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DESIGNING A DSP-BASED RAS SERVER

Part 1: RAS Server Background
by Shawn Arnold

Start ı The Little Picture ı RAS Port ı Host Controller Service Details ı RAS Port Design ı Sources and PDF

THE LITTLE PICTURE

At the heart of a RAS server is the RAS port. The RAS port is the point in the gateway where the translation of the data between the network and Telco system clouds occurs. RAS ports can serve as one of several types of translators.

In the most common case, the RAS server provides a modem port to a remote user for access to a given network. This network could be an office LAN, an ISP WAN, a retail store or bank WAN, or even the Internet.

For example, your ISP subscription permits you access to the ISP's network. In almost all cases, the ISP's network provides access to the Internet. In order to gain access to the ISP's network, the user must call, via your PC's modem, the local ISP access number. This call actually goes to a RAS server at the ISP's local point of presence. Via the RAS server at the local access point, the ISP subscriber gains access to the ISP's network and thus all the ISP services such as e-mail, chat rooms, newsgroups, and the Internet.

As a similar example, telecommuting works in much the same way. An employee can gain access to the company network remotely from home by calling a special access number which connects them to the company's RAS server for access to the office network (see Figure 1).

Figure 1ıIn a typical telecommuting scenario, a home office PC is connected to the main office LAN via a RAS server modem port.

Some RAS servers also provide outgoing services such as outgoing fax service. Not only can a RAS server provide outgoing fax services, but it can also provide incoming fax services.

An office worker might be editing a document within a word-processing application running on their PC or workstation. Their document may need to be faxed to an office in another location. Normally, they would have to print the document and manually feed it into the fax machine. The RAS server could provide a button within the word-processing application that could be clicked to fax the document directly from their application to the desired fax machine in the other office (see Figure 2).

Figure 2ıFor an off-network service, an application running on a company's network provides fax service via a RAS server fax port.

A new and growing application of RAS servers is for voice services. Basically, the RAS server is used as a voice port to a network. This RAS port application is referred to as "voice over IP" or VoIP. In this case, the RAS port is a voice coder. It compresses/decompresses digitized voice data for passage over a network.

Letıs say you call a friend who lives in another state. Instead of calling them directly and incurring long-distance Telco fees, you place this call via their ISP's local point of presence number. The ISP could route the voice call over their WAN or the Internet to the ISP's point of presence at the friendıs location. From this point of presence, the call would then be routed over the local Telco system to the friend's phoneıwithout incurring long-distance Telco charges (see Figure 3).

Figure 3ıWith VoIP services, a RAS server voice port provides access to a network for voice routing.

THE BIG PICTURE

Letıs examine how the RAS server works. Iım going to break down the RAS server into its basic building blocks and examine how each block functions and how each of the blocks play together.

As I said earlier, the RAS server provides a gateway for remote access to a network. You can see in Figure 4 that the RAS server is a gateway that sits on the boundary between the Telco system and a network. In most cases, the remote access is made via the Telco system. As mentioned earlier, this could be an ISP subscriber, telecommuter, and so on.

Figure 4ıThe RAS server acts as a gateway for remote access to a network. On one side of the gateway lies the network cloud, and on the other, the Telco system.

At the heart of the RAS server is the RAS port shown in Figure 5. The port provides the transformation of different data representations in the Telco system and in the network. Because of the nature of the Telco system, information passed over this media must be transformed into a special modulated from. The port provides the transformation service form data passing over the Telco system/network boundary. The RAS port performs the majority of the data processing required in a RAS Server.

Figure 5ıA RAS server is made-up of several parts. The most important part of the RAS server is the RAS port, which lies at the center of the system.

Depending on the type of connection being supported, the port will provide modem, fax, or voice services. Because the data processing is so numerically intense, DSPs become an attractive solution for the implementing of this so-called "data pump." Iıll show you later that DSPs indeed meet the critical design specifications for RAS ports and become the likely choice for RAS port implementation.

The RAS server stands at the boundary of the Telco system, which provides the media for making a remote access and a network (whether it be a LAN, WAN, or the Internet). On the Telco system side of the port is the Telco I/O interface. On the network side of the port is the Network I/O interface. Iıll look at both of these I/O interfaces in more detail later.

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