Lecture 15: Internet Topology and Structure
"Leased Lines" and Router InterconnectionsRevision (yet again!):
- The Internet is composed of many networks and/or subnets interconnected by
- Where the networks and/or subnets to be connected are not geographically
adjacent, it is common for a permanent point-to-point link to
be used. The historical terminology for a full-time telecommunications
interconnect of this kind is a "leased line".
The name derives from the ancient technology of renting a permanently
connected "phone line" plus the necessary modems at each end.
- Historically, a point-to-point inter-router link was normally a separate
subnet -- one with only two IP addresses allocated.
Organisations which wish to establish full-time links to the Internet for
their internal networks need to establish a router-to-router link to another
router which has Internet access. This can be done
"cooperatively", where one Internet-connected organisation allows another to
establish a link to its network, but nowadays is most commonly done by dealing
directly with an Internet Service Provider (ISP).
 The traditional "leased line"
provides a so-called basic carriage service. Such services are
not, in general, very profitable for a telecommunications company.
Example: Early AARNetThe Australian
Academic and Research Network (AARNet), connecting all Australian
universities and several research establishments, was originally established
(circa 1990) with a state-level router in each Australian capital city
connecting via leased-line services to the main AARNet hub router in Melbourne.
Another leased-line link ran from this router to the USA
In this architecture, AARNet retained ownership of all of the routers, and
the involvement of the prime telecommunications provider (at that time, Telstra)
was limited to providing basic carriage service. Thus, Internet access between
universities in Australia originally followed, to some extent, the cooperative
This architecture has now been replaced by a model whereby an C&W Optus operates as an
Internet Service Provider, connecting Regional
Networks (eg, the VRN in Victoria)
in each state to each other and to the Australian and International Internet.
 For more information, Roger Clarke has written
an excellent history of
Modern Internet StructureThe Internet in the USA has always used a
backbone topology, with the National Science Foundation Network
(NSFNET) providing the original backbone network.
In 1995, this structure was privatised, with a variety of commercial
organisations (National Service Providers (NSPs)) taking over
the backbone functionality. Interconnections between these organisations occur
at a number of Network Access Points (NAPs). A NAP can be
independently owned. NSPs are said to peer with other NSPs at
the NAPs, with (apparently) complex financial and/or cooperative arrangments in
place to "carry each other's packets". The worldwide Internet is now structured
similarly. We have:
NSPs (or tier-1
providers) provide ISP service to Regional Internet Service
Providers (or tier-2 providers) -- ie, R-ISPs are customers of
NSPs. R-ISPs can also negotiate private peering agreements, as
indicated in the diagram. Retail (also called access, local, or tier-3) ISPs
generally are clients of a Regional ISP and offer dial-in (and possibly
permanent) services to the public. At each level, we say that a provider has a
Point of Presence (POP) at a physical location if customers can
connect to it at that location.
Note that the distinction between each of these classes of provider can
become rather blurred, especially in Australia!
Permanent Internet Connections for BusinessA typical full-time business
(or home!) connection to the Internet involves:
- Choosing an ISP who offers permanent Internet access at a reasonable
pricing plan (with "livable" download limits...) in the geographical area
where the business is located -- ie, one which has a local POP.
- Choosing the basic-carriage technology (or "leased-line" technology) which
they will use to facilitate the data connection to the ISP. In Australia, this
is separated from the ISP charge because, for traditional communications
services like the PSTN, Telstra has an effective monopoly, since it owns
virtually all of the local loops. One of the conditions of Telstra's
semi-privatisation was that "value-added" competitors would have equal access
to the basic network. This restriction is removed in situations where the
Telstra monopoly doesn't apply. A good example is Cable TV-based access
- Managing several other issues, such as the purchase of a suitable
modem/router (which could be a dedicated box, or an ordinary computer system
running appropriate software), registering and delegating a domain name,
assigning IP addresses, setting up necessary servers and lots more.
Example: Telstra Internet DirectIt's obvious that selling "dumb"
telecommunications services isn't the way to make a profit in the new Internet
world! In about 1996 and 1997, all of the major telcos (or telecommunications
providers - eg Telstra and C&W Optus in Australia) moved to begin providing
IP service -- that is, to become NSPs/ISPs.
