1. Airmail: A Link-layer Protocol for Wireless Networks

2. System Goals
Provide a reliable link layer protocol for indoor and outdoor wireless systems
Importance of link layer error recovery in wireless channels
High error rate of wireless channels
Transport layer protocol is too slow to recover from losses
Congestion control mechanisms of transport layer are unnecessarily triggered, throughput is decreased
Approaches to improve link layer protocol performance
Automatic Repeat reQuest (ARQ)
Forward Error Correction (FEC)
Mobility and handoff processing

3. Related Issues Asymmetry in wireless channels
Mobile terminal has limited power and smaller processing capability
Characteristics of different wireless channels
Narrowband outdoor channels
Wideband indoor channels
Wideband outdoor channels

4. ARQ Incorporating Asymmetry
Concentrate intelligence in the base station and make the mobile terminal relatively dumb
Only the base station keeps timers
The base receiver sends its status to the mobile transmitter periodically
The status sent by the mobile receiver to the base transmitter is not periodical but event-driven
The mobile receiver combines several status messages into one status message

5. From Mobile Transmitter (MT) to Base Receiver (BR)
MT keeps sending packets until
Max buffer size reached
Retransmission request received
No more data to send
BR sends status messages to MT periodically
BR maintains a status timer to send status messages
BR also maintains a timer to detect packet losses (if status stops changing for a period of time)
MT retransmits the requested packets before sending any new packet

6. Packet Loss Detection If the forward channel (BR to MT) is bad
Status messages from BR to MT are lost
MT will run out of buffer and stop sending packets
BR’s status will stop changing and anomaly will be detected
If the reverse channel (MT to BR) is bad
Packets from MT to BR are lost
BR keeps sending status messages to MT
The status of BR will stop changing and anomaly will be detected

7. Data Transfer Scenario

8. From Base Transmitter (BT) to Mobile Receiver (MR)
BT keeps sending packets until
A whole block of packets sent
Retransmission request received
No more data to send
MR sends a status message after a whole block received
Status message is a bitmap, biti indicates if the ith packet in the block has been received
Transmission power conserved
BT retransmits the requested packets
Bandwidth conserved
If no status message from MR, BT sends Poll message to MR. If after several attempts there is no response from MR, BT will give up.

9. Data Transfer Scenario

10. Communicating Extended Finite State Machines (CEFSMs) Description
Representation of condition, input, output and update in EFSMs
Condition: {c}
Send message x to m1: m1!x
Receive message x from m1: m1?x
Either m1?x or m2?y: m1?x + m2?y
m1!x followed by m2!y: m1!x * m2!y
Update: [u]

11. EFSM of MT

12. EFSM of BR

13. Forward Error Correction (FEC)
Necessary for real-time applications
Three levels of FEC
Bit-level FEC
Achieved by hardware at physical layer
Use Viterbi algorithm
Byte-level FEC
Achieved at link layer
Add redundant bits in each packet to recover from bit errors
Use Reed-Solomon codes
Packet-level FEC
Add redundant packets to a block of data packets to recover from packet losses

14. FEC in Different Wireless Channels
Narrowband outdoor mobile channels
High random error rate
Bit-level and byte-level FEC is beneficial
Wideband indoor channels
Very low random error rate
Sometimes suffer from burst errors
Packet-level FEC is beneficial
Wideband outdoor channels
Suffer from both random errors and burst errors
All three levels of FEC are beneficial

15. Combination of Packet-level FEC and ARQ
Example:
W = 8, M = 2, N = 6; data packets: d0, d1, d2, d3, d4, d5; parity packets: p0, p1
If 6 packets or more are received, lost packets can be reconstructed
If less than 6 packets are received, lost packets cannot be reconstructed
Suppose only d0, d1, d4, p1 are received, retransmission request is sent in the form of (Lr = 2, Bitmap = 0010)
Lr is the low end of the window at the receiver, 0010 refers to d2, d3, d5
Transmitter moves Lt to 2, retransmits d2, d3, d5, transmits d6, d7, adds parity packets p0', p1'
Lt is the low end of window at the transmitter

16. Mobility and Handoff
When mobile terminal moves from one base station to another, packets maybe lost during transition, resulting in inconsistent states at the mobile terminal and the new base station, additional measures should be taken to deal with such situations

17. Handoff Processing
Mobile terminal sends handoff request to new base station
New base station requests state information from old base station
New base station attempts to derive its proper state from the information provided by the mobile terminal
New base station buffers data arriving from the network and mobile terminal. The transmit window of mobile terminal is increased during handoff (dynamic windowing)
New base station receives state information from old base station, updates its state and processes all buffered data according to normal protocol rules

18. Experimental Performance Results
The number of retransmitted packets is the same irrespective of the number of retransmission requests, and the number is exactly equal to the number of lost packets

19. Experimental Performance Results