5-7
Through time-interleaving three cell-phones now occupy the same
frequency bandwidth that one occupied just a few years ago.
Another area of rapid growth using TDMA is CATV reverse path.
Today’s CATV system includes many different signal and modulation
types. Each advanced service can have one or many different TDMA
signals assigned to a particular frequency.
When multiple users are accessing a network via modems, an
enormous amount of bandwidth would be occupied if each modem
were assigned one frequency channel. The majority of time the user
is hooked up to a network is spent idle. Most of the bandwidth would
be wasted. The ratio of authoring a typical email to transmitting a
typical email is at least a hundred to one. One possible answer
would be to use a low transfer rate. This system would work if no
large files need to be transmitted. A better answer is variable time
TDMA. Variable time TDMA gives more bandwidth when needed and
takes just a little bandwidth the rest of the time. More flexibility can
be achieved on cable modems due to the high fidelity connection.
Therefore, unlike IS-54B, the talk times can be variable. Fig4-4
illustrates that user ‘a’ is sending more data than users ‘b’ and ‘c’.s
Fig. 5-4: Variable Time Slot TDMA
B.
Current reverse path trouble-shooting strategies are not
capable of detecting problems inside of a TDMA modem’s frequency
allocation. Checking the level at each frequency is a good start for
characterizing the reverse path performance. A spectrum graph only
looks at each frequency for a small percentage of the total time. This
leads to two uncertainties.