Week 2 Flashcards
What are most communication systems compromised of?
Transmitter
Receiver
Multiple access allows what?
Allows many users to share the communications channel without interfering with one another.
Fixed Assignment Access?
Used for a voice oriented network.
Employed for allocating the traffic channels.
Fixed Assignment Access Traffic Channels?
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Spread Spectrum Multiple Access (SSMA)
Traffic?
Traffic is exchanged in both direction for a considerable length of time.
A signaling?
Control Channel exchanges short messages between the calling components to:
Set up the call
Terminate the call
Where is Random Access Method used?
used for data oriented network (packet switched)
Designed for bursts of data
Does not have a separate signaling channel
FDMA?
Frequency Division Multiple Access
What does FDMA do?
All users transmit at once but each user is assigned to its own channel around its carrier frequency
1G networks like AMPS
TDMA?
Time Division Multiple Access
What does TDMA do?
All users share same frequency band but each is user is assigned a unique time slot.
2G Cellular networks, GSM
SSMA?
Spread Spectrum Multiple Access
What does SSMA do?
SSMa employs signals that have a bandwidth which is several orders of magnitude greater than the minimum required bandwidth
PN?
Pseudo Noice
What does PN do?
Pseudo Noice sequence converts a narrowband signal to wideband noise like signal
Two major types of SSMA schemes?
Code Division Multiple Access
Frequency Hopped Multiple Access
CDMA?
Code Division Multiple Access
What does CDMA do?
Employs DSSS.
Signals all transmit at the same time
Signals all transmit the same frequency
Receiver uses code or signature sequence to isolate desired signal
What is ALOHA?
The first scheme developed for packet radio network.
A terminal transmits a packet when the packet arrives from the upper layer
Advantages of ALOHA?
Simple
Does not require sync between terminals
Disadvantages of ALOHA?
Low throughput under heavy load conditions
Slotted Aloha?
Sync version of ALOHA
Transmission time is divided into time slots
How can efficiency be increased for ALOHA?
If the transmitter senses the channel before transmission.
What happens when collision is detected?
The transmission is aborted
Jamming signal is transmitted
Retransmission procedure is initiated.
RTS?
Request to Send
What does RTS identify?
Source Address
Destination address
Length of data
