han the other one, the CMTS will only detect the strong signal and assume that no collision took place. If the two colliding upstream signals are the same strength, they will both be detected by the CMTS as garbled. The CMTS will then know that a collision took place and will instruct the cable modems to retransmit their packets (Ostergard, 1998). Ranging slots are also used to compensate for the differences in physical distance between the CMTS and each of the cable modems. The large geographic reach of a cable data network poses special problems as a result of the transmission delay between users close to head-end versus users at a distance from cable head-end. To compensate for cable losses and delay as a result of distance, the CMTS performs ranging, which allows each cable modem to assess its time delay in transmitting to the head-end. Large CATV networks can experience long delays in the millisecond range. The ranging protocol compensates for these delays by moving the clock of each cable modem forward or backward to make up for they delay. Ranging is performed periodically by the CMTS for each cable modem under its control. Three consecutive time slots are set aside for ranging. The CMTS commands the cable modem to transmit in the second time slot. The CMTS then measures the transmission time and gives the cable modem a small positive or negative correction value for its local clock. The two time slots on either side of the second time slot are required to insure that other traffic does not interfere with the ranging burst (Ostergard, 1998). The cable modem itself is comprised of the following major components; the Tuner, the Demodulator, the Burst Modulator, the Media Access Control (MAC) Mechanism, the Interface, and the Central Processing Unit (CPU). External cable modems have an on-board CPU to handle instruction processing. Internal cable modems are being developed that will use the PCs CPU much like the way...
