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 10, 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 $300 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 64Êkbps 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 #11.
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Phil Scott