Computer Networks - Channel Access Link

2.  Bandwidth of a medium that linking two devices is
greater than the bandwidth needs of the devices, the
link can be shared
 A channel-access scheme is based on a multiplexing
method that allows several data streams or signals to
share the same communication channel or physical
medium.
 channel access method or multiple access method
allows several terminals connected to the same multi-
point transmission medium to transmit over it and to
share its capacity.

3. Multiplexing Methods,
◦ FDM (Frequency Division Multiplexing) - Analog
◦ TDM (Time Division Multiplexing) - Digital
◦ WDM (Wave Division Multiplexing) – Analog
FDM
◦ Applied when the bandwidth of a link (in hertz) is greater
than the combined bandwidths of the signals to be
transmitted.
◦ signals generated by each sending device modulate different
carrier frequencies. These modulated signals are then
combined into a single composite signal that can be
transported by the link.
◦ A very common application of FDM is AM and FM radio
broadcasting

5. TDM
◦ TDM is a digital process that allows several connections to
share the high bandwidth of a link.
◦ Instead of sharing a portion of the bandwidth as in FDM, time
is shared.

8. WDM
◦ use the high-data-rate capability of fiber-optic cable.
◦ Using a fiber-optic cable for one single line wastes the
available bandwidth. Multiplexing allows us to combine
several lines into one.
◦ WDM is conceptually the same as FDM, except that the
multiplexing and demultiplexing involve optical signals
transmitted through fiber-optic channels.

9. Multiple Access Techniques,
◦ When nodes or stations are connected and use a common
link, called a multipoint or broadcast link, we need a
multiple-access protocol to coordinate access to the link.

10. Random Access Methods,
◦ There is no scheduled time for a station to transmit.
◦ Transmission is random among the stations.
Methods,
◦ ALOHA
 Pure ALOHA
 Slotted ALOHA
◦ CSMA
 1 – persistent
 Non persistent
 p - persistent
◦ CSMA/CD
◦ CSMA/CA

11. Controlled Access Protocols
◦ 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.
Types,
◦ Reservation
 Dynamic Time Division Multiple Access (Dynamic TDMA)
 Packet reservation multiple access (PRMA)
 Reservation ALOHA (R-ALOHA)
◦ Polling
◦ Token Passing

12. Channelization Techniques
◦ 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.
Types,
◦ FDMA (Frequency Division Multiple Access)
◦ TDMA (Time Division Multiple Access)
◦ CDMA (Code Division Multiple Access)

13. CDMA