Elements of a Dial-Up Remote Access Connection

A dial-up remote access connection consists of a remote access client, a remote access server and a wide area network (WAN) infrastructure as illustrated in Figure 7.1.

Figure 7.1 Elements of a Dial-Up Remote Access Connection

Remote Access Client

Windows 2000, Microsoft® Windows NT® 3.5 or later, Microsoft® Windows® 95, Microsoft® Windows® 98, Microsoft® Windows® for Workgroups, Microsoft® MS-DOS®, and Microsoft® LAN Manager remote access clients can all connect to a Windows 2000 remote access server. Almost any third-party Point-to-Point Protocol (PPP) remote access clients including UNIX and Apple Macintosh can also connect to a Windows 2000 remote access server.

Remote Access Server

The Windows 2000 remote access server accepts dial-up connections and forwards packets between remote access clients and the network to which the remote access server is attached.

Note

The term remote access server as it is used in this chapter refers to a Windows 2000 Server computer running the Routing and Remote Access service and configured to provide remote access.

Dial-Up Equipment and WAN Infrastructure

The physical or logical connection between the remote access server and the remote access client is facilitated by dial-up equipment installed at the remote access client, the remote access server, and the telecommunications infrastructure. The nature of the dial-up equipment and telecommunications infrastructure varies depending on the type of connection being made.

PSTN

The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the analog phone system designed to carry the minimal frequencies to distinguish human voices. Because the PSTN was not designed for data transmissions, there are limits to the maximum bit rate of a PSTN connection. Dial-up equipment consists of an analog modem for the remote access client and the remote access server. For large organizations, the remote access server is attached to a modem bank containing up to hundreds of modems. With analog modems at both the remote access server and the remote access client, the maximum bit rate supported by PSTN connections is 33,600 bits per second, or 33.6 kilobits per second (Kbps).

Figure 7.2 illustrates a PSTN connection.

Figure 7.2 Dial-Up Equipment and WAN Infrastructure for PSTN Connections

Digital Links and V.90

The maximum bit rate of the PSTN is a function of the range of frequencies being passed by PSTN switches and the signal-to-noise ratio of the connection. The modern-day analog phone system is only analog on the local loop, the set of wires that connects the customer to the central office (CO) PSTN switch. Once the analog signal reaches the PSTN switch, it is converted to a digital signal. The analog-to-digital conversion introduces noise on the connection known as quantization noise.

When a remote access server is connected to a CO using a digital switch based on T-Carrier or ISDN rather than an analog PSTN switch, there is no analog-to-digital conversion when the remote access server sends information to the remote access client. There is no quantization noise in the downstream path to the remote access client, and therefore, there is a higher signal-to-noise ratio and a higher maximum bit rate.

With this new technology, called V.90, remote access clients can send data at 33.6 Kbps and receive data at 56 Kbps. In North America, the maximum receive bit rate is 53 Kbps due to Federal Communications Commission (FCC) power rules.

To obtain V.90 speeds, the following must be true:

• The remote access client must call using a V.90 modem.

• The remote access server must be using a V.90 digital switch and be connected to the PSTN using a digital link, such as T-Carrier or ISDN.

• There cannot be any analog-to-digital conversions in the path from the remote access server to the remote access client.

Figure 7.3 illustrates a V.90-based PSTN connection.

Figure 7.3 Dial-Up Equipment and WAN Infrastructure for V.90 Connections

ISDN

The Integrated Services Digital Network (ISDN) is a set of international specifications for a digital replacement of the PSTN providing a single digital network to handle voice, data, fax, and other services over existing local loop wiring. ISDN behaves like an analog phone line except that it is a digital technology at higher data rates with a much lower connection time. ISDN offers multiple channels; each channel operates at 64 Kbps and because the network is digital end-to-end, there are no analog to digital conversions.

Dial-up equipment consists of an ISDN adapter for the remote access client and the remote access server. Remote access clients typically use Basic Rate ISDN (BRI) with two 64-Kbps channels, and large organizations typically use Primary Rate ISDN (PRI) with 23 64-Kbps channels.

Figure 7.4 illustrates an ISDN connection.

Figure 7.4 Dial-Up Equipment and WAN Infrastructure for ISDN Connections

X.25

X.25 is an international standard for sending data across public packet switching networks. Windows 2000 remote access supports X.25 in two ways:

1. The remote access client supports the use of X.25 smart cards, which can connect directly to the X.25 data network and use the X.25 protocol to establish connections and send and receive data. The remote access client also supports dialing into a packet assembler/disassembler (PAD) of an X.25 carrier using an analog modem.

2. The remote access server only supports the use of X.25 smart cards.

For more information about the configuration of X.25 and PADs, see Windows 2000 Server Help.

Note

X.25 smart cards are adapters that use the X.25 protocol and can directly connect to an X.25 public data network. X.25 smart cards are not related to smart cards used for authentication and secure communications.

Figure 7.5 illustrates an X.25 connection.

Figure 7.5 Dial-Up Equipment and WAN Infrastructure for X.25 Connections

ATM over ADSL

Asymmetric Digital Subscriber Line (ADSL) is a new local loop technology for small business and residential customers. Although ADSL provides higher bit rates than PSTN and ISDN connections, the bit rate is not the same in the upstream and downstream directions. Typical ADSL connections offer 64 Kbps from the customer and 1.544 megabits per second (Mbps) to the customer. The asymmetric nature of the connection fits well with typical Internet use. Most Internet users receive a lot more information than they send.

ADSL equipment can appear to Windows 2000 as either an Ethernet interface or a dial-up interface. When an ADSL adapter appears as an Ethernet interface, the ADSL connection operates in the same way as an Ethernet connection to the Internet.

When an ADSL adapter appears as a dial-up interface, ADSL provides a physical connection and the individual LAN protocol packets are sent using Asynchronous Transfer Mode (ATM). An ATM adapter with an ADSL port is installed in both the remote access client and remote access server.

Figure 7.6 illustrates an ATM over ADSL connection.

Figure 7.6 Dial-Up Equipment and WAN Infrastructure for ATM over ADSL Connections

Remote Access Protocols

Remote access protocols control the connection establishment and transmission of data over wide area network (WAN) links. The operating system and LAN protocols used on remote access clients and servers dictate which remote access protocol your clients can use.

There are three types of remote access protocols supported by Windows 2000 remote access:

1. Point-to-Point Protocol (PPP) is an industry-standard set of protocols providing the best security, multi-protocol support, and interoperability.

2. Serial Line Internet Protocol (SLIP) is used by older remote access servers.

3. Microsoft RAS protocol, also known as Asynchronous NetBEUI or AsyBEUI, is a remote access protocol used by legacy remote access clients running Microsoft operating systems, such as Microsoft® Windows NT® 3.1, Windows for Workgroups, MS-DOS, and LAN Manager.

Table 7.1 summarizes the remote access protocols and their use in Windows 2000.

Table   7.1 Remote Access Protocols and Their Use in Windows   2000

LAN Protocols

LAN protocols are the protocols used by the remote access client to access resources on the network connected to the remote access server. Windows 2000 remote access supports TCP/IP, IPX, AppleTalk, and NetBEUI. For more information, see "Remote Access and TCP/IP and IPX" later in this chapter.