Lecture 15: Internet Topology and Structure

"Leased Lines" and Router Interconnections

• The Internet is composed of many networks and/or subnets interconnected by routers.

• 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"^[1]. 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).

^[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

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This architecture has now been replaced by a model whereby an C&W Optus operates as an Internet Service Provider, connecting Regional Networks (eg, the VRN in Victoria) in each state to each other and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

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:

Note that the distinction between each of these classes of provider can become rather blurred, especially in Australia!

Permanent Internet Connections for Business

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

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

• 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 longer available.

• 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...

^[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

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:

ISDN Primary Rate Access

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 "Committed Information Rates"

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

Useful links

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.