There are several options for obtaining an Internet connection. A long-popular way for home users to connect to the Internet is via dial-up connections on phone lines. For higher speeds, alternatives are a Digital Subscriber Line (DSL), an Integrated Services Digital Network (ISDN) line, or a cable modem. Satellite connections are also possible. Table 4-1 compares the capabilities of the different methods. Not every connection type is available in all locations. Depending on the type of access and the equipment that connects to the provider, Internet communications may use Ethernet, serial port (RS-232), or USB. Ethernet is fast and flexible, and an Ethernet network enables multiple computers to share a connection. Hardware support for RS-232 is very inexpensive. Most microcontrollers have an on-chip UART and require only a TTL-to-RS-232 converter. A computer that connects to the Internet via an RS-232 connection to a modem doesn’t have to support Ethernet at all. Instead, the computer can use the Point-to-Point Protocol (PPP) to send and receive IP datagrams over the RS-232 connection. Generally, a USB connection isn’t practical for small embedded systems. USB modems must connect to a PC or other USB host, while most USB-capable embedded systems are USB devices. Also, USB modems typically come with driver software for Windows only.

Dial Up

A dial-up connection is available anywhere there is phone service. A modem provides an interface between a computer that wants to access the Internet and an ordinary phone line (Network article 4-1). To make a connection, the computer instructs the modem to dial a number that connects to a modem at the ISP. The ISP’s modem in turn connects to a router with an Internet connection. A PC’s modem may be on the motherboard or an expansion card, or the modem may connect to the PC via an RS-232 or USB port. An embedded system may also contain a modem or connect to an external modem, usually via RS-232. The computer uses the Point-to-Point Protocol (PPP) to manage the modem connection and to send and receive IP datagrams over the serial link. Rabbit Semiconductor’s Dynamic C has an optional module with libraries and example code for PPP communications. For TINI users, the com.dalsemi.tininet.ppp package supports PPP. RFC 1661: The Point-to-Point Protocol (PPP) defines the protocol. Limitations of dial-up connections are a maximum speed of 56 kilobits per second and the need to provide a phone line for the connection. Advantages are low cost and availability anywhere there is phone service. In general, a dial-up connection isn’t the best option for a server because of limited speed. But dial up can be useful for some computers that occasionally communicate on the Internet. For example, a series of data loggers might periodically dial in to send readings to a central computer that is on the Internet and programmed to accept the communications from the data loggers. A system with a dial-up connection may also communicate by sending and receiving e-mail. Multiple systems can share a dial-up account if each calls in turn. A computer that connects to an ISP via dial-up may also use Ethernet to connect to a local network.

DSL

DSL uses a conventional phone line with equipment at each end to enable the line to carry voice and Internet communications at the same time. Although the exact setup can vary with the provider, Network article 4-2 shows a typical configuration, where the customer’s site has a DSL modem and a splitter. In the upstream direction, a splitter combines phone and Internet traffic on a single pair of wires. In the downstream direction, the splitter routes the phone and Internet traffic onto the appropriate wires inside the customer’s premises. Another name for the splitter is network interface device (NID). The line carrying Internet traffic in the customer’s premises connects to a DSL modem, which has a USB or Ethernet connection to the customer’s computer. At the phone company’s central office, phone traffic is routed to and from the company’s switching equipment, and Internet traffic is routed to and from a DSL Access Module (DSLAM). The DSLAM interfaces to the company’s DSL equipment, which connects to the Internet. DSL connections often use Point-to-Point Protocol over Ethernet (PPPoE). PPPoE requires logging on with a user name and password but doesn’t require dialing a phone number to connect to the ISP. Dynamic C’s PPP module supports PPPoE and includes an example application. DSL has several variants with differing speed and distance limits. Not all providers offer all variants. Two popular options are asymmetric DSL (ADSL) and single-line, or symmetric DSL (SDSL). With ADSL, traffic in each direction has a different speed, with the downstream speed typically much faster than the upstream speed. Embedded systems that host busy Web or FTP servers will probably find SDSL, with equal speeds in both directions, more suitable. The speed of a connection varies with the DSL variant, the distance from the phone company’s central office, and the quality of the phone line. Theoretically, ADSL can support speeds as high as 6.1 Mb/s downstream and 1.5 Mb/s upstream. In practice, speeds are likely to be equal to or less than 1.5 Mb/s downstream and 384 kb/s upstream. The theoretical maximum for SDSL is 2 Mb/s in each direction. The maximum distance between the customer and the central office is around 18,000 feet for ADSL and 18,000 to 22,000 feet for SDSL.

ISDN

Like DSL, ISDN connections can use conventional phone lines. ISDN has two main variants. With Basic Rate Interface (BRI) ISDN, the phone line carries two 64-kb/s “B” channels that can be combined for a single 128-kb/s connection. A separate lower-speed “D” channel carries signaling information. As Network article 4-3 shows, the computer that wants to communicate over the Internet connects via Ethernet, RS-232, or USB to an ISDN terminal adapter, which in turn connects to a network termination. The customer’s phone line connects the network termination to a switch at the phone company’s central office, which routes the traffic to and from the ISP. It’s also possible to use one ISDN channel for voice traffic and the other for a 64-kilobit Internet connection. If BRI ISDN isn’t enough, Primary Rate Interface (PRI) ISDN has 23 channels and speeds of up to 1.544 Mb/s. A BRI connection requires a T1 line, which is a special 4-wire phone line that carries digital data from the central office to the customer.

Cable Modem

A cable modem doesn’t use phone lines, but instead uses a connection to a cable-TV provider that offers Internet access. The same cable can carry TV broadcasts and Internet traffic. As Network article 4-4 shows, the computer that wants to communicate over the Internet connects via Ethernet or USB to a cable modem. The cable modem in turn connects to a filter and splitter, then connects via coaxial cable to a neighborhood concentrator, which has a high-speed connection to the cable company’s facility. The cable’s bandwidth is divided into channels. Each TV channel uses a 6-Mhz portion of the bandwidth. Internet traffic typically uses bandwidth above the TV channels for downstream traffic and bandwidth below the TV channels for upstream traffic. With a cable modem, you share bandwidth with other customers in the neighborhood. So the performance of a cable-modem connection depends in part on the network speed provided by the account and in part on how much other traffic there is at the same time. Most cable-modem connections are asymmetrical, with higher downstream speeds. Typical network speeds for cable modems are from 256 kb/s to 1.5 Mb/s downstream and up to 384 kb/s upstream. Most providers encrypt the Internet traffic so customers who share a connection can’t view each others’ data.

Because cable-TV providers market to residential customers, cable Internet may be unavailable at a business location. Because of the expense of running cable, cable Internet may be unavailable in remote locations.

Satellite

Another option for obtaining Internet access, especially for remote areas, is a satellite link (Network article 4-5). Early offerings of Internet access via satellite were downstream only, requiring a phone-line connection for upstream data. Newer systems offer 2-way communications via satellite. Download speeds range between 150 to 500 kb/s, with upstream speeds of around 50 kb/s. The low-speed upstream communications make satellite links less than ideal for hosting a server. The satellite dish requires a view of the southern sky. The satellite modem may connect via Ethernet or USB to a customer’s computer.