  Recent studies on the practice of shaping subscribers' traffic by ISPs give a
new insight into the actual performance of broadband access networks at a
packet level. Unlike metro and backbone networks, however, access networks
directly interface with end-users, so it is important to base the study and
design of access networks on the behaviors of and the actual performance
perceived by end-users. In this paper we study the effect of ISP traffic
shaping using traffic models based on user behaviors and
application/session-layer metrics providing quantifiable measures of
user-perceived performance for HTTP, FTP, and streaming video traffic. To
compare the user-perceived performance of shaped traffic flows with those of
unshaped ones in an integrated way, we use a multivariate non-inferiority
testing procedure. We first investigate the effect of the token generation rate
and the token bucket size of a token bucket filter on user-perceived
performance at a subscriber level with a single subscriber. Then we investigate
their effect at an access level where shaped traffic flows from multiple
subscribers interact with one another in a common shared access network. The
simulation results show that for a given token generation rate, a larger token
bucket provides better user-perceived performance at both subscriber and access
levels. It is also shown that the loose burst control resulting from the large
token bucket --- up to 100 MB for access line rate of 100 Mbit/s --- does not
negatively affect user-perceived performance with multiple subscribers even in
the presence of non-conformant subscribers; with a much larger token bucket,
however, the negative effect of non-conformant subscribers on the
user-perceived performance of conformant subscribers becomes clearly visible
because the impact of token bucket size and that of token generation rate are
virtually indistinguishable in this case.
