Some concerns have been raised if 802.11 Medium Access Control (MAC)
performs well enough in multi-hop ad hoc networks (Xu and Saadawi, 2001). One
possible improvement to loss of bandwidth in multiple hops is a combination of random
access and polling protocols. A combined random access and polling MAC layer has
separate channels for access and transfer of data, and consequently a node which has
gained access can transfer data over several hops without decreased bandwidth.
Goldsmith and Wicker (2002) suggest this approach for 802.11 ad hoc network
and mentions Packet Reservation Multiple Access (PRMA) (Goodman et al., 1989) and some other variants. One variant that is not mentioned in Goldsmith and Wicker (2002)
is Inhibit Sense Multiple Access with Polling (ISMA/P) (Chakraborty and Wager,
1999). The protocol is an overlay on Global System for Mobile Communications (GSM)
but the same method can be applied over any multiplexing scheme Time Division
Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) or Code Division
Multiple Access (CDMA) and bandwidth can be increased from that of ISMA/P. The
behavior of ISMA/P is analyzed in Jormakka (2003) and its behavior is well understood.
The study describes a new MAC protocol, Inhibit Sense Multiple Access/with
Reservation for Ad hoc networks (ISMA/RA ). It can be considered as a modification of ISMA/P.
The basic idea of ISMA/RA is to divide the time axis into slots. The TDMA
approach for ad hoc WLAN networks is not new, it is used, e.g., in HiperLAN/2, but the
solution and performance issues (as in Habeta et al., 2002) in HiperLAN are quite different than in ISMA/RA. |
