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Chapter 5Upstream channel capacity and characterisation pHowever, with higher offered loads (above ≈ 47% of the cc), short packets cause morerequests to be sent in the contention access region, resulting in more collisions (lessthroughput). This effect is better appreciated in Figure 5.22. This figure shows thepercent of the bandwidth wasted by reservation requests, which resulted in collision. Itis evident that with a 64-byte packet size, as the offered load increases, a considerableamount of bandwidth is wasted. For instance, with offered loads over 30% of the cc,approximately up to 8% was consumed only by collisions, which reduced considerablythe bandwidth available for data packets. As a result of this reduction in the reservationaccess region, the stations are unable to transmit all data packets due to the high risk ofcollision among reservation request, which cause the station’s buffers to build up almostunbounded. Thus, very large packet access delays are evident with an offered load aslow as 28% of the cc (produced by 27 stations) as indicated in Figure 5.21.With a 128-byte packet size, there was also a sizeable amount (up to ≈ 4%) wasted bycollisions. However, with large packet sizes, (e.g. 1024 and 1518 bytes) the bandwidthwasted by collisions only represented a tiny portion of the link capacity (under 0.05% ofBandwidth Comsumed by CollidedContention Slots (%)987654321Pk_Size : 64 bytesPk_Size : 128 bytesPk_Size : 256 bytesPk_Size : 512 bytesPk_Size : 1024 byte sPk_Size : 1518 byte s3 Mbps Upstream32 Kbps kbps isochronous Isochrous streamsTree Algorithm018 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69umbe r of Active StationsFigure 5.22 – Bandwidth consumed by collisions vs. No. of active stationsfor increased packet size.5-34