This page describes some of the technological issues in a way that is intended to be accessible to non-techies who may be interested to learn more. It is not intended as a justification for any design decisions, nor to give detail of how Flexilink works.
Traditionally, networks have been divided into circuit-switched and packet-switched technologies.
An example of circuit switching is the telephone system. There are three distinct phases to communication: setting up the call, having a conversation with the person at the other end, and disconnecting.
In the first phase, the user tells the system the destination to which they wish to be connected, and the system finds a route to the destination and sets it up so that the sounds from each user's microphone are delivered to the other user's earpiece. Originally that was done by humans pushing plugs into sockets; in digital systems it is done by computers writing configuration data into special-purpose hardware.
Note that finding and setting up the route is a very different process from conveying the sounds along it. In the original systems, one was done by humans and the other by the electric circuit. In digital systems, one is done by a computer and the other by some electronics called the "routing fabric". In both cases, finding the route requires some thought, whereas conveying the sounds is a very simple process.
When the user disconnects, the system releases the resources that were occupied by the call.
An example of a packet-switched network is the postal service. A postcard, for example, carries both the sender's message and the destination address. At a sorting office, the address is read and a decision made as to where the card needs to go next, and the card is put in the appropriate mailbag.
Note that there is no real separation of the tasks of deciding the route and conveying the message. Also, there is no disconnection phase because there is nothing to disconnect. To put it another way, packet switched systems are "stateless" whereas circuit switched systems are "stateful" because the process of setting up a call changes the state of the system.
An application designed for one kind of service can still use the other. A single packet can be sent on a circuit-switched system by connecting a call, sending the data, and disconnecting (think of sending a fax); in practice this scenario would be unusual, because even fetching a simple web page requires the exchange of several packets. Conversely, a stream of data can be sent on a packet switched system by putting it in a succession of packets and putting the same address on each one; think of liquid fuels, which can be carried by pipeline or by loading them into a fleet of lorries.
A network consists of "nodes" (the circles in the drawing above) joined by "links" (the lines). In the telephone system, the nodes are the exchanges and the links are the cables. The limitations to how much data can be carried are the capacity of the links and rate at which the nodes can pass on the data.
In earlier circuit-switched systems, such as analogue telephones and ISDN, a link would carry a certain number of "channels", and a call would use up one channel on each link over which it passed, for the duration of the call. This was OK for voice calls, but when systems began to be used for data which did not form a continuous stream (such as a dial-up Internet connection) occupying a channel during the periods when there was nothing to send would be wasteful.
Where most of the traffic is small bursts of data with long pauses in between, as when retrieving web pages to read on a browser, packet-switched systems clearly make more efficient use of the links provided there is enough traffic to keep the link busy, and assuming the channel is kept open in between page accesses.
However, if the link is kept busy the packets (which arrive at random intervals) will often have to be queued up in the nodes waiting their turn to get on the link, and sometimes the queue will overflow the node's memory and some packets will be lost. Also, the "headers" which contain the addresses and other administrative information consume a significant proportion of the capacity of the link, especially if the packets are small.
The traffic carried by networks can be categorised as follows:
Voice: sound, often limited to the range 300Hz to 3kHz, as part of a two-way conversation
Audio: sound, covering the full frequency range, transmitted in one direction
Video: moving images, transmitted in one direction
Data: static objects (text, still images, etc) being copied from one location to another
(The narrow frequency band for voice is the result of the limitations of the early systems; it is adequate for speech except that it makes "S" and "F" sound the same.)
Originally, the analogue telephone system was developed to carry voice. It was gradually replaced by ISDN, which offered a very similar service in a digital format.
Meanwhile, data traffic began to grow, originally encoded as tones but, with ISDN, also able to be carried directly in the voice channels. These channels carried 56 or 64kbit/s, so if the application needed more it had to use several channels, and if it needed less some of the capacity was wasted.
The next technology was Asynchronous Transfer Mode, or ATM. That was also circuit switched but allowed channels to be of whatever capacity the application needed, and also allowed any capacity a call didn't use to be used for other traffic instead of being wasted. The basic technology was good and worked well, but the equipment was overly-complex and difficult to manage, and the system was ineptly marketed.
Internet Protocol (IP) is a packet-switched technology which was developed around 1980. In the 1990s it was the obvious choice for corporate networks, where most of the traffic was data. More recently it has also become dominant in public telecommunications networks, including 4G (or LTE) mobile networks. The mobile industry's use of IP in 4G has revealed a number of fundamental problems, and they have begun to look for an alternative (more detail here).
IP is used with two other protocols, TCP and UDP; the combinations are often referred to as TCP/IP and UDP/IP, though "TCP/IP" is also used to refer to the whole set of protocols, even when TCP is not used.
TCP is a "connection-oriented" protocol, which means that it appears to the application as circuit-switched, and indeed as a byte stream rather than packets. The "circuit" is two-way, and each end sends an acknowledgement of the other end's packets; if a packet is not acknowledged, it is assumed to have been lost, and is sent again.
UDP is "connectionless", so keeps the packet-switched model, and actually adds very little to the service provided by IP.
Continuous media such as audio and video are carried using an additional protocol layer called Real-Time Protocol, or RTP. This is normally used with UDP; it can also be used with TCP, but the mechanisms TCP uses to ensure that all the packets arrive correctly can add significantly to the delay. When used over Ethernet, the various headers and framing symbols for the Ethernet, IP, UDP, and RTP layers add up to nearly 100 bytes per packet.
SIP is the Session Initiation Protocol, which is used to establish, manage, and terminate multimedia communications. The media themselves are transferred using RTP.
Flexilink is a circuit-switched system, but without the disadvantages of earlier circuit-switched systems. Two basic services are offered: "foreground" or "synchronous" for continuous media such as voice, audio, and video, and "background" or "asynchroous" for data. More details are available here.
Note that in each case the corresponding service on the Internet is connection- (or session-) oriented, using SIP in one case and TCP in the other. The Flexilink call connection procedures will interwork with SIP and with TCP to allow establishment of calls which use Flexilink for some parts of the route and IP for others. The QoS guarantees will, of course, only apply over the Flexilink part of the route, but if the IP part is restricted to a small private network with sufficient overcapacity the performance should still be adequate.
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