FrameworkInside&OutApplication

• ATM Quick Review1. Understand why ATM is a good candidate2. Are you on the right way?(Don’t just think about ATM)

• Tag Switching On ATM 1. How does it work over ATM? 2. Are Tags over ATM panacea?• Conclusions

ATM Review

• Packet Networks(IP pushing routers) 1. User data+overhead bits (routing,flow,error)

2. Packets: protocol dependant(200—65,545bytes)

3. Results: larger overhead, smaller user data

• Cell Networks(ATM Stressing switches)

1. User data(in form of cells)+a small header

2. Cells: use ATM switches (53bytes)

3. Results: Fast and efficient switching

ATM Review Cont…

• ATM is great:1. ATM network=ATM switches+ATM endpoints

2. Well-defined job of ATM switches3. Three-layer architecture4. Virtual connections:

a. connection orientedb. two types of connections: VC and VP

c. switching refers to: VCI and VPI Q: IS ATM A GOOD CANDIDATE?(where to cut in?)

ATM Review Cont…

• Cutting-in Points for Tag switching:1. Fast forwarding—table lookup, similar to ATM2. Improved performance—upgrading ATM at low cost3. Easy network evolution—implemented in stages over ATM

ATM Review Cont…

• “Tags” control over all, once implemented?

NO!!

1.Traditional ATM is not impacted

(“ship in the night”)

2. No Problem with other network if properly configured and upgraded.

(“mixed network”)

Tag Switching over ATM

• How Tag switching works in ATM?

1. Implementing two components

2. Using VPI & VCI and input port# as index

3. Swap them off with outgoing VPI & VCI and output port#

4. You done! ATM switches are now ATM-LSRs

Tag Switching Over ATM Cont…

• Are “tags” in ATM are panaceas?(several points worth seriously considering……)

1.Time-to-Live (TTL)

2. “Hot-tag” problems

3. Hardware limitation

Tag Switching over ATM Cont…

• TTL—loop-related problem1. In IP: TTL decremented by 1/hop

2. ATM: tag values are encoded in data link layer(AAL header) and forwarded by layer-2 ATM switches(no TTL field)

3. Once loop forms, ATM efficiency lost!

Tag Switching over ATM Cont…

• “hot-tag”—resource problem1.Tags are resources, everybody wants them!2.What if lack of outgoing tags?

What if incoming tags “illegal”? What if tag assignments conflicting each other?

3.Possible solutions: ATM strip off the labels and use IP ATM could get confused and just discard

Tag Switching over ATM

• Hardware limitation—Encoding problem

ATM switchessay:don’t expect me do too much!

1.ATM switches are different from a “generic MPLS encapsulation” hardware

2.ATM switches have no capability for encoding translation(ATM network encoded the same way)

3.ATM switches VPI & VCI encoding cannot perform label merging( application limitation)

Conclusions

• Pushing and Stressing will continue… 1. Net has changed, fired by WWW

2. Random, any-to-any traffic and multimedia3. Who will win?

• Tag switching comes in and expanding…1. Faster forwarding 2. Improved scalability 3. Improved performance for low cost 4. Ease of network evolution