Lecture 15: Internet Topology and Structure


"Leased Lines" and Router Interconnections

Revision (yet again!):

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).

[1] 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 AARNet

The 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[2]

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 model.

This architecture has now been replaced by a model whereby an C&W Optus operates as an Internet Service Provider, connecting Regional Networks (eg, the VRN in Victoria) in each state to each other and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.

[2] For more information, Roger Clarke has written an excellent history of AARNet.


Modern Internet Structure

The 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:

NSP, R-ISP and
Local ISP Hierarchy
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 Business

A typical full-time business (or home!) connection to the Internet involves:

  1. 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.

  2. 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 technologies.

  3. 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 Direct

It'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 lecture discussion...


"Access" Technologies

Access (or, more traditionally "leased line") service can be provided by a variety of technologies[3]:

[3] For lots of tutorial information, see Telstra's information page at http://www.telstra.com.au/internetdirect/access.htm.


Digital Subscriber Line (DSL) Technologies

ADSL 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:

Structure of DSL hardware
Aspects of this will be discussed in the lecture.


ISDN Primary Rate Access

ISDN 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:
ISDN Primary-rate interface
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 the NT2.

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 speed.


Frame Relay and ATM Access

Frame 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 Service

ATM 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 links

Kim Davies' Australian ISP List has details of all (known) ISPs operating in Australia.

Telstra Bigpond Direct

NSFNET history

Ameritech's Chicago NAP

Google Links to Internet History pages

Another bit of NSFNET history

The aus.net.access newsgroup sometimes has interesting discussions about Internet access in Australia.

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Copyright 2004 by Philip Scott, La Trobe University.
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