§1.2.1Access Networks

Where the access network starts and stops

An access network access network Simple The network that joins your device to the first router on the way to everywhere else. Precise The network that physically connects a host to the first router (the edge router) on a path from that host to any other distant host. introduced in ch. 1 — open in glossary is the network that physically connects a host to the first router on a path from that host to any other distant host. That first router is called the edge router edge router Simple The first router between your device and the rest of the Internet. Precise The first router on a path from a host to any other distant host; the point where the access network meets the wider Internet. introduced in ch. 1 — open in glossary .

The definition is narrow on purpose. The access network is not “your ISP (Internet Service Provider) Internet Service Provider Connects hosts, and other ISPs, to the rest of the Internet. introduced in ch. 1 ”. It is the specific stretch of wire, cable, fibre or air between your device and the point where the wider network begins.

The book groups access networks by the setting they are used in: home, enterprise, and wide-area mobile wireless. We take them in that order.

Home access

More than 80 % of households in Europe and the USA had Internet access as of 2020. The two most common kinds of broadband residential access are DSL (Digital Subscriber Line) Digital Subscriber Line Home Internet access over the existing telephone wire. introduced in ch. 1 digital subscriber line Simple Home Internet access that sends data over the ordinary telephone wire. Precise A residential broadband access technology in which a DSL modem exchanges data with a DSLAM in the telco's central office over the existing twisted-pair telephone line. Three frequency bands share the line: 0–4 kHz voice, 4–50 kHz upstream data, 50 kHz–1 MHz downstream data. introduced in ch. 1 — open in glossary and cable.

DSL, and why it shares a wire with your telephone

A home usually gets DSL from the same local telephone company that provides its telephone line. When DSL is used, your telephone company is also your ISP (Internet Service Provider) Internet Service Provider Connects hosts, and other ISPs, to the rest of the Internet. introduced in ch. 1 .

Your DSL (Digital Subscriber Line) Digital Subscriber Line Home Internet access over the existing telephone wire. introduced in ch. 1 modem sends digital data over the existing telephone line to a DSLAM (Digital Subscriber Line Access Multiplexer) Digital Subscriber Line Access Multiplexer The device in the telco central office that serves hundreds or thousands of DSL lines. introduced in ch. 1 in the telephone company’s local CO (Central Office) Central Office The telephone company's local building where DSL lines terminate. introduced in ch. 1 . The modem turns digital data into high-frequency tones; the DSLAM turns the tones from many houses back into digital data. Hundreds or even thousands of households connect to a single DSLAM.

The clever part is that the one telephone line carries data and ordinary telephone calls at the same time, by putting them in different frequency bands.

This makes one DSL link behave as if it were three separate links, so a telephone call and an Internet connection can use it at the same time. The technique is called frequency-division multiplexing, and §1.3.2 explains it properly.

At your end, a splitter separates the data from the telephone signal and sends the data to the DSL modem. At the telephone company’s end, the DSLAM does the same and sends the data on into the Internet.

Real rates are often lower than the standards allow. A provider may deliberately limit a residential rate when it sells tiered service — different rates at different prices. Three physical things also limit the maximum. The first is the distance from the home to the central office. The second is the thickness of the twisted-pair wire. The third is how much electrical interference the line picks up.

Cable, and the neighbours

While DSL reuses the telephone network, cable Internet access reuses the cable television network. You get it from the same company that provides your cable television.

Optical fibre connects the cable head end to neighbourhood-level junctions, and traditional coaxial cable runs from there to individual houses and flats. Because both fibre and coaxial cable are used, the arrangement is called HFC (Hybrid Fiber Coax) Hybrid Fiber Coax The cable access network: fibre to the neighbourhood junction, coaxial cable to the home. introduced in ch. 1 hybrid fiber coax Simple Cable Internet access: fibre to the neighbourhood, then coaxial cable to each home. Precise The cable access network architecture in which optical fiber connects the cable head end to neighborhood-level junctions and coaxial cable reaches individual homes. Each junction typically supports 500 to 5,000 homes, and the medium is a shared broadcast medium. introduced in ch. 1 — open in glossary . Each neighbourhood junction typically serves 500 to 5,000 homes.

Cable access needs a cable modem, which usually connects to the home computer through an Ethernet port. At the head end, the CMTS (Cable Modem Termination System) Cable Modem Termination System The head-end device that turns the analog signals of many cable modems back into digital data. introduced in ch. 1 does the same job as the DSLAM: it turns the analog signals from many homes back into digital data.

