THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

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

HOST CONTROLLER SERVICE DETAILS

The host controller must drive the network data transfers to and from the RAS port device. The RAS port provides the network data in a byte/character format that the network protocol layers understand. Embedded in the stream of byte/characters are the network IP/TCP and PPP frames, which are used by the network devices.

Depending on the chosen design, the host controller will either have to poll the RAS port devices to determine when data transfers are required or it can be interrupt driven via a hardware signal by the RAS port devices whenever host controller data transfer services are required.

Also, depending on the chosen design and modem chip set, the data transfers can consist of single word transfers to/from a RAS port device register or, more preferably, complete buffer transfers to shared memory or via DMA transfers directly to/from the RAS port device memory.

The host controller must configure the port for the correct type of call service. Assuming the RAS port is a reprogrammable device, the host ensures the appropriate setup of the required port service, depending on the type of call. As mentioned previously, the call type could require modem/data, fax, or voice configurations.

Because in most cases the call type is not known a priori, the host controller must dynamically configure the RAS port device in real time. This is an important consideration in the design of the RAS server.

Usually it is the host controller's responsibility to set up the physical connection between the RAS port device and the remote device (this includes both incoming and outgoing calls). In the case of an incoming call, the actual detection comes from the Telco system I/F. However, the Telco system I/F will notify the host controller of the incoming call. The host controller must then respond by allocating a RAS port to the call and setting up the port device to handle the specific call type. The host must provide firmware to the port device to configure it for the specific call type (modem, fax, or voice). Also,the host must be sure that the Telco and network data are directed to and from the allocated RAS port.

For an outgoing call, the host performs the proper tasks to place the call and ensure a connection to a remote device. Although the RAS port may provide services such as DTMF tone generation and detection, the host must orchestrate these services to ensure the call is successfully connected.

Similar to incoming calls, the host controller allocates a RAS port to the call and sets up the port device to handle the specific call type. The host does so by providing firmware to the port device to configure it for the specific call type. And again, after the connection is established, the host makes sure the Telco and network data are directed to and from the allocated RAS port.

As youýll see, the Telco system I/F includes a sophisticated transceiver that not only provides for a physical connection to the Telco system trunk line, but also provides all the Telco system data in a more readily usable form. This level of sophistication comes at the cost of added control. These sophisticated Telco system I/F transceivers usually require an additional controller.

This control can be provided by a separate controller, specifically designed in to control the transceiver. To reduce the complexity of the hardware, the control functions could be performed by the RAS port host controller. In this case, the host controller would have to perform all the tasks to maintain the I/F transceiver.

Note that even if a separate controller is chosen, the host controller still has to maintain and interface to the separate controller. Although the controlling burden may be significantly reduced, this is still a required task of the host controller.

Assuming the RAS port is chosen to be a reprogrammable port device, the host must provide for the loading of firmware into the port device. This could range from a static one-time load at powerup, to dynamic loading where specific modules of firmware are dynamically requested by the port and supplied by the host controller.

There may be cases where the RAS port has a level of sophistication that enables it to access a shared EEPROM or flash-memory resource on its own. In that case, the level of host intervention and support is eliminated.

TELCO I/F BLOCK

The Telco I/F is the RAS server block that provides the interface to the Telco system. The Telco data in and out of the RAS port device passes through this I/F.

A trunk line to the RAS server is provided by the Telco system in the form of serially channelized data. The I/F trunk usually takes one of two formsýT1/E1 trunk line or ISDN trunk line.

As you can see in Figure 7, the Telco I/F is made up of three main sub-blocks: the trunk line transceiver, the transceiver controller, and the serial bus. Because the physical signals on the Telco trunk lines are robustly designed for adverse conditions of distance, interference, and minimal cost of material, the physical signals are not compatible with standard digital electronic logic circuits. For this reason, special transceiver chips are available that allow the physical connection of the trunk lines to an electronic device such as a port device.

Figure 7ýOn the Telco side of the RAS server, the Telco I/F block is made-up of three important pieces-the trunk line transceiver, the transceiver controller, and a serial bus.

The trunk line signals are also sophisticated. A lot of information is packed into a single 2-wire serial signal. This information includes clock timing, framing information, the actual data, and switch signaling information. Therefore, the transceiver chip not only provides the physical connection, but also breaks down the sophisticated serial signal into a signal more manageable for the delivery of the serial data to the RAS port device.

Depending on the direction of data flow, the transceiver breaks down the sophisticated trunk line signal and provides the following signals to the RAS Port device:

• serial clock
• frame sync
• PCM Tx & Rx data
• telco switch signaling information

Because the trunk line signals are so sophisticated, the analysis of these signals by the transceiver chip is also sophisticated. So much so that a controller processor is usually required to manage and set up the operation of the transceiver chip.

Note that in some cases, the controller tasks can be moved onto the network side host controller.

The Telco bus refers to the physical connection between the Telco I/F and the RAS port devices. The Telco bus supports multiply ports. This connection is a serial connection to the ports. Usually this is in the form of a connection to a serial port in the RAS port device. The serial bus signal is in TDM form. Each port is assigned a timeslot(s) on the serial bus.

The data on the trunk lines is samples of a digitized analog signal. The digitized signal is that of an analog modulated signal carrying the data being transferred between the remote site and the network over the Telco system media. The analog signal is digitized at an 8.0-kHz rate and is logarithmically compressed down to an 8-bit sample. This digitized stream of samples is referred to as the pulse-code modulated (PCM) stream.

PREVIOUS • NEXT