Optimization Problems in Wireless Coding Networks Alex Sprintson Computer Engineering Group Department of Electrical and Computer Engineering

Agenda An overview of the COPE A new forwarding architecture for wireless networks by Katti et. al. Sigcomm 2007 Optimization problems in wireless coding networks Joint work with Salim El Rouayheb and Asad Chaudhry

COPE New Architecture For Wireless Networks Substantially improves throughput Inserts a coding layers between MAC and IP Identifies coding opportunities Benefits from them by mixing packets

Background Bob and Alice Relay Requires 4 transmissions

Background Bob and Alice Relay Requires 3 transmissions XOR

Broadcast medium Each packet is broadcasted, for free, in a small neighborhood around its path

Coding opportunity Each node stores the overheard packets for a short time Each node tells its neighbors which packets it has heard by annotating the packets it sends

Opportunistic coding When a node transmits a packet it uses its knowledge of what its neighbors have heard

COPE Overview Opportunistic Listening COPE sets all nodes in promiscuous mode Nodes snoop on all communications over the wireless medium Store the overheard packets for a limited period of time T (default T=0.5 s).

COPE Overview Each node broadcasts reception reports to tell its neighbors what packets are stored Reception reports are sent by annotating the data packets the node transmits A node that has no data packets to transmit periodically sends the reception reports in special control packets

Opportunistic coding The key question is what packets to code together to maximize throughput The node should maximize the number of native packets delivered in a single transmission, while ensuring that each intended nexthop has enough information to decode its native packet Use a simple heuristic We analyze this problem in the second part of the talk

Learning Neighbor State How does a node know what packets its neighbors have? By using reception reports Problems At times of sever congestion, reception reports may get lost in collisions At time of light traffic they can arrive too late A node cannot relay on reception reports and may need whether a neighbor has a particular packet May need to guess whether a neighbor has a particular packet

Intelligent guessing Leveraging the routing protocols Routing protocols compute the delivery probability between every pair of nodes This information needed for identifying good paths Occasionally, a node can make an incorrect guess

Coding gain Defined as a ratio between the number of packets required by the current non-coding approach to the minimum number of transmissions used by COPE The gain is equal to 1.33 in the Bob and Alice experiment Relay Requires 3 transmissions XOR

Coding+MAC gain Experiments show that the throughput improvement sometimes greatly exceeded the coding gain The interactions between coding and MAC produces a beneficial side effect For example, in the Bob and Alice example, the MAC divides the bandwidth equally between the three users However, the router needs to transmit twice as many packets Halve the packets transmitted by the edge nodes are dropped at the router queue

Coding+MAC gain COPE allows the router to XOR packets and drop them twice as fast Coding and MAC gain is higher for many topologies

Practical considerations Never delay a packet Transmit when a wireless channel is available If no coding opportunities exists, do not wait for the arrival of a new packet. Let the node to opportunistically overload each transmission with additional information

Practical considerations (cont.) Dealing with packets of different lengths Give preference to XORing packets of the same length Coding small packets with large packets reduces bandwidth savings Empirical studies show that the packet distribution in the Internet is bimodal with picks at 40 and 1500 bytes Dealing with packets of two different sizes

Practical considerations (cont.) Internet packet size distribution

Practical considerations (cont.) Packet reordering We would like to limit reordering packets from the same flow Because TCP considers it as a congestion signal Consider packets according to their order in the queue Reordering can arise because of the need to retransmit packets which were lost in quessing COPE has a module that puts TCP packets in order before delivering them to the transport layer

Pseudo-broadcast has two modes: broadcast and unicast Broadcast mode cannot be used because of poor reliability and lack of backoff Unicast mode: retransmission of the packet until a synchronous ack is received Solution: unicast packets which are intended for broadcast COPE require each nexthop node to ack the packet

Packet format Packet ID- a 32-bit hash of the packet's source IP address and IP sequence number

Performance Large Scale Experiment 20 nodes 2 floors Pick sender and receiver randomly Packet size based on actual measurement Flow arrival are Possion

Large Scale Exp. Offered load in Mb/s Scare Coding Opp. At Low Demands Demand Up / Congestion Up / Gain Up

Findings Network coding does have practical benefits, and can substantially improve wireless throughput. When the wireless medium is congested and the traffic consists of many random UDP flows, COPE increases the throughput by 3-4x. Hidden terminals create a high collision rate that cannot be masked even with the maximum number of retransmissions. In these environments, TCP does not send enough to utilize the medium, and thus does not create coding opportunities.