Fibre to the home

FTTH (Fiber To The Home) Fiber To The Home Access with an optical fibre path running from the central office to the house. introduced in ch. 1 fiber to the home Simple Home Internet access with an optical fibre running all the way to the house. Precise An access technology providing an optical fiber path from the central office directly to the home, using either an active optical network (AON) or a passive optical network (PON) for distribution. It can potentially provide access rates in the gigabits per second range. introduced in ch. 1 — open in glossary is simpler to describe: run an optical fibre from the central office directly to the house. It can provide Internet access rates in the gigabits per second range.

There are two competing ways to distribute the fibre. An AON (Active Optical Network) Active Optical Network An FTTH distribution architecture; essentially switched Ethernet. introduced in ch. 1 is essentially switched Ethernet, covered in chapter 6. A PON (Passive Optical Network) Passive Optical Network An FTTH distribution architecture using a passive neighbourhood splitter shared by up to about 100 homes. introduced in ch. 1 — used in Verizon’s FiOS service — works as the diagram above shows: each home has an ONT (Optical Network Terminator) Optical Network Terminator The device in the home that terminates the optical fibre. introduced in ch. 1 , connected by its own fibre to a neighbourhood splitter. The splitter combines fewer than about 100 homes onto a single shared fibre, which runs to an OLT (Optical Line Terminator) Optical Line Terminator The device in the central office that converts between optical and electrical signals. introduced in ch. 1 in the central office. The OLT converts between optical and electrical signals and connects to the Internet through a telephone company router.

At home you connect a router — usually a wireless one — to the ONT.

The simplest arrangement of all is called direct fibre: one fibre leaving the central office for each home. It is used less often, because sharing is cheaper.

5G fixed wireless

The newest option removes the cable entirely. 5G fixed wireless sends data by radio from the provider’s base station to a modem in the home, using beam-forming to aim the signal. A WiFi (IEEE 802.11 wireless LAN) IEEE 802.11 wireless LAN Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. introduced in ch. 1 router connects to that modem, possibly bundled into the same box.

The attraction is not only speed. It avoids installing cabling from the central office to the house, which is expensive and prone to failure. Chapter 7 covers 5G cellular networks.

Access in the enterprise, and increasingly at home

On corporate and university campuses, and more and more in homes, a LAN (Local Area Network) Local Area Network A network covering one building, campus or home. introduced in ch. 1 local area network Simple A network covering one building, campus or home, joining nearby devices together. Precise A network used to connect a host to the edge router within a campus, company or home. Ethernet and 802.11 wireless (WiFi) are by far the most prevalent LAN technologies. introduced in ch. 1 — open in glossary connects a host to the edge router. There are many LAN technologies, but Ethernet is by far the most common in corporate, university and home networks.

Ethernet users connect to an Ethernet switch with twisted-pair copper wire. That switch, or a network of interconnected switches, is connected in turn to the larger Internet. Users typically have 100 Mbps to tens of Gbps to the switch; servers may have 1 Gbps to 10 Gbps. Chapter 6 covers Ethernet in detail.

Increasingly, though, people connect wirelessly. In a wireless LAN, users send and receive packets through an access point access point Simple The radio box that wireless devices talk to in order to reach the wired network. Precise The device in a wireless LAN that wireless users send packets to and receive packets from, and which is itself connected into the enterprise's or home's wired network. A wireless LAN user must typically be within a few tens of meters of it. introduced in ch. 1 — open in glossary that is connected into the wired network — most likely by Ethernet. A wireless LAN user must usually be within a few tens of metres of the access point. Wireless LAN access based on IEEE (Institute of Electrical and Electronics Engineers) Institute of Electrical and Electronics Engineers The body whose 802 committee standardises Ethernet and WiFi. introduced in ch. 1 802.11 technology, known as WiFi (IEEE 802.11 wireless LAN) IEEE 802.11 wireless LAN Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. introduced in ch. 1 , is now almost everywhere: universities, offices, cafés, airports, homes, even aeroplanes. 802.11 today provides a shared rate of more than 100 Mbps.

Ethernet and WiFi began in offices and universities, but they are now common at home too. Many homes combine broadband residential access — a cable or DSL modem — with these inexpensive wireless technologies.

Wide-area wireless access: 4G and 5G

Mobile devices send and receive packets through a base station operated by a cellular network provider, using the same wireless infrastructure as cellular telephony. The difference from WiFi is range: a user need only be within a few tens of kilometres of the base station, rather than a few tens of metres.

Telecommunications companies have invested enormously in fourth-generation (4G) wireless, which provides real-world download speeds of up to 60 Mbps. Fifth-generation (5G) networks are already being deployed. Chapter 7 covers the principles of wireless networks and mobility, along with WiFi, 4G and 5G.

Putting them side by side

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