CSE 123 Lecture Notes - Lecture 24: Alohanet, Bit Rate, Frequency-Division Multiple Access
28 Nov 2018
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Fixed Partitioning
Need to share media w/ MULTIPLE nodes (n)
- Multiple simultaneous conversations
Simple solution
- Divide channel into multiple, separate channels
- Physically separate
Bitrate of the link is split across channels!
- No one sender cannot achieve FULL bit rate of the medium!
- Can ONLY send/receive on their own channel
Frequency Division (FDMA)
Divide bandwidth of f Hz into n channels EACH w/ bandwidth f/n Hz
- Easy to implement
- Unused channels go “idle”
- “Approximately” f/n - want to make sure no one channel touches another!
Time Division Multiple Access (TDMA)
Divide channel into rounds of n time slots each
- Assign different hosts to different time slots WITHIN a round
- Unused time slots “idle” again!
Hosts get FULL capacity for their particular time slot
- Good if you want to send in “bursts”
ALL clocks have to be synchronized so no one host fights with another!
Code Division (CDMA)
Do NOTHING to “physically” separate the channels
- ALL stations transmit at same time in same frequency bands!
- So-called spread-spectrum techniques
- Taking your message and “spreading” it across the whole spectrum
Sender modulates their signal on top of unique code
- Like Manchester modulates on top of clock
- Bit rate of resulting signal MUCH LOWER than entire channel
Receiver applies code “filter” to extract desired sender
- ALL other sender seem like “noise”
Problem w/ Channel Partitioning
Not well suited for random access usage
- I just want to send whenever I want to send! (i.e. random access)
Instead, we want media access schemes for more common situations
- Not ALL nodes want to send ALL the time
- Don’t have a fixed # of nodes
“Potentially” higher throughput for transmissions
- Active nodes get FULL channel bandwidth
Aloha - protocol to communicate between the islands!
Document Summary
Need to share media w/ multiple nodes (n) Bitrate of the link is split across channels! No one sender cannot achieve full bit rate of the medium! Can only send/receive on their own channel. Divide bandwidth of f hz into n channels each w/ bandwidth f/n hz. Approximately f/n - want to make sure no one channel touches another! Divide channel into rounds of n time slots each. Assign different hosts to different time slots within a round. Hosts get full capacity for their particular time slot. Good if you want to send in bursts . All clocks have to be synchronized so no one host fights with another! All stations transmit at same time in same frequency bands! Taking your message and spreading it across the whole spectrum. Sender modulates their signal on top of unique code. Like manchester modulates on top of clock. Bit rate of resulting signal much lower than entire channel.
