Cable Modems
Upstream Characteristics
Upstream is the term for the data transmitted from the subscriber's modem to the
CMTS. Upstream is usually bursts, and therefore multiple subscribers can
transmit on the same frequencies.
The frequency range is 5-42 MHz, and the bandwidth is 2MHz.
The modulation method can be either QPSK (2 bits/symbol) or 16-QAM (4 bits/symbol), with the later being faster, but also more sensitive to errors.
Each modem transmits in time slots, which may be marked as reserved, contention or ranging.
Reserved Slots
Reserved slot is a time slot that is reserved to only one modem. Other modems are not allowed to transmit in that time slot. The CMTS allocates the reserved timeslots to modems through a bandwidth allocation algorithm.
Reserved slots are normally used for relatively long data transmissions.
Contention Slots
Every modem connected to the CMTS is allowed to transmit in contention slots. If two subscribers decided to transmit data to the CMTS at the same time, the packets will collide and the data will be lost. In that scenario, the CMTS will detect the collision and notify the involved modems about it. The modems will then retransmit the data in other (random) time.
Contention slots are normally used to short data transmission, such as a request from the CMTS to allocate reserved time slots for longer transmission.
Ranging Slots
Since different modems are located in different distances from the CMTS, the time delay varies from modem to modem. To compensate for this, modems use "ranging protocol", that effectively moves the modem's clock back and forth.
To do this, a number (normally 3) of consecutive time-slots are set aside for ranging every now and then. The cable modem is commanded to try transmitting in the 2nd time-slot. The CMTS measures this, and tells the cable modem a small positive or negative correction value for its local clock. The two time slots before and after are the "gap" required to insure that the ranging burst does not collide with other traffic.
Cable Modems
Upstream Characteristics