Control Access & Channelization
Managing medium access through coordination and division.
Traffic Load & Access Methods
One important thing to keep in mind regarding the evolution of multiple access protocols is the relationship with network traffic load. As we move from random access methods to controlled and then to channelization, we are accommodating increasing levels of traffic.
Controlled Access
In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations.
Reservation
- Purpose: Stations make a reservation before sending data to avoid collisions.
- Timeline Structure: Divided into Reservation Interval (fixed time) and Data Transmission Period (variable time).
- Slot Allocation: If there are M stations, the reservation interval has M slots. Each station has 1 slot.
- Process: A station wanting to send transmits a 1-bit signal in its slot. After the reservation interval, everyone knows who wants to speak, and they transmit in order.
Polling
Works with topologies where one device is designated as a Primary Station and others as Secondary Stations. All data exchanges go through the primary.
Select Function
Used when the Primary has something to send. It sends a SEL frame to alert the secondary.
Poll Function
Used when the Primary wants to receive. It asks each device if it has data. Secondary responds with NAK (no data) or Data.
Token Passing
Stations are organized in a logical ring. A special packet called a Token circulates through the ring.
- Rule: Only the station holding the token can transmit data.
- Token Management:
- Time Control: Holding time is limited to prevent monopolization.
- Token Monitoring: Detecting lost tokens or failed stations.
- Data Transmission: If a station receiving the token has no data, it immediately passes it to the successor.
Channelization
Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we discuss three channelization protocols: FDMA, TDMA, and CDMA.
Frequency-Division Multiple Access (FDMA)
In FDMA, the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data.
In other words, each band is reserved for a specific station, and it belongs to the station all the time.
Time-Division Multiple Access (TDMA)
In TDMA, the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data.
Each station transmits its data in its assigned time slot.
Code-Division Multiple Access (CDMA)
CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link.
It differs from TDMA because all stations can send data simultaneously; there is no timesharing. Each station uses a unique code pattern.
(Differentiated by Code)