mills@huey.udel.edu.UUCP (11/28/86)
Folks, I have been evaluating a new Trailblazer packet-ensemble modem made by Telebit for possible use to connect IP hosts together via ordinary dial-up lines. This interesting modem packetizes serial-asynchronous data on multiple carriers and is theoretically capable of speeds to 14 Kbps. It operates in buffered, half-duplex mode with error control by retransmission. I connected a pair of Trailblazers between two fuzzballs in the same calling area operating with the SLIP protocol at 4800 bps. For comparison a leased line operating with conventional full-duplex, synchronous modems at 4800 bps was also available between these machines. This is a short note describing the results of delay and throughput tests. The tests used ICMP Echo/Echo Reply messages with lengths randomly distributed in the range 40-256 octets and were conducted in the same manner as described in RFC-889. Each test accumulated 512 samples, where a sample consisted of one ICMP Echo/Echo Reply volley across the link with no other traffic on the link. The samples were then displayed on a bitmap display as a scatter diagram of length versus delay and saved in a file (telbit.bit on udel2.udel.edu in Sun format if you're interested). The linear regression line was then computed to determine the intrinsic delay and throughput as a function of packet length. From the scatter diagram it is apparent that the Trailblazer packetizing mechanism is multi-modal, in that different packetization algorithms are used for the shorter packets and for the longer ones. By trial and error it was discovered that the boundary between the two lies at about 125 octets, so separate regression lines were then computed for each region. The (one-way) results are shown below along with the results for the conventional 4800-bps modem: Trailblazer 4800-bps modem 40-125 125-256 40-125 125-256 ------------------------------------------------------- min delay (ms) 756 1093 103 247 max delay (ms) 1084 1388 244 467 slope (bps) 2075 3551 4802 4767 Obviously, the Trailblazer performance doesn't look too impressive compared to the 4800-bps modem when both are operated at the same interface speed. One reason for this is that the Trailblazer packetizes data internally upon submission and depacketizes it before delivery. Unfortunately, the particular fuzzball interfaces used here are simple character-at-a-time devices that do not work reliably above 4800 bps. If the interface speeds could be increased at both ends of the link without limit, the Trailblazer delays could in principle approach values given by subtracting twice the delays in the fourth column from those in the second and third columns of the table. This leaves about 500 ms to be accounted for by the Trailblazer packetization and transmission protocols, including the turnaround and resynchronization procedures necessary for half-duplex operation. Not bad, but not thrilling either. Inspection of the scatter diagram suggests the 3551-bps slope for the Trailblazer may be characteristic beyond the 256-octet limit of the measurements. Above this value; however, the modem begins to flow-control the source, at least in the test configuration, where the telephone line is only about three miles long. The Trailblazer line speed can be estimated as follows: of the (1388-1093) = 295 ms to transmit a (256-125) = 131-octet message, (467-247) = 220 ms are consumed in the interface, so that 75 ms represents the time required for transmission. Therefore the Trailblazers are humping data at 131*8/.075 = 13973 bps, which could be predicted on the assumption of good local line quality. I conclude the peformance of the Trailblazer, while potentially brilliant, is badly eroded in applications where the data already are packetized. This would seem a shame, considering the thunderous horsepower of the modem circuitry (M68000, TMS3020, oodles of memory), and could be easily remedied by the inclusion of a parallel port and appropriate handshaking protocol. Dave -------