Telstra OnRamp

A very recent perturbation in the Australian ISDN market is Telstra's introduction of its OnRamp service.

Quoting from the OnRamp sales blurb:

All OnRamp products are based on the European ETSI standards so they are compatible with a wide range of hardware equipment. The adoption of the ETSI standards will also assist Telstra to make ISDN more widely available and generally reduce prices.

OnRamp products will be progressively available in exchanges from a date to be set in 1997. The first batch of OnRamp products will be basic rate - providing two digital line equivalents.Customers will have the choice of choosing OnRamp Duo and customising it to their individual needs by utilising a range of optional features. Alternatively, they can choose one of the "pre-packaged' solutions : OnRamp SOHO or OnRamp Premier (with more to come). A range of OnRamp primary rate products will also be released in mid 1997.


ISDN Macrolink

The ISDN Microlink, and the currently available OnRamp products provide Basic Rate Access to the ISDN (2B+D). 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" (or "nailed up") point-to-point circuit or service (SPC). 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 (currently:) via 4 "aggregated" B channel semi permanent links to Bundoora, in Melbourne, giving a total usable bit rate of 252 kbps.

NB:
This situation should improve dramatically during 1997 with the commissioning of a dedicated high-speed microwave link between the two campuses.

ISDN semi-permanent links are priced very competitively and have now replaced older dedicated service products, such as DDS (Digital Data Service) and DATEL, for virtually all applications.

[1] Using clever "aggregation" techniques.


Frame Relay

Frame Relay is a new data service which has only recently become available from Telstra[2].

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.

[2] 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"[3]. 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.

[3] Fear, Steve in "Australian Communications", Oct 1995, P.63


Cable Modems

One of the biggest changes to have occurred in the Australian communications market in recent times has been the "roll out" of cable TV infrastructure in the large cities by Telstra and Optus, and soon in many country areas by Northgate Communications.

The (so called) cable modem is a device which the cable TV companies can use to exploit their investment in new cabling by offering access to the Internet, or other data services.

Cable modems exploit the "data" channel which is reserved by the cable operator for this purpose. This channel is shared by all subscribers in a designated service area (500 to 2000 homes?), so although each cable modem operates at high bit rate (in the order of 1Mbps, depending on who you believe), actual throughput would normally be much lower at peak times.

The large scale adoption of cable modems will depend on:


SLIP and PPP

The various technologies described in this lecture are all used to provide point-to-point links.

Regardless of the technology, IP packets are always carried over the link using either SLIP (Serial Line IP) or PPP (Point-to-Point Protocol) encapsulation.

SLIP is a very old (non) standard, and defines a very rudimentary method of defining IP packet boundaries in asynchronous links. Its use is discouraged in new installations, and it is almost never used for router-to-router links.

PPP is a variation of HDLC (see lecture 10) which is the preferred modern standard for carriage of IP packets on point-to-point data links of all kinds. PPP is much more complex than SLIP, but provides vastly more services. Some of these include:


ATM - The Future

The telecommunications industry is generally agreed that one day all 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 #12.
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Phil Scott