er, 64QAM and 256 QAM are susceptible to interfering signals, making them unable to support noisy upstream transmissions. Quadrature Phase-Shift Keying (QPSK) is a digital frequency modulation technique used for sending data upstream over coaxial cable networks. QPSK is suitable for sending data upstream over a cable data network because it is fairly resistant to noise. Depending on the amount of cable RF spectrum allocated, upstream channels may deliver 500 Kbps to 10 Mbps, using 16 QAM or QPSK modulation techniques, with 16QAM being the fastest transfer method of the two (Salent, 1999). Upstream cable modem traffic is always sent in bursts. Each modem transmits upstream bursts in time slots. These time slots can be designated as reserved, contention, or ranging slots. As the name implies, a reserved slot is a time slot that is reserved to a particular cable modem. No other cable modem is allowed to transmit in this reserved time slot. The CMTS allocates the reserved time slots to the various cable modems under its control through a bandwidth allocation algorithm. Reserved slots are normally used for longer data transmissions (Ostergard, 1998). Contention time slots are open for all cable modems to transmit in. If two cable modems attempt to transfer simultaneously in the same contention slot, their packets collide and the data is lost. The CMTS detects the collision and signals that no data was received, which makes the each cable modems try to retransmit the data after waiting a random length of time.Ranging is the process of automatically adjusting transmit levels and time offsets of individual cable modems. Ranging is performed to insure that bursts coming from different modems line up in the right time slots and are received at the same power level at the CMTS. A uniform power level for bursts reaching the CMTS facilitates collision detection. If two cable modems transmit at the same time, but one is much weaker t...
