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 ?
- Expense: ATM switches and NIC much more expensive than Ethernet: existing Ethernet equipment in IP networks could not be reused
- 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
- Cell overhead: the large ATM cell header provides a big overhead
- 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 ?
- The host requiring a connection sends a connection request to first ATM switch
- This ATM switch then finds path to destination and forwards request to all switches along path
- Each pair communicates with the next in sequence on path to choose matching VPI/VCI and store in respective tables.
- Only if all switches agree to establishing the SVC, is successful acknowledgement provided, otherwise request denied.
- 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
- Faster forwarding because of indexing in place of routing table lookup
- 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.
- 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
