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Archives
 July 1999 Issue
Focus on Telephony
By Justin J. Junkus
Concentration: A Topic for Serious Thinkers
Obviously, the title to this column is a play on words. When we are talking about solving a problem, for example, concentration means focused thought. In telephony, concentration is more funneling than focusing.
It means that, in any possible call path, callers will be using shared resources to reach the other party. If youre planning to offer telephony service, its wise to give serious consideration to how the resources are allocated to keep service at an acceptable level.
A little background
Concentration is a relatively new phenomenon to the cable industry, and its age in cable roughly correlates to when we stopped being cable TV and became cable telecommunications.
Broadcast services, by their nature of going out to everyone tuned to the appropriate channel, do not need individual communication paths. Two-way services do need this capability, however, because each subscriber must be able to send and receive unique information.
Dedicated paths from subscribers to their call destinations do not have to be permanent, however. In fact, the cost of providing private lines for every possible connection is prohibitive and was the driving force for the invention of manual cordboards and then automated telephony switches. Common sense confirms the economicsafter all, its not very likely that everyone will be on the phone at the same time.
So, subscribers to two-way services contend for various pieces of equipment needed to complete a call. I referred to this phenomenon in my March column on switched telephonys network interfaces when I mentioned the concentrated mode of operation of the TR-008-compliant remote digital terminal (RDT).
Concentration has a long history in telephony, but before we get into that part of it, lets look at how much concentration can be tolerated, and how the affects on service are measured.
How it works
Concentration is based upon a specialized branch of probability theory called traffic engineering. Traffic engineering is aimed at giving the subscriber the illusion of exclusive use of telephony equipment. How many subscribers will, on average, receive this illusion is known as the grade of service.
A typical benchmark for grade of service is that 99 percent of the calls presented to the piece of equipment will experience less than 3 seconds delay before receiving service. This is known as P.01 Grade of Service: "P" for probability of delay, ".01" for the 1 percent that misses the objective.
Other grades of service also are possible, such as P.03 and P.05. The higher the number, the less equipment the service provider needs. The tradeoff, of course, is that 3 percent or 5 percent of the callers, rather than 1 percent, have to wait for equipment availability.
Just how serious and noticeable this delay becomes depends on the number of calls per second offered to the system. There are extensive tables showing grades of service for various call volumes and quantities of equipment.
Network design requirements
Service providers must build their networks based on the highest call volume for the busiest day of the year, and that occurs (if youre lucky) only once a year.
On the other hand, if the high-day, busy-hour load is exceeded for some reason, the probability of any one subscribers delay in receiving service becomes greater. This situation might occur during a natural disaster, such as a tornado hit.
Now lets look at the choice of busy hour and busy day. Call volume is measured in hundred call seconds, abbreviated CCS. As there are 3,600 seconds in an hour, the maximum traffic that can be handled by any piece of equipment in one hour (in use all the time) is 36 hundred call seconds, or 36 CCS.
The traffic engineer needs to pick the most likely busy hour and estimate the number of CCS that will be offered during it to each pool of common equipment. This traffic load is the sum of call setup time and holding (in-use) time for all the calls drawing upon the pool.
Skill, training and experience still come into play because the traffic engineer needs to understand factors that might change the estimate of the networks load. Data calls over a telephony modem are a good example of such factors. They tend to have much longer holding times than, say, an ordinary person-to-person telephone conversation.
As I mentioned earlier, concentration has a long history. I am indebted to Dan Paone, director of product marketing for Antec, for his excellent white paper on this history. Much of the flow of thought that follows comes from reading his document.
How it all began
Concentration was first introduced into one of the earliest telephony switching systems, Step-by-Step. "Steppers" included a piece of equipment called a line finder. This device recognized an off-hook condition and provided the connection to a dial tone generator. It was equipped on a 1:10 basis for the number of subscribers being served by the switch.
In the first 100 years of telephony, this method for sharing common equipment needed to provide telephone service permeated into every type of telecommunications switch.
On the other hand, subscriber loopsthe distribution plant that connects the subscribers to the central office that houses the switchremained as dedicated pairs of wire per subscriber. Concentration didnt become part of the subscriber loop until the introduction of digital loop carrier (DLC) systems using multiplexing technology.
Initially, even DLCs didnt provide concentration. Rather, they shared lines by giving each subscriber line the exclusive use of high-speed lines between remote and central office terminal equipment in the pair gain system for a fraction of the total time. This fraction of time became known as a "timeslot."
In the late 1970s, engineers devised a system that allowed lines not only to share the high-speed lines in the pair gain system, but also to contend for their timeslots by assigning 48 subscriber lines to 24 possible timeslots. This contention provided a 2:1 concentration on the line side of the DLC. Over time, TR-008-compliant DLC systems grew to serve a greater number of subscriber lines by adding more high-speed lines between the DLC and the switch. Line side concentration, however, remained at 2:1.
Todays situation
In the early 1990s, Bellcores TR-303 standardized an improvement to DLCs called the Next Generation Digital Loop Carrier, or NGDLC. This development included a timeslot interchanger (TSI), which allowed any line to use any available timeslot in the set of all high-speed lines going to the switch.
Pooling of timeslots allowed higher overall concentration levels, but more importantly, markedly decreased the probability that any one line would find all paths to the switch busy.
Up to 2,016 lines could compete for 672 possible timeslots to the switch, and each line might receive a path via any of the 672 timeslots. The particular timeslot used by a subscriber line could be different for every call (dynamic concentration). This use of the TSI in the NGDLC is commonly referred to as switch-side concentration.
Circuit-switched cable telephonys host digital terminal (HDT) is a mutation of the NGDLC. As I discussed in the March column, the HDT functions very much like its telephony cousin and provides a standard interface to a digital telephony switch.
When both line-side and switch-side concentrations are combined, circuit-switched telephony systems can provide up to a 5:1 dynamic concentration ratio for subscriber lines. - CT
Justin Junkus is president of KnowledgeLink, a consulting and training firm specializing in the cable telecommunications industry. To discuss this topic further, or to find out more about KnowledgeLink, you may e-mail him at .
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