Multiplex communications – Channel assignment techniques – Using time slots
Reexamination Certificate
1999-10-15
2004-09-07
Chin, Wellington (Department: 2664)
Multiplex communications
Channel assignment techniques
Using time slots
C370S431000, C370S337000
Reexamination Certificate
active
06788702
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the scheduling of transmissions without collisions in ad hoc computer networks with radio links, in which routers can have both hosts and networks attached to them.
BACKGROUND
Ad-hoc networks (i.e., multi-hop packet radio networks) are an ideal technology to provide a seamless extension of the Internet to the wireless mobile environment. In ad-hoc networks, nodes (e.g., stations or packet radios) can be mobile and communicate with one another either directly or through intermediate nodes, without relying on any preexisting network infrastructure. The self-configuring, dynamic-connectivity, multihop-propagation and fully-distributed nature of ad-hoc networks makes them very attractive for many new applications but also introduces difficult problems at the link and network layer.
Many medium-access control (MAC) protocols have been developed for wireless networks. The carrier-sense multiple access (CSMA) protocol was the first to be used in multihop packet-radio networks. A limitation of CSMA in multihop networks is that sources hidden from one another cannot detect their respective transmissions, which degrades CSMA's performance to that of the pure ALOHA protocol. F. A. Tobagi and L. Kleinrock, “Packet switching in radio channels: Part II—the hidden terminal problem in carrier sense multiple-access modes and the busy-tone solution,” IEEE Trans. Commun., vol. COM-23, no. 12, pp. 417-33, 1975. Many MAC protocols have been proposed and implemented to solve the hidden-terminal problems of CSMA.
The hardware characteristics of packet-radios are such that a packet-radio cannot transmit and listen to the same channel simultaneously; therefore, collision detection (e.g., CSMA/CD as described in R. M. Metcalfe and D. R. Boggs, “ETHERNET: Distributed packet switching for local computer networks,” Communications of the ACM, vol. 19, no. 7, pp. 395-403, 1976) cannot be used in a single-channel packet-radio network. The throughput of CSMA protocols is very good, as long as the multiple transmitters within range of the same receivers can sense one another's transmissions. Unfortunately, “hidden terminal” problems (see Tobagi et al., supra) degrade the performance of CSMA substantially, because carrier sensing cannot prevent collisions in that case.
The busy tone multiple access (BTMA) protocol, id., was the first proposal to combat the hidden-terminal problems of CSMA. BTMA is designed for station-based networks and divides the channel into a message channel and the busy-tone channel. The base station transmits a busy-tone signal on the busy-tone channel as long as it senses carrier on the data channel. Because the base station is within a line of sight to all terminals, each terminal can sense the busy-tone channel to determine the state of the data channel. The limitations of BTMA are the use of a separate channel to convey the state of the data channel, the need for the receiver to transmit the busy tone while detecting carrier in the data channel, and the difficulty of detecting the busy-tone signal in a narrow-band channel.
A receiver initiated busy-tone multiple access protocol for packet-radio networks has also been proposed. C. Wu and V. O. K. Li, “Receiver-initiated busy-tone multiple access in packet radio networks,” ACM SIGCOMM 87 Workshop: Frontiers in Computer Communications Technology, Stowe, Vt., USA, Aug. 11-13 1987. In this scheme, the sender transmits a request-to-send (RTS) to the receiver, before sending a data packet. When the receiver obtains a correct RTS, it transmits a busy tone in a separate channel to alert other sources nearby that they should back off (i.e., refrain from transmitting). The correct source is always notified that it can proceed with transmission of the data packet. One of the limitations of this scheme is that it still requires a separate busy-tone channel and full-duplex operation at the receiver.
Several protocols have been proposed based on three-, four- or even five-way “collision-avoidance” handshakes done with small control packets and meant to avoid data collisions when sources of data packets cannot hear one another. The collision-avoidance approach in each of these schemes follows the basic philosophy first introduced by Tobagi and Kleinrock in SRMA (Split-Channel Reservation Multiple Access). F. A. Tobagi and L. Kleinrock, “Packet switching in radio channels: Part III—polling and (dynamic) split-channel reservation multiple access,” IEEE Trans. Commun., vol. COM-24, no. 8, pp. 832-845, 1976. In SRMA, and subsequent collision-avoidance protocols, a sender node sends a request-to-send (RTS) packet to the intended receiver, either sensing the channel before sending the RTS or not sensing the channel before the RTS transmission. A receiver that hears a clean RTS responds with a clear-to-send (CTS), and the sender can send a data packet after hearing a clean CTS. Surprisingly, Tobagi and Kleinrock's work on SRMA is not referenced by most subsequent proposals based on collision avoidance.
Other collision-avoidance schemes that have been proposed include the following: U.S. Pat. No. 5,319,641 discloses a method to improve CSMA p-persistent protocols by introducing a random waiting time that stations must wait listening to the channel once they have packets to send. The method disclosed does not work in networks with hidden terminals. U.S. Pat. No. 4,661,902 discloses a method that amounts to an implementation of SRMA over a single channel in which stations use carrier sensing before sending RTSs. P. Karn, “MACA—a new channel access method for packet radio,” in ARRL/CRRL Amateur Radio 9th Computer Networking Conference, pp. 134-40, ARRL, 1990 discloses a technique that amounts to SRMA running over a single channel in which a request-to-send (RTS) packet is sent without carrier sensing. Karn includes no description of how to support packet trains, however. U.S. Pat. No. 5,231,634 discloses a method that also applies SRMA's basic approach over a single channel. The RTS specifies the length of the impending data packet.
U.S. Pat. No. 5,502,724 discloses a method that extends the collision avoidance handshake to allow for multiple data packets to flow among a pair of communicating stations. A station that intends to establish a connection with a second station senses the channel. If the channel is idle, it sends a connection request (CR) packet to the intended receiver station. The CR specifies the number of data packets that the connection includes. The intended receiver sends a connection confirm (CC) packet to the sending station; the CC also specifies the number of packet in the connection. After the exchange of correct CR and CC packets the sending station may send one or multiple data packets and the receiving station may send an acknowledgment packet specifying which data packets were received correctly. To end the connection, the sending station sends a disconnect request (DR) and the receiving station issues a disconnect confirm (DC). Stations that receive a CR packet back off for an amount of time that is long enough for the advertised number of data packets to be sent to the receiver. After receiving a CR or CC, a station can attempt to access the channel when a timer proportional to the number of packets to be sent in the connection expire, or when it receives a DR or DC packet.
Some limitations with the method disclosed in U.S. Pat. No. 5,502,724 are that the method cannot ensure collision-fee transmissions of data packets, even with the transmission of CC packets by the receiver. The need for feedback from the receiver to its neighbors on a packet by packet basis was demonstrated by C. L. Fullmer and J. J. Garcia-Luna-Aceves in “Solutions to Hidden Terminal Problems in Wireless Networks”, Proc. ACM SIGCOMM 97, Cannes, France, Sep. 14-18, 1997. Because the CC packet sent by the receiver may collide with another packet at a neighbor of a receiver, the CC packet does not provide sufficient feedback to hidden nodes; furthermore, the need for feedback packets to be
Beyer David A.
Fullmer Chane L.
Garcia-Luna-Aceves J. Joaquin
Chin Wellington
Fox Jamal A.
Nokia Wireless Routers, Inc.
Squire Sanders & Dempsey L.L.P.
LandOfFree
Protocol for neighborhood-established transmission scheduling does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Protocol for neighborhood-established transmission scheduling, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Protocol for neighborhood-established transmission scheduling will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3207365