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.
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 (currently:) via 4 "aggregated" B channel semi permanent links to Bundoora, in Melbourne, giving a total usable bit rate of 252 kbps.
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.
- NB:
- This situation should improve dramatically during 1997 with the commissioning of a dedicated high-speed microwave link between the two campuses.
[1] Using clever "aggregation" techniques.
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.
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
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:
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:
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...