Notes on data communication and network-Channel Access

Channelization is a method for multi-channel access. In this method, different sites share the available bandwidth of the link in terms of time, frequency, or encoding, three channels are discussed here: FDMA, TDMA, and CDMA.

1. Frequent Multi-Channel Access (FDMA)

In FDMA, available bandwidth is divided into frequency bands. Each site uses the frequency band allocated to it to send data. In other words, each frequency band is reserved for specific sites, this band is always a site, and each site uses a band-pass filter to limit the frequency of the transmitter. To prevent conflicts between sites, the specified bands are separated by a small protection band. Describes the concept of FDMA.

Data of the four sites is transmitted through the channel at any time. Only analog signals can be transmitted using FDMA.

2. time division multiplexing (TDMA)

In TDMA, all sites share the channel bandwidth over time. Each site is assigned a time slot. Only in this time gap can it send data. Each site transmits data in the specified time slot. Describes the concept of TDMA.

At a certain time point, only one site is transmitting data.

3. Code-Based Multi-Channel Access (CDMA)

Unlike FDMA, CDMA only has one channel that occupies the entire bandwidth of the link. Unlike TDMA, all sites can send data at the same time without time sharing issues.

CDMA refers to communication using different codes. For example, there are many people in a room who can speak English, while others do not understand English. The other two can speak Chinese, others do not understand Chinese, and so on. It is easy for a few people to communicate, but they need to use different languages.

Assume that D1 and D2 are the data from site 1 among the four sites, and so on. The code assigned to the first site is C1, the second is C2, and so on. We assume that the assigned encoding has two features:

1. The two encodings are multiplied by 0.

2. The encoding itself is multiplied by 4. (Number of sites)

How to send data to four sites:

If site 2 wants to know what site 1 is talking about, it will multiply the data in the channel with C1 and divide it by 4:

(D1 * C1 + D2 * C2 + D3 * C3 + D4 * C4) * C1/4 = (D1 * C1 * C1 + D2 * C2 * C1 + D3 * C3 * C1 + D4 * C4 * C1)/4 = 4 * D1/4 = d1

(Because C1 * C2, C3 * C1, C4 * c1 are both equal to 0)