Network Interconnects

We have already seen that the Internet consists of many networks (or subnets) interconnected with routers.

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

Each of the LANs in the previous diagram is a distinct network (or subnet) for purposes of IP addressing.

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:

In fact, it is possible to configure routers so that the interconnecting point-to-point link does not waste an entire subnet. Maybe see later.

Point-to-Point Data Services

In almost every case, the point-to-point links used to connect two geographically separated networks must be supplied by a licensed telecommunications provider[1].

In Australia there are only two such providers, Telstra and Optus. Of these, only Telstra currently has a large 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 special circumstances.

PSTN as a Data Service

In lecture 11, we saw how modems are used to facilitate data transfer over the phone system.

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

The charges for use of the PSTN are (still!) the same regardless of whether it is used for voice or data transfer. The components are (Telstra prices):

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 $500 for 28.8 kbps (V.34) and dropping rapidly.

A perturbation which may (drastically) affect Telstra's PSTN pricing in the future is Optus' recently announced intention to offer local call service using its new "Cable Television" infrastructure.

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

Integrated Services Digital Network (ISDN)

ISDN provides an all-digital replacement for the PSTN. ISDN is now widely available from Telstra, and most phone users now have the option of choosing it. Optus has, apparently, no intention of offering ISDN service.

Recall from lecture 11, the basic ISDN "Microlink" 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.

* 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

An ISDN service typically has the following components:

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.

Basic ISDN Charging

ISDN charging resembles the PSTN mechanism, except almost universally more expensive:

Microlink installation:: $360 (approx, per service)
Microlink annual access:: $900 (approx, per service)

In addition, all ISDN calls are time charged, with rates increasing with distance. Costs for local calls are comparable with the PSTN flat-rate figure of $0.25 for calls less than a few minutes. There is an extra charge imposed on ISDN data calls (as opposed to telephony calls), since the entire 64Ękbps is not necessarily needed when voice signals are compressed.

If a non-ISDN computer is to be connected, a terminal adapter must be leased or purchased. The cheapest of these currently (April 1996) retail for between $AU500 and $AU1000.

Thus, ISDN is still a somewhat expensive option for the private user. The higher cost must, however, be evaluated in terms of the higher data rates available compared to current PSTN modems.

This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #12.

Phil Scott