Data Communications Networking Devices - 4th Ed.pdf

5.3 GATEWAYS __________________________________________________________________ 589the LAN cabling distance, since the communications controller must be cabled toan MAU under lobe length distance restrictions. In comparison, a local control unitcan be connected via coaxial cable to a communications controller, which enablesthe gateway distance to be extended. If a remote control unit is used, that devicefunctions similar to a remote communications controller since both the TRAandthe TIC cabling distances to a MAU are governed by lobe distance restrictions.In a local environment the communications controller and control unit are bothchannel-attached to a host computer, and their data transfer capabilities are similar.In a remote environment the transmission rate of a control unit is restricted to amaximum of 56 kbps. In comparison, a remote communications controller canoperate at T1/E1 data rates, providing over 20 times the data transfer capability of acontrol unit. Thus, the TIC can provide a higher level of throughput when used ata remote location.The biggest difference between the use of a TRAand a TIC is in the area of PUand LU support. The NCP on a communications controller can support up to 9999PUs per TIC. Then, each gateway PC functioning as a PU will support a groupingof LUs based upon the gateway software used. Another key difference between theuse of a TRAand a TIC concerns the method of gateway communications.When a control unit TRAprovides a connection to the Token-Ring network thecommunications controller polls the control unit and the control unit polls eachdownstream PU, with each gateway polling its LUs. When a TIC is used eachdownstream PU requests service from the TIC by using a `dial-up' service when itrequires service. Thus, this can considerably reduce the polls ¯owing on anattached local area network and results in the TIC being able to theoreticallysupport up to 9999 PUs.3278/9 coaxial connectionArather outdated and limited function gateway is based upon the use of a 3278/9coaxial adapter card. Instead of emulating a 3X74 control unit like SDLC and X.25gateways, the coaxial adapter permits a gateway PC to be connected to a port on a3X74 control unit. That port can be con®gured as a distributed function terminalDFT) port. When used in this manner the DFT port provides access to ®vesessions as it represents ®ve LUs. Gateway software then divides the ®ve SNAmainframe sessions among contending workstations on the local area network. Thismeans that a coaxial adapter-based gateway is limited to providing a maximum of®ve simultaneous host sessions. Similar to the other gateways described in thissection, a Token-Ring or Ethernet adapter card would be installed in the gateway toprovide a connection to the local area network. Figure 5.30 illustrates the hardwareused to provide a 3278/9 coaxial cable gateway.Although a coaxially connected gateway is limited in its session support, itoperates at coaxial cable data transfer rates to the control unit. If a local control unitis used the operating rate of a coaxially connected gateway can approach 2 Mbps. Incomparison, SDLC and X.25 type gateways are limited to a 56 kbps data transferrate. Thus, coaxially connected gateways can provide a high level of SNAaccessperformance for small local area networks when such networks are at the mainframelocation. In addition, this method eliminates the necessity to obtain a TIC or