Solutions to Chapter 4 - Communication Networks

Communication Networks (2 nd Edition)
Chapter 4 Solutions
which is 4x 1/3 x 80% STS-3 or 16/15 STS-3. Adding the other 20% of the traffic which is
destined to the higher tier link and has to traverse some of the lower tier links in both directions,
the average link capacity in the lower tier links should be 16/15 STS-3 + 2 x 20% STS-3 or 22/15
STS-3, which is almost 1.5 STS-3 on average.
On the higher tier ring, the traffic from each subring will be routed equally to each other subring.
Thus the bandwidth required on each link will be four times the average traffic between two
subrings or 4 x 1/3 of the total traffic from each subring. Because each ring produces 4×0.2 ×
STS-3 amount of traffic, the bandwidth required on each link of the higher tier ring is on the order
of 4/3 × 0.8 × STS-3 ≈ STS-3.
b. Discuss the bandwidth requirements that are possible if 80% of the traffic generated by each
site is destined to sites in other rings.
If 20% of the traffic is to stay within the same ring, then the required bandwidth on the higher tier
ring will be increased. The required bandwidth on the lower tier rings will not change significantly,
as each source still produces and receives STS-3 of traffic.
The higher tier ring will now support 4 times more traffic. Thus, the bandwidth requirement is
around STS-12.
15. Compare the efficiency of BLSR and UPSR rings in the following two cases:
a. All traffic originating at the nodes in the ring are destined for a given central node.
Considering that the total capacity of working and protection lines in both BLSR and
UPSR is the same, the total bandwidth utilized in both BLSR and UPSR in this scenario is
the same. The difference is the distribution of the traffic and unused capacity across the
ring in the two cases.
As an example we consider the ring shown in the figure below, consisting of four nodes.
We first consider the UPSR scenario. The last hop before the central node is the
bottleneck in this case limiting the traffic in the other hops on the working ring shown with
solid lines. Assuming equal distribution of bandwidth among the nodes, if the last hop is
fully utilized the hop prior to the last hop will be utilized up to 2/3 of its capacity and the
one before that will be utilized up to 1/3 of its capacity. The remaining hop will not be
utilized at all. The lines on the protection path will be utilized similarly but in the opposite
order, shown with dotted lines. As a result the sum of working and protection lines
together will be utilized only to half of the total capacity all across the ring.
UPSR
BLSR
Leon-Garcia/Widjaja 8