Download Solution - Dr. Wissam Fawaz

LEBANESE AMERICAN UNIVERSITY
Electrical and Computer Engineering Dept
COE 799
Optical Networks
Fall 2011
W. FAWAZ
Assignment I (Solution Key)
Problems
a. The user data rate for OC-3 is stated to be 155.52 Mbps. Show how
this number can be derived from the SONET OC-3 parameters.
b. To accommodate lower data rates than STS-1, SONET has a system of
virtual tributaries (VT). A VT is a partial payload that can be inserted
into an STS-1 frame and combined with other partial payloads to fill
the data frame. VT1.5 uses 3 columns, VT2 uses 4 columns, VT3 uses
6 columns, and VT6 uses 12 columns of an STS1 frame. Which VT can
accommodate: 1) A DS1 service, 2) European CEPT-1 service, and 3)
A DS-2 service
c. What is the available user bandwidth in an OC-12c connection.
d. Let us assume that the routing tables in an IP administrative domain
remain unchanged for a long time. In this case, the path between two
computers, say A and B, within the same domain will not change.
Therefore, all of the packets from A to B will follow the same path. Can
we say that during that time, the IP network behaves like a
connection-oriented packet-switching network? Why?
e. Explain how bursty data transmission over a circuit-switched network
leads to a poor utilization of the connection?
Solution
a. The SONET STS-3 frame consists of 810 x 3 bytes and is
transmitted 8000 times per second. This gives a total data rate
of 810 x 3 x 8000 x 8 = 155.52 Mbps.
b. VT1.5 is the virtual tributary that carries one DS1 signal. VT2 is
the one used to carry an E1 signal. Finally, VT6 transports a DS2
signal.
c. An STS-12c frame has twelve STS-1 frames with 3 columns
overhead for each STS-1 frame. Thus, the STS-12c payload = 90
* 12 * 9 – 3 * 12 * 9 = 9396 bytes. We subtract 1 column of
POH so that at the end we have: 9396 – 1*9 = 9387 bytes sent
every 125 μs. That translates into a user bandwidth of: 600.768
Mbps.
d. In a connection-oriented packet-switched network. All packets
associated with a connection follow the same path. In the given
scenario the IP network can be visualized as a connectionoriented packet-switched network. However, in a connection
oriented packet-switched network, a connection is associated
with a certain level of quality of service, whereas in the IP case
there is no quality of service guarantees.
e. Circuit switching involves three phases: circuit establishment,
data transfer and
circuit disconnection. In circuit switching,
channel capacity is dedicated for the entire duration of the
connection even when no data is being sent. Circuit switching,
therefore, is not a good choice of solution for bursty data
transmission. The source emitting the data is actively
transmitting for a period of time and then becomes idle and it
does not send any data. This cycle of being in active and silent
states repeats until the source complete its transmission. In
such cases, the utilization of the circuit-switched connection is
low because the link is not utilized during the silent periods.
Packet switching is appropriate for bursty traffic, since packets
emitted from various sources all share the same links.