Rodrigo Garcés

Collision avoidance and resolution multiple access for multichannel wireless networks

We introduce and analyze CARMA-MC (for collision avoidance and resolution multiple access for multiple channels), a new stable channel access protocol for multihop wireless networks with multiple channels. CARMA-MC relies on the assignment of a unique channel and a unique identifier to each node to support correct deterministic collision resolution in the presence of hidden terminals. CARMA-MC dynamically divides the channel of each node into cycles of variable length; each cycle consists of one or more receiving periods and a transmission period. During the receiving period, stations with one or more packets to send compete for the right to acquire the floor of a particular receivers channel using a deterministic tree-splitting algorithm. Each receiving period consists of collision resolution steps. A single round of collision resolution (i.e., a success, and idle or a collision of control packets) is allowed in each contention step. The receiving period is initiated by the receiver and takes place in the channel assigned to the receiver station. The channel utilization and packet delays are studied analytically and by simulation.

Collision avoidance and resolution multiple access with transmission groups

We introduce a stable multiple access protocol for broadcast channels shared by bursty stations, which we call CARMA-NTQ (for collision avoidance and resolution multiple access with non-persistence and transmission queues). Like previous efficient MAC protocols based on tree-splitting algorithms (e.g., DQRAP), CARMA-NTQ maintains a distributed queue for the transmission of data packets and a stack for the transmission of control packets used in collision resolution. However, CARMA-NTQ does not require the mini-slots commonly used in protocols based on collision resolution. CARMA-NTQ dynamically divides the channel into cycles of variable length; each cycle consists of a contention period and a queue-transmission period. The queue-transmission period is a variable-length train of packets, which are transmitted by stations that have been added to the distributed transmission queue by successfully completing a collision-resolution round in a previous contention period. During the contention period, stations with packets to send compete for the right to be added to the data-transmission queue using a deterministic first-success tree-splitting algorithm, so that a new station is added to the transmission queue. A lower bound is derived for the average throughput achieved with CARMA-NTQ as a function of the size of the transmission queue and the number of queue-addition requests that need to be resolved. This bound is based on the upper bound on the average number of collision resolution steps needed to resolve a given number of queue-add requests.

A near-optimum channel access protocol based on incremental collision resolution and distributed transmission queues

We introduce a new stable multiple access protocol for broadcast channels shared by multiple stations, which we call the incremental collision resolution multiple access (ICRMA) protocol. ICRMA dynamically divides the channel into cycles of variable length; each cycle consists of a contention period and a queue-transmission period. The queue-transmission period is a variable-length train of packets, which are transmitted by stations that have been added to the distributed transmission queue by successfully completing a collision-resolution round in a previous contention period. During the contention period, stations with one or more packets to send compete for the right to be added to the data-transmission queue using a deterministic tree-splitting algorithm. A single round of collision resolution is allowed in each contention period. Analytical results show that collision resolution in ICRMA is much more efficient than DQRAPs. Simulation and analytical results show that ICRMAs throughput is within 5% of the throughput achieved by the ideal channel access protocol based on a distributed transmission queue and incremental collision resolution.

Mobile connectivity protocols and throughput measurements in the Ricochet Microcellular data network (MCDN) system

We describe the protocols implemented in the Ricochet MCDN system to provide continuous connectivity to mobile users traveling up to 70 mph. These protocols are general in nature for any frequency-hopping microcell-based system, particularly those that follow the FCC part 15.247 rules [9] and operate in unlicensed spectrum. We also present throughput measurements as a function of velocity and describe a model to predict those numbers based upon the protocols implemented. The MCDN system is a mesh-based system of microcells that are connected wirelessly to an interspersed mesh of wired access points (WAPs) that cover approximately 12 square miles on average [7]. The average microcell density is approximately 5-6 per square mile, with 3-8 overlapping cells at each point. Since the system is entirely packet-based, we have instantaneous hand-off between microcells as there are no complicated cellular-type negotiations for circuits required as all of the information needed to route the packet through the system is included in the header; however, when traveling through the mesh of microcells at a high rate of speed, the mobile unit must acquire new microcells fast enough to ensure continuous connectivity. The system must also know how to route packets to the mobile unit as it drops old cells and acquires new ones, as well as being able to contact a moving mobile unit. This paper discusses the acquisition, registration, and routing protocols that make this possible and reviews performance data of typical mobile users.

Collision avoidance and resolution multiple access (CARMA)

The collision avoidance and resolution multiple access (CARMA) protocol is presented and analyzed. CARMA uses a collision avoidance handshake in which the sender and receiver exchange a request to send (RTS) and a clear to send (CTS) before the sender transmits any data. CARMA is based on carrier sensing, together with collision resolution based on a deterministic tree-splitting algorithm. For analytical purposes, an upper bound is derived for the average number of steps required to resolve collisions of RTSs using the tree-splitting algorithm. This bound is then applied to the computation of the average channel utilization in a fully connected network with a large number of stations. Under light-load conditions, CARMA achieves the same average throughput as multiple access protocols based on RTS/CTS exchange and carrier sensing. It is also shown that, as the arrival rate of RTSs increases, the throughput achieved by CARMA is close to the maximum throughput that any protocol based on collision avoidance (i.e., RTS/CTS exchange) can achieve if the control packets used to acquire the floor are much smaller than the data packet trains sent by the stations. Simulation results validate the simplifying approximations made in the analytical model. Our analysis results indicate that collision resolution makes floor acquisition multiple access much more effective.