Telstra Corporation's permanent Internet service is offered as its Internet
Direct (previously BigPond Direct) service. Pricing is based on a
installation charge, a fixed monthly charge plus a data volume charge. In
addition, customers must (usually) acquire an "Access Method", see below.
On interesting observation on most Australian full-time Internet (IP) services
in Australia is that pricing is based on a mix of monthly charges plus a cost
for volume of data received and/or sent, perhaps with a no-fee
threshold. This can be contrasted with the normal (traditional?) USA practice of
charging on "pipesize" alone. This has some interesting implications, see
"Access" TechnologiesAccess (or, more traditionally "leased line")
service can be provided by a variety of technologies:
- Traditionally: a permanently connected (or "nailed up") 'phone line and
two modems -- in Australia, the ancient DATEL service worked this way. Bit
rates up to 72kbps were available. Note that the term "nailed up" has survived
to this day to describe any permanently available data circuit. Many
businesses use a modern-day equivalent of this technology by taking advantage
of Australia's "untimed local call" charging scheme, using a dial-in modem but
holding the connection permanently open.
- DDS was an early (and fiendishly expensive) digital "leased line" service
in Australia, with bit rates between 2400 and 48kbps. The original AARNet was based on this service. It's no
- DSL (Digital Subscriber Line) technologies are rapidly becoming the most
common access method used by small and medium-sized businesses in Australia.
- ISDN-based services. For example, a Telstra ISDN service can dedicate one
or more B Channels as a permanently available data link, see later.
- Asynchronous Transfer Mode (ATM) and Frame Relay services, satellite,
wireless, etc, etc...
 For lots of tutorial
information, see Telstra's information page at http://www.telstra.com.au/internetdirect/access.htm.
Digital Subscriber Line (DSL) TechnologiesADSL and SDSL are relatively
new access technologies, based on the use of unused communications bandwidth in
the wire "local loop" used to provide basic analog telephone service. DSL offers
possible data rates from several hundred kbps up to the low Mbps range.
One difference between DSL services and other types is that, due to the way
commercial access to telephone exchange DSL equipment is structured, ISPs can
charge a single monthly fee including both "Basic Access" and Internet charges.
The system diagram looks like:
Aspects of this will be
discussed in the lecture.
ISDN Primary Rate AccessISDN Primary Rate Access
product (eg Telstra OnRamp30)
gives 30 x B channels, at 64kbps each and an additional 64 kbps D (or
signalling) channel, conceptually:
The number of B channels
can be increased in multiples of 10.
The NT2 (typically a PABX) connects to a special OnRamp NT1, at a point
called the T interface. An S interface can be provided on the customer side of
Note that in the USA and Japan, the primary rate service is instead 20B+D
over a 1.544 Mbps (called a T1) bearer.
Where all of the available B channels are dedicated as a "leased line"
point-to-point data service, users refer to an E1 (2Mbps) or T1 connection
Frame Relay and ATM AccessFrame Relay and ATM are specialised types of
access (data) service which have only recently become available in
Australia,(some capital cities only) from the major suppliers. They provide
higher data rate access, typically from a few Mbps up to 155Mbps for ATM.
Frame Relay "Committed Information Rates"Access to a frame relay
network is typically available at a "port speeds" of 2Mbps. The port speed is
the rate of the point-to-point physical link between a user's premises and the
frame relay service.
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 an agreed
"Committed Information Rate" (CIR). This is typically much less
than the actual port speed, even down to 0bps. It is possible for a frame relay
user to transmit up to the port speed -- in other words, continuously. However,
the network is engineered so that reliable frame delivery becomes less and less
probable as the average offered data rate rises above the CIR: ultimately, the
network is permitted to drop frames.
ATM ServiceATM service is commonly used as an "integrated"
(data/voice/video) service connecting multiple sites of larger businesses. A
"virtual circuit" (perhaps of specified "sustained information rate" (SIR)) is
configured within the ATM network, terminating at the ISPs POP. Use of
technologies such as this only make sense in terms of an overal "communications
architecture" for an organisation.
Useful linksKim Davies'
Australian ISP List has details of
all (known) ISPs operating in Australia.
Telstra Bigpond Direct
Ameritech's Chicago NAP
Links to Internet History pages
Another bit of NSFNET
The aus.net.access newsgroup sometimes has
interesting discussions about Internet access in Australia.
Copyright © 2004 by Philip
Scott, La Trobe University.