Why are the packet sizes used realistic + why it is relevant
realistic packet sizes due to being mean + max packet size in Ethernet which is commonly useed to backhaul the 802.11 system
General trend for MAC eff. as function of MSDU size + PHY rate implications (3)
- MAC eff. reduces w/ increasing data rate + decreasing packet size
- High rate PHY suffer low MAC eff. (compression effect) unless large packet sizes are used
- Packet aggregation might be used but has QoS implications
Why provision for multi-slot packets?
What constraints does standard have?
Why are they necessary?
(5)
- Multi-slot transmission makes MAC more eff.
- Reduces overhead of silent period since only one is needed after several time slots instead of every one
- Standard constrains multi-slots to be 3 or 5, ensures that even/odd allocations of slots to M/S resume at end of multislot packet
- Are symmetric or asymmetric between UL + DL
- Longer packets not supported as prob. of error becomes too large
Principle of ‘HARQ’ that enhances performance comapred to ARQ (3)
- Combined w/ FEC
- HARQ doesn’t discard erroneous data
- Tries to combine w/ info received later to deduce correct packet
Chase Combining Method (2)
- stores erroneous data + tries to combine multiple copies of the same bits to deduce correct values
- Via soft or hard combining of multiple copies
Incremental redundancy Method (2)
- stores past received data
- only transmits some additional redundant bits (not same again) at the highest coding rate necessary to enable correct decoding @ receiver
Of
- ‘basic’ HARQ
- Chase Combining
- Incremental Redundancy
which are mandatory, optional and incompatible? (3)
- Basic HARQ - used in 802.11 PHY
- Chase Combining - optional, doesn’t need any features added / changed to accomodate for it
- Incremental Redundancy - requires change, therefore incompatible
Benefits of LA in wireless comms + improvements to QoS? (2)
- Avoids need to design system @ trade-off between throughput + PER
- Parameters of system can be adapted accoding to the link quality between QoS dimensions
Benefits of DRA in wireless comms + improvement to QoS? (4)
- exploits multi-user diversity in space, time + frequency
- allows the best resources to be allocated to individual devices
- improves throughput and / or reliability
- trade-off between maximising sum network eff. + individual device fairness
Adv / disadv of a multihop wireless network in comp. to single hop fromthe perspectives of owner + operator (6)
- Multihop improves range/coverage of a network
- Benefits:
- access to those out of range of infrastructure
- lowers infrastructure costs
- Disadv:
- increased pwr consumption of user devices (relays)
- additional protocol overheads for scheduling + resource allocation
- protocols more complex + less eff. of b/w
What is ACL? (3)
- Asynchronous ConnectionLess
- ‘best effort’ variable rate
- ‘whatever is left’ after the SCO service scheduling
- best suited to for fastest data rate
Challenges in multihop networks for scheduling + resource allocation? (4)
- More complex as more links to accomodate
- Links share resources w/o excessive interference
- In CC networks, controller needs allocations relayed to nodes outside of range
- In DC, co-ordination is impaired by the hidden node problem
What is SCO? (3)
- Synchronous Connection Orientated
- provides limited but fixed bit rate connections (finite #)
- intended for CBR traffic e.g. voice
MAC Frame Diagram
- Downlink
- TTG - Transmit Turnaround Gap
- Uplink
- RTG - Receiver Turnaround Gap
- Preamble
- FCH - Frame Channel
- TDM - Time DIvision Multiplexing
- DL Map
- UL Map
- DL Data
- UL Data
- SS Gap
Top Level Function of MAC frame
- TTG - placed between DL + UL to allow BS to switch between transmitter + receiver, vie versa for SS
- RTG - placed to allow BS to reverse process
Mid-Level, DL MAC Frame (3)
- Preamble - allows synchronisation + channel estimation
- FCH - (framing channel) DL + UL Maps explaing how the frame has been allocated
- Decided by BS, maps used to announce decisions to SSs
- TDM - Contains DL data
Bottom-level, DL MAC frame
- DL Map - start + end time of DL data phases in TDM. incl. modulation + coding used in each
- UL Map - (as above) + which SS is scheduled to transmit
- DL Data Phase:
- each DL data uses differnet modulation + coding schemes
- 1st DL uses most robust of modulation
- SSs should NOT schedule to receive data transmission @ too high a rate
Mid-Level, UL
- UL Data - TDM impossible, each SS transmits its data during the UL data frames
- SS Gaps - Silent Periods
- allows for signals from different SSs arriving @ BS @ different propagation delays
- otherwise UL signals would collide
