CS2302 Computer Networks Unit - I (Channel Access on links)

UNIT I

Network architecture – layers – Physical links – Channel Access on links– Hybrid multiple access techniques – Issues in the data link layer –Framing – Error correction and detection – Link-level flow control

Channel Access on links

·       Channel access on links is a multiple access method where the available bandwidth of a link is shared in time, frequency or by code between different stations to make efficient use of high-speed telecommunications lines.

·       Some form of multiplexing is used for multiple-access method.

·       Various multiple access techniques are

1.     Frequency Division Multiple Access (FDMA)

2.     Time Division Multiple Access (TDMA)

3.     Code Division Multiple Access (CDMA)

Frequency Division Multiple Access

·       Frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands.

·       Each station is allocated a band to send its data.

·       Each frequency band is reversed for a specific station that band belongs to the station all the time.

·       FDMA is a data link layer protocol that uses FDM at the physical layer.

Fig: Frequency division multiplexing

 

Time Division Multiple Access

·      Time division multiplexing is possible when the achievable data rate of the medium exceeds the data rate of digital signals to be transmitted.

·      In time-division multiple access (TDMA), the entire bandwidth is one channel. All the stations share the bandwidth of the channel in time.

·      Each station is allocated a time slot during which it can send data.

·      The main problem with TDMA lies in achieving synchronization between the different stations.

• Each station needs to know the beginning of its slot and the location of its slot.

Fig: Time division multiplexing

Code Division Multiple Access

·       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 at the same time without timesharing.

·       CDMA simply means communication with different codes used with spread spectrum based on coding theory.

·       Each station is assigned a code, which is a sequence of numbers called chips.

·       We start with data signal with rate D. where called as bit data rare.

·       We break each bit into k chips according to a fixed pattern that is specific to each user, called the user’s code.

·       The new channel has a chip data rate of KD chips per seconds.

 

Fig: code division multiplexing