Shared Access Networks
12 important questions on Shared Access Networks
What are the ethernet topologies?
- coax segments of max 500m
- segments can be joined by repeaters
- 4 repeaters between hosts (so 5 cables)
- total reach of 2500m
- signal is broadcast over entire network
- manchester coding
10 Base-T
- twisted pair point to point (limited to 100m)
- hub is multiway repeater
- mimics broadcasting bus
- robust
- Max size of the network is drastically reduced
What is the ethernets frame format?
- allows receiver to synchronize with the signal
2 Addresses: src & dest
- unique, 48 bit unicast address assigned to each adapter
- broadcast: all 1s
- multicast: first bit is 1
- promiscuous mode: adapter delivers all frames to host
Type (16 bits)
- size (<=1500) or demux key (1500>) to identify higher level protocol
Body
- at least 46 bytes, at most 1500 bytes.
CRC (32 bits)
Explain ethernet transmit algorithm: CSMA / CD
If line is idle
- send immediately
- upper bound message size of 1500 bytes
- must wait 9.6 microsec between back to back frames
If line is busy
- wait until idle and transmit immediately
If collision
- transmit 32 bit jamming signal, then stop transmission
- random delay and try again (exponential backoff)
Minimum frame length required
- minimum frame is 64 bytes
- to know for sure whether collision occurred or not
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What are Wireless Links?
- all transmissions share the same "wire"
- Solutions: dividing the medium according to space / frequencies. Medium access control
Frequencies
- frequency bands reserved for specific purposes
- frequencies allocated to organisations in geographical areas
- license exempt bands
Issues
- power: mobile devices have limited power. So limited distance
- Noise: other devices generate signals in same range: lots of bit errors
What is spread spectrum?
FHSS (Frequency Hopping Spread Spectrum
- transmit over pseudo-random sequence of frequencies
- sender and receiver share RN generator + initial seed
Direct Sequence Spread Spectrum (DSSS)
- represent each bit by multiple bits
- sender and receiver know RN generator to generate pseudo random bits
- transmitted values are spread over frequency band that is n times wider
Pros and Cons for FHSS and DSSS
- Simple implementation
- tolerance of interference
- protections against eavesdropping
- simultaneous transmission over multiple access points
- scalable
FHSS Cons
- lower rate and range
- complex hand-off from cell to cell
- vendors seem to prefer DSSS
DSSS
- n-bit chipping code
- difficult to correct itself
What are the leading wireless technologies?
- typical link length: 10 m
- typical data range: 2 Mbps
- typical use: link a peripheral to a coputer
- wired analogy: USB
Wi-Fi
- 100 m
- 54 Mbps
- link computer to a wired base
- Ethernet
3G Cellular
- tens of kilometers
- hundreds of Kbps (per connection)
- link a mobile phone to a tower
- DSL
Explain Base Station vs Ad hoc network
- base has no mobility
- connected to (wired) distribution system
- all communications is routed via base station
- host mobility: none, restricted to one, handover between base stations
- base point also called (Wireless) Acces Point (AP)
Ad hoc network
- no base station
- direct communication between peers
- forwarding via chain of peers
- mesh can be extended incrementally
Explain Medium Access Control in IEEE 802.1
- physical carrier sensing (listen) and random backoff (if busy)
- exponential backoff if no ACK returns (collision assumed)
Problem: hidden and exposed nodes
- hidden nodes: A and C are unaware of each other. Collission if both send frame to B. A and C cannot detect collision
- Exposed nodes: B and C are aware of each other. If B sends to A, C could conclude it cannot send to D. Wrong conclusion: no interference
Optional: Collision Avoidance
- similar to Ethernet but rather Collission Avoidance (CA) instead of Detection (CD)
Explain IEEE 802.11: Frame Format
- type: RTS, CTS, probe, data frame, ACK, ...
- ToDS, FromDS bits
- duplicate frame? Encrypted frame?
Adresses: four 48 bit addresses
Duration: how long will channel be occupied?
Sequence control: because of handover, frames can arrive in wrong order
Data: up to 2312 bytes
32-bit CRC
Remark: no preamble? No frame delineation? No demux key? No sliding window?
Explain IEEE 802.11: Preamble
- different sizes (up to 192 bits)
Purpose
- synchronization
- start frame delimiter
- frame length
- speed (data rate)
- modulation selection
- CRC 16
Transmitted at slow speed: eg 1Mbps
Multitude of choices allows system to adapt for current signal conditions: eg adapt rate in case of errors
Remark: can this be considered a layer-1 header?
Explain IEEE 802.11: Addresses
- ultimate destination
- sending AP
- receiving AP
- original source
Interpretation depends on ToDS/FromDS bits. (00, 01, 10, 11)
Only use of 4 addresses in special occasion (11). When the distributor is itself a WiFi network instead of ethernet. From AP1 to AP2 but after AP2 still has to go to Des and want to know original Srce
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