Reservation
In
this method, a station needs to reserve the channel before sending frames. The
time is divided into slots, in each time slot a reservation frame is send
first. If there is n-systems connected to the channel then the reservation
frame contain n-mini slots, each of them corresponding to a particular station.
If a station needs to send a data frame then it set its reservation mini slot.
The stations, which are set reservation can send data frame after the
reservation frame. The following figure illustrates reservation in a four-node
network. Since there are four systems, the reservation frame contains four
mini-slots.
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| Reservation |
Polling
Polling is use in networks where one device is designed as
the primary station and all others are secondary stations. All data exchange in
the network is handling by primary station. That means, if a secondary station
need to send some data then it must send the data to the primary station, the
primary station then forward this data to the actual destination. Primary device
control the link and all the secondary stations follow its instructions. Always
the primary device is the initiator of a session.
In this
method there exist two functions: poll
and select. If the primary station wants
to receive data, it asks the secondaries if they have anything to send, this is
called poll function. If the primary
wants to send data, it tells the secondary to get ready to receive; this is called select function.
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| Polling |
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| Select function |
Token Passing
In token
passing method, the nodes in the network are organized in a logical ring. In
this method, a special packet called token circulates through the network. When
a station has some data to send, then it must need to wait until it receives
the token. If the token is received from its predecessor then it send data and if
no more data to send then it passes token to the next station. If a station has
no data to send while receiving token then it simply forward the token to its successor.
Two high-speed Token Ring networks are FDDI (Fiber Distributed Data Interface)
and CDDI (Copper Distributed Data
Interface).
Next : TDMA FDMA and CDMA
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