UCEN 3133 Advanced Computer Networks - Tutorial 1

1. Explain how connection oriented switching (as exemplified by technologies like ATM or MPLS) provides an advantage of speed over connectionless packet switching technologies (such as TCP/IP) ?

Connection oriented switching uses the label on a packet as an index into an internal table to forward the packets onwards. The process of indexing into a table is faster than searching a table (which is the mechanism used in routing), particularly when the indexing can be implemented in hardware

 

2. Describe briefly 4 underlying reasons why ATM was not adopted on a larger scale. What factor makes MPLS more attractive to a broader adoption in comparison to ATM ?

1. Expense: ATM switches and NIC much more expensive than Ethernet: existing Ethernet equipment in IP networks could not be reused
2. Connection set up latency: Since ATM is connection oriented, the packet request to set up a PVC has to pass through many switches before acknowledgment
3. Cell overhead: the large ATM cell header provides a big overhead
4. Specification of QOS requirements difficult: not all applications know beforehand the QOS that they require during a session.

MPLS was designed to build on top of the existing IP infrastructure (particularly layer 2 – Ethernet), rather than replace it completely – which makes it easier for companies with legacy IP infrastructure to migrate.

3. Explain how a Switched Virtual Circuit (SVC) established in an ATM network ?

1. The host requiring a connection sends a  connection request to first ATM switch
2. This ATM switch then finds path to destination and forwards request to all switches along path
3. Each pair communicates with the next in sequence on path to choose matching VPI/VCI and store in respective tables.
4. Only if all switches agree to establishing the SVC, is successful acknowledgement provided, otherwise request denied.
5. The request setup SVC is done through signalling control and request messages, sent across reserved connections for control traffic

4. Briefly describe 3 advantages offered by switching across an IP infrastructure

1. Faster forwarding because of indexing in place of routing table lookup
2. Aggregated route information.  IP routing table look up once when packet arrives at edge router in ISP, and packet is assigned a label for further forwarding.
3. Manage aggregate flows through a Service Level Agreement (SLA) easier with labels. Each label is equivalent to the Forward Equivalence Class (FEC)

5. Consider 2 hosts, X and Y,  in an ATM network with 3 intermediating ATM switches, A, B and C between them. The order of connection in a newly formed PVC is X -> A -> B-> C-> Y. Given below are the switching tables for all 3 switches:

Switch C

Old VPI/VCI	Interface	New VPI/VCI
0	0	5
1	0	3
2	1	2
3	1	4
4	2	1
5	0	0

Switch B

Old VPI/VCI	Interface	New VPI/VCI
0	1	0
1	0	5
2	0	4
3	0	1
4	2	2
5	0	3

Switch A

Old VPI/VCI	Interface	New VPI/VCI
0	0	5
1	0	1
2	1	3
3	2	2
4	0	4
5	1	0

Given that the VPI/VCI leading into Y has the value of 4, what is the value of the VPI/VCI at the initial end of X ?

Answer:

From Switch C table, the old VPI/VCI resulting in 4 going into Y is 3

From Switch B table, the old VPI/VCI resulting in 3 going into Switch C is 5

From Switch A table, the original VPI/VCI resulting in 5 going into Switch B is 0

Therefore, initial end of X has VPI/VCI of 0