortunate conflict here in the usage of the word "'block". In the telephone world it describes the action of preventing a call being set up due to lack of resources. In the data world a "block" is a logical piece of data which is kept together for transport through the network.

Chapter 4. Traffic Characteristics 63

correction mechanism can often only be done by context²⁵ (since you don't know who the sender is until you are sure there are no errors in the block), and will require retransmissions for recovery. Voice, on the other hand, cannot tolerate the time delays inherent in recoveries and does not care about occasional errors or bursts of errors. (Voice and human language are very redundant indeed.)

Power Demands

Problems caused by fluctuations in the demand for power, should not happen in modern digital systems.

Statistics shows us that when many variable (or varying) things are added up then the mean (average) becomes more and more stable (has less and less variation). For example, in voice calls if one takes the power demands on a trunk amplifier for a large number of calls, then the requirement is very stable indeed and well known. The dynamics of speech, when added up over many calls, produces remarkably stable system demands.

When data is used instead of voice, many things change. Call duration is usually cited (data calls are generally much longer than voice) but there are other problems. When modems are used for data communication over a telephone channel there are no "gaps between words". The modem produces a constant, high level signal. If too many modem calls happen to be multiplexed on a single interexchange (frequency division) trunk, then the additional electrical power, required by the multiplexors and amplifiers can be so great as to cause the device to fail. (Power supplies are designed to supply only enough power for voice calls.) This restriction will go away with the advent of digital systems, but it was the cause of PTT hesitancy about allowing modems to be connected

arbitrarily around the telephone system without consideration of their effects on that system.

Volume of Data

If telephone calls are to be regarded as 64 Kbps full-duplex, then not even the largest organisation today (1991) transmits enough data to be more than ten percent of its telephone traffic. Most organisations transmit less than five percent, and of all the communications traffic carried over public communication lines perhaps one or two per cent is data. This is very important since anything that is done in the system to accommodate data traffic, if it adds cost to the voice part of the system, will be very hard to justify because of the large cost added to the total system for small benefit.

It is perfectly true that data traffic is growing rapidly and voice traffic is not, but there is a very long way to go. Particularly in that the number of interfaces to the public networks being used for voice versus the number of interfaces being used for data is a more important criteria than the number of bits sent. This ratio of the number of interfaces is even more biased in the direction of voice traffic.

25 At the link level, the sender is always known regardless of the content of the block. Later when released from the context of the link, the only identification for the block is the routing information within the block itself.

Balanced Traffic

Most voice calls involve a two way conversation (albeit that some people talk more than others!). This means that for voice transmission, the traffic is usually reasonably well balanced.

Not so for data. Even without the obvious example of file transfer (which is one way), traditional (IBM 3270 style) interactive data traffic involves very short (perhaps 30 to 50 bytes) input and large output (typically 500 bytes but often 2000 bytes or more). In graphics applications the unbalance is even greater than this.

Echo Cancellation

In traditional (analogue) voice systems, the problem of suppressing echoes is extremely important. In a digital full-duplex system, it would seem that echos were no longer a consideration.

This is not completely true. Some echoes can be generated within a full-duplex backbone network with analogue subscriber loops. As noted in section 2.3.3, "The Subscriber Loop" on page 35 these loops can generate large echoes and this will be a problem if the network delay exceeds about 40 milliseconds.