The number of B channels can be increased in multiples of 10.
The NT2 (typically a PABX) connects to a special Macrolink NT1, at a point called the T interface. An S interface can be provided on the customer side of the NT2. For a Microlink, the T and S interfaces are identical.
Note that in the US and Japan, the primary rate service is instead 20B+D over a 1.544 Mbps (T1) bearer.
Such links are called "semi permanent" because they are not a "fixed" physical link within the ISDN network but a permanent call set up by the ISDN terminal equipment.
The link between Bendigo and the Internet is (apparently:) via 4 "aggregated" B channel semi permanent links to Bundoora, in Melbourne. Until very recently (like, yesterday), the link from Bundoora to the rest of the Internet was also carried on 4 B channels.
ISDN semi-permanent links are priced very competitively and have substantially replaced other dedicated service products, such as DDS and DATEL, for many applications.
[1] Using clever "aggregation" techniques.
0-12 km $2172 101-200 km $9720For 4 B channels, the prices become:
0-12 km $6900 101-200 km $29388 ... etc.This may be compared with, for example, an older DDS (see last lecture) link at 2400 bps between Bendigo and Melbourne (1991 prices):
Installation: $2180 Annual access: $3528 Transmission: $1728 Total: $7436
[2] 1995 prices.
It is based on a model whereby either LAN frames or, more commonly, higher-layer packets (such as IP datagrams) are transported through the frame relay network in a "point-to-point" manner using "permanent virtual circuits" (PVCs) to define the two endpoints, eg:
Note that frame relay standards also allow for "switched virtual circuits", but these are not (yet?) available in Australia.
[3] Although it has been available from some time from smaller independent operators.
Internally, the frame relay network is engineered on the basis that not all nodes will continuously attempt to transmit at their full port speed all of the time. In fact, each port is only guaranteed reliable service at its agreed "Committed Information Rate" (CIR). This is typically less than half of the actual port speed in bps.
The charge for a PVC is based on the agreed CIR, not on the actual port speed. To minimise the cost, it is even possible to have a CIR of 0 bps. However, it is still permissible to transmit up to the port speed - reliable delivery just becomes less likely above the CIR, when the network is permitted (even though this is unlikely) to drop frames.
Pricing for Telstra's Frame Relay service is very complex, having improved from "totally indecipherable" to "still incomprehensible"[4]. However indications are that for many links, especially where a low CIR is specified, overall costs will be significantly less than for ISDN semi permanent links.
[4] Fear, Steve in "Australian Communications", Oct 1995, P.63
Most existing digital systems (eg, ISDN) are synchronous, in that their bit rates are structured to fit into a strict hierarchy. This leads to inefficiency when "slots" in the hierarchy are unused when, for example, a station has no data to send.
ATM encapsulates all information into cells, which are a total of 53 bytes in size: 5 bytes for header and 48 for "payload". Because these cells have a fixed, simple format they can be switched very quickly. Typical ATM systems operate at 155 Mbps.
The current market for ATM is principally in the area of "LAN emulation" systems. It is not clear when the telecommunications carriers will begin to introduce it to ordinary consumer networks. In addition, there remain some difficult unsolved problems in the ATM system architecture.
Watch this space...