Point 1 above is the most crucial driver. To further explain its effect, if we inspect
Figure 13.10 in more detail, we see that each AR is dual-homed (either via
intra-office fiber or via transport over the OL) to two BRs. There are a variety of
architectural options to accomplish this. (1) There is just one BR per backbone CO.
In this case, for each AR co-located with a BR, it uses intra-office fiber to link to the
BR. The link to the mated BR must be transported over the other network transport
layers (IOS, SONET ring, WDM, etc.). A remote AR (an AR not co-located with
any BR) would use network transport to connect to each of two BRs. (2) Alter-
natively, there are dual-BRs in each backbone CO. Here, a co-located AR would use
intra-office fiber