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DESIGNING A DSP-BASED RAS SERVER

Part 1: RAS Server Background
by Shawn Arnold

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

RAS PORT

Letýs take a closer look at what each of the services offered by the RAS port really includes. A modem port provides remote access to networks. A modem port must provide:

• data modulations standardsýV.90, K56 Flex, V.34, V.32bis, V.32, V.22bis, V.22
• data protocol standardsýerror correction: V.42, LAPM, MNP 1-4; data compression: V.42bis, MNP 5
• some DLC processingýPPP packetization support

A fax port provides outgoing and incoming fax services. More specifically, a fax port needs to provide:

• data modulation standardsýV.17, V.29, V.27ter, V.21 ch2
• fax command setýclass 2 T.30
• optional fax command set extensionsýECM, in-line format conversion, BFT, extra fine mode
• image compressionýT.4 1D, T.4 2D, T.6

A voice port provides access to networks for routing voice calls, so a voice port must provide voice compression/codec standards (e.g., G.711, G.722, G.728, G.723, G.723.1, G.729). In many cases, the RAS port must provide services to assist in answering or originating calls over the Telco system. The RAS port must provide DTMF and MF tone generation and detection, as well as call progress services.

An obvious question becomes, why the RAS port must support such a variety of standards and protocols? In general, the RAS port must be able to connect to any possible remote device that could call and attempt to connect to the RAS port. A caller may have a 10-year-old modem, or they may have a state-of-the-art modem that supports the newest and fastest modem standards. One important feature of the RAS server is its compatibility with the widest variety of devices that could possibly attempt connection to the port.

NETWORK I/F BLOCK

The Network I/F refers to the RAS server block, which provides the interface to the network. The network data in and out of the RAS port device passes through this I/F. The network provides a byte/character data to the RAS server. As you can see in Figure 6, the Network I/F is made up of two main sub-block areasýthe host port controller and the network bus.

Figure 6ýOn the network side of the RAS server is the Network I/F block. This block is made-up of two important pieces-the host controller and the network bus.

The host refers to the main processor or controller that supports the operation of each port and directs the data flow between ports and the network. This host controller can usually handle and control several RAS ports. In this case, "several" means anywhere from 24 to as many as 100 port devices. The host controller processor is usually connected in parallel to the RAS port devices that it supports.

If the RAS port is the heart of the RAS server, then the host controller is the brains of the RAS server. The host is responsible for many things. Depending on the specific design, the host can be responsible for any combination of the following:

• driving the data transfers
• setting up a RAS port device; modem, fax, voice
• answering incoming or originating outgoing calls
• controlling the Telco system I/F transceiver
• dynamically downloading firmware to the RAS port device

The network bus refers to the physical connection between the RAS port devices and the host controller. The host controller supports multiple ports via a parallel connection to the RAS port devices. This parallel connection is usually to a port device parallel port that allows shared memory access or DMA to port device's memory.

Data on the network side of the RAS port is the byte/character form of the data being transferred between the network and the remote user. These bytes/characters often makeup the network packets being transferred between the network and the remote application.

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