Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

Extending Seamless IP Multicast Edge-Coverage 938000007000001-hop-750kMMARP-750k1-hop-375kMMARP-375k1-hop-187kMMARP-187k600000Bandwidth (bps)50000040000030000020000010000005 10 15 20 25 30 35 40 45Distance (m)Fig. 4. Effective data rate achieved at increasing distanceAs it is shown in Fig. 4, both approaches are able to deliver the transmittedbandwidth at short distances. For those cases the link-layer contention is lowand the signal quality is good enough.In the single-hop trials, due to the degradation of the signal strength withincreasing distance, the achieved bandwidth is lower as the distance increases.This is clearly assessed both for the achieved bandwidth and the packet deliveryratio in the ‘1-hop’ cases of Fig. 4, and Fig. 5. These results are basically theexpected behaviour as long as it is commonly known that (particularly in indoorscenarios) the signal strength usually decreases at a rate inversely proportionalto d 2 , d 3 and even in some cases d 4 in really bad indoor conditions. In our case,it is clearly shown that the bandwidth and packet delivery ratios rapidly drop tozero for distances around 30 m and beyond. As expected for the 1-hop dedicatedlink, given a fixed distance, the difference between the achieved bandwidth andthe one being used at the source is bigger at higher data rates.In the case of the multihop MMARP-based multicast ad hoc access network,it can be noticed that the performance at increasing distances degrades muchslower than 1/d 2 . This is because the average distance in each of the intermediatehops is lower than in the single-hop trial. Thus, the mean signal strength is higherand the achieved bandwidth and packet delivery ratio are higher as well.However, as Fig. 4 shows, MMARP only manages to achieve a 100% deliveryratio at distances at which only one or two of the MMARP nodes are needed.At a distances higher to 30m some packet losses come up. These losses aremainly due to the well-known hidden terminal problem which happens amongthe nodes MMARP 1, MMARP 2 and MMARP 3. As long as IEEE 802.11bdoes not implement layer 2 acknowledgements of multicast frames (as it does forunicast traffic), each time a collision happens the packet is lost.It is also particularly noticeable that the trial with the higher bandwidth inthe multihop case performs much worse than the others. This is because, dueto contention, the effective bandwidth, even in the ideal case, is lower than the