ISDN Macrolink

The ISDN Microlink (last lecture) is also called Basic Rate Access to the ISDN. For corporate users, the Macrolink or Primary Rate Access service provides 30 B channels and a 64 kbps D channel:

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.


ISDN "Semi Permanent" Service

In either a Microlink or Macrolink installation, one or more[1] B channels may be dedicated as a "leased line" permanent point-to-point service. These services are very commonly used to interconnect routers in the Internet.

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.


ISDN Semi Permanent Charging

Because an ISDN semi-permanent link only exists in the context of an existing ISDN installation (Microlink or Macrolink), those costs are not included here. Typically[2], for a single 64 kbps link:
0-12 km         $2172
101-200 km	$9720
For 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.


Frame Relay

Frame Relay is a new data service which has only recently become available from Telstra[3].

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.


Frame Relay "Committed Information Rates"

Access to the Telstra frame relay network is available at "port speeds" of 64 kbps, up to 1.984 Mbps.

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


Other Data Services

Other Telstra Data Services are continually being introduced. For example:

FastPac
designed to interconnect LANs at 2 Mbps or 10 Mbps, provides a point-to-point data service. Fastpac was introduced just a few years ago, and is now being withdrawn.
DDS Flexnet
This appears to be an enhanced DDS service whereby Telecom provides a completely integrated voice/data network under the management control of the customer. Current status is unknown.
Radio Data Services
The GSM digital mobile radio service is based on an 18 kbps bit stream, and can be used to carry digital data. Currently only short (160 byte) messaging is available, but this will change soon.
Cable modems, etc
with both Telstra and Optus rolling out their "broadband" networks, initially targeted at "Pay TV", it will not long before they both offer high speed, full duplex data services using this infrastructure. It is not yet clear at all how such services will be priced, nor what bit rates will be available.

ATM - The Future

The telecommunications industry is generally agreed that all future operations (voice, TV, data, etc) will one day be based on so-called "Asynchronous Transfer Mode" digital transmission.

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...


This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #13.
Phil Scott