TECHNICAL
“The average number of simultaneous calls in progress during a period is known as the traffic flow, or intensity, and is measured directly in units known as Erlangs .” Others have published extensively on traffic engineering from the switched circuit perspective. One such paper introduces probability and queueing theory in some depth, particularly with Chapter 15 “Traffic measurements” which has offered some weight to my current thought process. Note that the ITU reference is dated 2005 and I cannot find an updated document within ITU archives. For planning purposes, the BT document referred to above uses the busiest hour of the day, the busy hour, as the period T, and unless stated otherwise ‘Erlangs of traffic’ refer to a period of one hour. During my telephony days, the weekly daily hours were 09:00-10:00 and 15:00-1600. Although the BT and ITU approaches and any measurement process are rarely used today (if at all), there are models which can be referenced i.e., they have rigour in definition and use. More importantly, the definition and to a lesser extent the measurement approach has been agreed within international standards bodies. Note that the BT reference document mentioned above was published in the early 60s. As a further observation, traffic planning within a circuit switched network was invariably within a single management domain – being able to “observe” the ingress and egress of a flow assists considerably.
Continuing in that it is useful to go further (for illustrative purposes) to reduce Figure 1. to a black box as illustrated in Figure 2.
The move from circuit switching to packet switching (and thence the Internet) Some background, in that my early days with BT were heavily focused on telephony and circuit switching. During this period, I was aware of the developments within BT on packet switching (EPSS), but I was not aware of the work being conducted on packet switching in the USA, France and also the UK. From the US perspective, luminaries such as Baran, Roberts, Cerf, Khan, Kleinrock, and Day are worthy of mention and from the UK and France, Davies and Pouzin are also identified as pioneers. Skipping over the many developments in networking (both circuit and packet switched), the development of the ARPAnet and the eventual adoption of the Internet Protocol Suite (IPS) has led to today’s Internet and its relative complexity. To expand upon my comment re complexity, I must offer my sincerest thanks to Angelos Antonopoulos, et al, for permission to use an example from their recent paper. As the title suggests, the paper examines the relationships between Internet stakeholders and net neutrality. Figure 3. highlights the multi-tier structure of an Internet model and indicates that the number of stakeholders involved within an Internet delivery chain can be extensive. As illustrated, the notion of a single provider managing the delivery of an Internet service is a non-sequitur and should be continually challenged (by all).
Figure 2: A Black Box approach
The Black Box model is just that and is used to illustrate/simplify my thought process. Isenberg captured this concept very well i.e., “Just Deliver the Bits, Stupid” and would now suggest that any engineering company which is in the bit delivery business as being a “Bitco.”
Statement
To set the scene, I have extracted some words from an early IET reference:
“Time-honoured representations of network traffic by pure- chance streams, which have worked well for many years, are now insufficient. The new data networks display a complexity of behaviour which requires a corresponding richness of input, and this has required the development of a multitude of traffic descriptions.”
Circuit Switching
In my early days with BT (1971 onwards) and, at the risk of “teaching granny how to suck eggs”, the public telephone network in the UK was managed by the predecessor of BT (with the honourable exception of Kingston upon Hull). One advantage of this single entity approach (read management domain) was that telephone traffic and demand planning was well understood and adhered to sound engineering principles. In retrospect, the demand planning/ forecasting unit was an invaluable group within BT.
Traffic within a circuit switched network – a definition.
The unit of “Traffic Flow” within a circuit switched network is introduced at, and quoting directly from this document, is
Figure 3: An example of Internet complexity
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DECEMBER 2025 Volume 47 No.4
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