Network Interconnects

Where a router is used to connect two geographically adjacent LANs (see last lecture), it is simply equipped with two appropriate LAN interfaces. Where the LANs to be connected are not near one another, it is normal for a point-to-point link to be used, thus:

IP Addressing in Point-to-Point Links

Not so obvious is the fact that the point-to-point link is also a network (or subnet), albeit one with only two hosts. For example, between the Bundoora and Bendigo campuses of La Trobe University, we have:

Point-to-Point Data Services

In Australia there are only two such providers, Telstra and Optus. Telstra currently has the largest presence in the data communications marketplace.

Telstra has, at various times over the past decades, offered the following (generally point-to-point) data services:

PSTN

or "Public Switched Telephone Network", a flash name for the ordinary old phone service.

DATEL

a "nailed up" or "leased line" variation on the PSTN service. Officially deceased.

DDS

the original "Digital Data Service", now largely obsoleted by ISDN.

AUSTPAC

a "packet switched" data service providing "connect on demand" point-to-point links specifically for data transfer.

ISDN

The Big New Thing.

[1] Although there are some exemptions from this rule in certain circumstances.

PSTN as a Data Service

Modems are sometimes (although rarely) used to build router-to-router links of the kind illustrated in slide 2 of this lecture. More commonly, they are used to connect to dial-in routers, a technology used by retail Internet Service Providers to deliver Internet access to isolated computer systems.

The usual configuration of a dial-in router is such that hosts which connect via modem appear to be directly connected to the ISP's own LAN:

PSTN Charging

• Provision of phone service x 2 (installation approx $200 +
• Phone calls (local @ flat rate, $0.25, long distance @ temporal STD rates). Note that there is no restriction on the duration of a local (flat rate) phone call, and that there is wide variation in the definition of "local". Also, STD calls may be made using Telstra or Optus, although comparison of relative rates is difficult.
• Modems, two required. Currently approx $300 for 28.8 kbps (V.34) and dropping rapidly.

Also, see later for discussion on "cable modem" technology.

Integrated Services Digital Network (ISDN)

Recall from lecture 11, the basic-rate ISDN service provides:

An ISDN service is integrated in the sense that each B channel can be used to carry either a digitised voice (or video, FAX, etc) call, or a 64 kbps data call. The D channel is normally used for signalling (call setup, etc), but may also be used as a 16 kbps data link under some circumstances. Although....

* In Australia, the ISDN interface is owned by the telecommunications provider, eg Telstra. This is not the case in some other countries.

ISDN System Diagram

The S-Bus is a type of dedicated LAN. It uses 4 (or 6) wires, and can extend between 100 and 600 metres from the NT1. A Terminal Adapter (TA) is needed for DTEs without ISDN interface.

Telstra Basic-Rate ISDN Services

Pricing for ISDN services is similar in principle to an "ordinary phone service" (except more expensive), thus:

• Installation charge
• Monthly rental
• All call charges are time charged, with differential rates for data and telephony calls.

Note, however, that nothing is ever simple in these deregulated times, and that Telstra offers pricing in the context of a range of plans, having various combinations of monthly rental and allowances for free calls. Furthermore, it's worth remembering that a basic-rate ISDN service actually provides the equivalent of two phone services.

Current pricing information is available on Telstra's Web pages: http://onramp.telstra.com.au/cost.htm