Cable Modem Theory

by: Rolf V. Østergaard

What Is Upstream - Page 9

Upstream is the term used for the signal transmitted by the Cable Modem. Upstream is always bursts, so many modems can transmit on the same frequency. The frequency range is typically 5-65 MHz or 5-42 MHz. The bandwidth per channel may be e.g. 2 MHz for a 3 MBit/s QPSK channel.

The modulation forms are QPSK (2 bits per symbol) and 16-QAM (4 bits per symbol), with the later being the fastest, but also most sensitive to ingress. One downstream is normally paired with a number of upstream channels to achieve the balance in data bandwidths required.

Each modem transmits bursts in time slots, that might be either marked as reserved, contention or ranging.

Reserved slots

A reserved slot is a time slot that is reserved to a particular Cable Modem. No other Cable Modem is allowed to transmit in that time slot. The CMTS (Head-End) allocates the time slots to the various Cable Modems through a bandwidth allocation algorithm (notice: this algorithm is vendor specific, and may differentiate vendors considerably).

Reserved slots are normally used for longer data transmissions.

Contention slots

Time slots marked as contention slots are open for all Cable Modems to transmit in. If two Cable Modems decide to transmit in the same time slot, the packets collide and the data is lost. The CMTS (Head-End) will then signal that no data was received, to make the Cable Modems try again at some other (random) time.

Contention slots are normally used for very short data transmissions (such as a request for a number of reserved slots to transmit more data in).

Ranging slots

Due to the physical distance between the CMTS (Head-End) and the Cable Modem, the time delay vary quite a lot and can be in the milliseconds range. To compensate for this all Cable Modems employ a ranging protocol, that effectively moves the "clock" of the individual Cable Modem forth or back to compensate for the delay.

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 (Head-End) 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.

The other purpose of the ranging is to make all Cable Modems transmit at a power level that makes all upstream bursts from all Cable Modems arrive at the CMTS at the same level. This is essential for detecting collisions, but also required for optimum performance of the upstream demodulator in the CMTS. The variation in attenuation from the Cable Modem to the CMTS can vary more than 15dB.