NET_12.TXT ISOLATED BACKBONE TRUNKS ------------------------ The ideal backbone trunk will have infinite capacity and be collision-free. The collision-free requirement can be achieved. Anything approaching infinite capacity would require microwave or fiber optic resources beyond our present reach. Looking at economics of equipment availability and what present technology allows, we'll have to be satisfied with a lesser circuit capability. Several arrangements are superior to the simplex backbone trunk commonly used. One such is isolated simplex links. With isolated simplex links, there are only two radios on a link frequency. This is accomplished by pairing up a set of radios on a different band for each link in the network. ------------------------------------------------------------------------------- ISOLATED SIMPLEX BACKBONE TRUNKING SCHEME NODE A NODE B NODE C NODE D NODE E ====== ====== ====== ====== ====== 51.89 <---------> 51.89 51.97 <-------> 51.97 51.82 446.1 <--------> 446.1 446.3 <--------> 446.3) Figure 12-1 ------------------------------------------------------------------------------- Figure 12-1 avoids collisions from adjacent node transmitters. For instance, NODE C is frequency isolated from nodes A and E. Probability of collisions between NODE B to NODE C and NODE C to NODE D are minimal if DCDs are working correctly. The 51 and 446 MHz pairing reserves the option of using 145 and 223 MHz for LANs. The next configuration upgrade is to a half-duplex isolated backbone trunk. Advantages include zero probability of collisions and is easily upgraded to full-duplex. ------------------------------------------------------------------------------- ISOLATED HALF-DUPLEX BACKBONE TRUNKING SCHEME NODE A NODE B NODE C NODE D NODE E ====== ====== ====== ====== ====== 446.2 tx ------> 446.2 rx 446.4 tx -----> 446.4 rx 441.2 rx <------ 441.2 tx 441.4 rx <----- 441.4 tx 441.3 tx ------> 441.3 rx 441.5 tx ----> 441.5 rx 446.3 rx <------ 446.3 tx 446.5 rx <---- 446.5 tx Figure 12-2 ------------------------------------------------------------------------------- The isolated half-duplex configuration is an excellent choice for a cost- effective backbone. High gain beams can be used at 440 MHz to restrict pattern coverage away from distant nodes which may be reusing the frequency. Also, a reasonable balance between transmitter powers and antenna gains can be achieved in order to meet the 40 dB or better, fade margin requirements. The ultimate upgrade to full-duplex can be done on a link by link basis by the installation of either duplexers or two separate antennas. Since it emulates landline practice, the ultimate configuration is full-duplex. Isolated full-duplex links will yield the highest throughput. But operating in-band, as in the half-duplex example above, requires expensive duplexers. One way to get around the duplexer cost is to link cross-band full-duplex. ------------------------------------------------------------------------------- ISOLATED FULL-DUPLEX CROSS-BAND BACKBONE TRUNKING SCHEME NODE A NODE B NODE C NODE D ====== ====== ====== ====== 147.5 TX ------------> 147.5 RX 446.1 RX <------------ 446.1 TX 147.52 RX <------- 147.52 TX 446.2 TX -------> 446.2 RX 147.54 TX -------> 147.54 RX 446.3 RX <------- 446.3 TX *Depending on geographical configuration, * 446.1 TX ----> there will be a point where frequency * 147.50 RX <---- pairs can be reused. Figure 12-3 ------------------------------------------------------------------------------- The frequencies in figure 12-3 were selected because they fall in the general portion of each band that's designated as simplex. Of course the actual frequencies used will need to be very carefully coordinated. With this configuration, repeater channels on either band, and which might be located at the site, should be able to co-exist. In case there are voice repeaters on-site, there may be a need to add cavity filters to the backbone trunk radios. In such situations, less costly cavities of the band-pass variety may provide adequate filtering. Note that the frequency pairing scheme in the example is such that dual receivers and dual transmitters in their respective bands are paired at each site. This is an interference avoidance technique. The main precaution in selecting transmitter frequencies is to avoid combinations that will result in a mix producing a receiver IF interference signal when two or more transmitters are keyed simultaneously. Advantages of 446/147 MHz cross-banding include the ability to use directional (higher gain) link antennas, availability of low-cost radios capable of 9600 baud operation, and the convenience of adding 9600 baud LAN ports on 223 MHz without serious band/interference problems. The above configurations will be two to three times more expensive than a non- isolated simplex system, but the throughput rate will be five times greater than simplex, even at 1200 baud. Because of its non-self interfering configuration, higher data rates will be linearly reflected (minus node transit time, which is a constant) in throughput.