The number of B channels can be increased in multiples of 10.
The NT2 (typically a PABX) connects to a special OnRamp 30 NT1, at a point called the T interface. An S interface can be provided on the customer side of the NT2. For a OnRamp2, 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.
In its more recent OnRamp products, Telstra currently offers its OnRamp Xpress service. This is similar to a SPC, but charging is much more complicated!
OnRamp Xpress charging is based on the idea of a monthly capped charge. Example:
A business purchases an OnRamp Xpress service to connect permanently to a local ISP at 64 kbps:
| On Ramp 2 | $50 per month | Xpress 1 Plan Fee | $90 per month | Xpress 1 limited cap | $135 per month | Total Xpress charges | $275 per month |
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.
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.
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:
In this model, a local Internet Service Provider (ISP) would obtain a leased line (or similar) connecting them to an "upstream" (wholsesale) ISP such as connect.com.au. Large companies could deal directly with these wholesale ISPs.
In 1996 and 1997, the major telecommunications providers (Telstra and C&W Optus.) moved to begin providing IP service.
Telstra Big Pond is now an Australia-wide Internet Service Provider competing in the retail market with local ISPs. It can also provide permanent links for large-scale operators.
On interesting observation on the Telstra (and C&W Optus.) Internet (IP) services is that charging is based on volume of data received.
For example, the Australian Vice Chancellors have contracted C&W Optus. to provide Internet service to AARNet. C&W Optus. charges AARNet $120 per gigabyte for Australian traffic, and $180 per gigabyte for International traffic. Telstra is rumoured to charge a flat $190 per gigabyte.
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.
In Australia, C&W Optus. has a nationwide ATM network. Its Spinnaker product "wholesales"" ATM-based IP carrier service to Internet Service Providers, using a volume-based charging scheme. It also offers frame relay-based IP service (at 2Mbps).
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 12) 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: