Airmail: A Link-layer Protocol for Wireless Networks

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

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

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

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

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

Data Transfer Scenario

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.

Data Transfer Scenario

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]

EFSM of MT

EFSM of BR

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

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

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

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

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

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

Experimental Performance Results