Lecture 13: Point-to-Point Data Links
Network Technology -- Point-to-Point Data Links
These are the most basic data communications links, eg:
Virtually all computers support such data links, using their so-called
serial ports. Typically, these ports are able
to operate at any standard bit rate between 300 bps and 230400 bps.
The most common hardware interface standard used in such data links is
called RS232: in fact, the term RS232 port is widely
used as a synonym for serial port.
A connection may be "direct" or may use a modem.
Modems
A modem allows a point-to-point data link to be carried over the
analog telephone system (PSTN, also called POTS, or Plain Old
Telephone System), thus:
The modem can:
- Dial the telephone number of another modem under program control, or
answer the phone
- Negotiate a connection at the highest common bit rate that each
modem can support
- perform"on the fly" data compression
- perform (under some conditions) error detection and correction
RS232 Interface
This interface was originally based on a 25 pin
connector[1]. It defines:
The RS232 interface was designed to connect a modem (DCE) to a computer
(DTE). To make a direct (DTE-DTE) link, a null modem
cable (or null modem adapter) must normally be used.
[1] 9 pin connectors are also used, but
there is no standard pin assignment for this -- the PC/AT pinout is the
most common.
Asynchronous vs Synchronous Framing
All data links have to provide byte-level
synchronisation of the sender and receiver.
In an asynchronous system, such as is normally used
with modems, each character (or byte) sent on a data link is prefixed
with an extra start bit and has a stop
bit appended, thus:
In synchronous systems, a whole block
or frame of data is sent as a single unit, with a
synchronising preamble of a few bytes. This has much lower overhead,
and is by far the most common in "leased-line" data links, see later.
PSTN For Internet Access
Modems are nowadays most commonly used for dial-in
Internet access, connecting to a dial-in router, a
technology used by retail Internet Service Providers
(see later) to deliver Internet access to isolated computer systems.
The usual configuration of a dial-in router is that hosts which
connect via modem appear[2] to be
directly connected to the ISP's own LAN.
[2] That is, their network (and subnet)
numbers are the same as those of the computers on the ISP's internal
LAN.
The Point-to-Point Protocol (PPP)
In order to use a modem-based point-to-point link to carry IP packets,
some kind of data link protocol is needed. Originally
this was achieved using a very rudimentary "non-standard" protocol
called SLIP. The current protocol for this purpose
is PPP.
A PPP "frame" looks like:
The "FLAG" bytes delineate frame boundaries, and the data field normally
contains an IP datagram.
When a PPP link is started, the protocol specifies a moderately complex
set of "state transitions". Some of these states are:
- Establishment
- the PPP software exchanges
Link Control Protocol messages, negotiating basic
parameters for the link.
- Authenticate
- PPP includes facilities for
verification of (eg)
username/password pairs (PAP) to ensure that the link initiator is
permitted to establish this data link.
- Network
- various essential network configuration
variables are set during
this phase, eg IP addresses and netmasks.
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 physically near one
another, it is normal for a point-to-point link to be used, thus:
See later for discussion of "leased line" links, commonly used in this
context.
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 and routing.
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, until
recently the Bundoora and Bendigo campuses of La Trobe University were
linked as follows:
Here we see that subnet 131.172.239.0 has been assigned to the
point-to-point link between between the Bendigo and Bundoora
campuses.
Integrated Services Digital Network (ISDN)
ISDN provides an all-digital replacement for the PSTN. ISDN is now
widely available in Australia from Telstra Corporation (although not
from other telco providers), and most Telstra customers now have the
option of choosing it instead of PSTN.
The basic-rate ISDN service provides, conceptually:
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 can be used as a 16 kbps data link
under some circumstances. Although....
Note that in Australia, the ISDN interface is owned by the
telecommunications provider (or Telco), eg Telstra. This is not the
case in some other countries.
ISDN System Diagram
An ISDN service typically has the following physical components:
The S-Bus is a kind 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.
Charging Policies
There is probably no more volatile marketplace at present than that for
telecommunications services -- in many sectors (eg mobile
communications),
competition has resulted in large price reductions. This has, in
general, not been observed in the "fixed service" market, where
established telco monopolies such as Telstra in Australia are able to
maintain comparatively high prices.
In general, PSTN and ISDN services are charged in a similar way:
- Installation (or connection) cost, usually between $200 and $500 per
service in Australia, depending on location, etc.
- Annual (or monthly) "access" charge. Typically between $15 and $70
per month in Australia, depending on selected "plan".
- Call costs, at widely varying rates depending on location, distance,
"plan". etc.
- Note that for ISDN, all calls are time charged, with differential
rates for data and telephony calls. In Australia, a strange
disturbance in the political "force" means that local PSTN calls
continue to be charged at a flat rate. This will
change. I think...
This lecture is also available in
PostScript format.
The tutorial for this lecture is Tutorial
#12.
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Copyright © 2000 by
Philip Scott,
La Trobe University.