Enhancing LAN Efficiency with Switches and Hubs: A Guide to Spanning Tree and ATM Design Considerations

1. Lecture 13 Extended LANs and ATM Homework 3.1, 3.3-3.7, 3.9-20

2. Switches and Hubs • Switches • Only forward traffic to a network containing the addressee • Hubs • Forward traffic to everyone • Switched networks scale much better

3. Broadcast and Multi-cast on Switched Networks • Broadcast address-how’s it different from ethernet? • Multi-cast address-

4. LAN Switches and Bridges (Datagram)

5. Bridges Learn the Port of Each Host! • Hosts must sign in with the bridges to build the routing table—but this is transparent to the users • Routing table starts empty • Addresses not in the table are handled as broadcasts • The first message you send sets up your entry (address and port) in the routing table

6. Multicast • Bridges have a jointly-held multi-cast address • Members of a multi-cast address send a message using their multicast address to the bridges’ multicast address

7. An Extended LAN with loops • Broadcast packets loop endlessly

8. Spanning Tree-Dealing with loops • Idea is to temporarily deselect redundant ports to eliminate loops • Bridges are labeled: B1, B2, B3 etc • Bridge with smallest label becomes the root bridge • Each bridge computes distance to the root for each port—number of hops • Bridge on each LAN with smallest distance to the root becomes a designated bridge-using the port closest to the root

9. How spanning tree eliminates loops in an extended LAN

10. Running Spanning Tree • Bridge maintains dynamic values of : • the root id • the minimum distance to the root (in hops) • These numbers are determined by exchanging configuration messages (X,d,Y) • Y=myid • d=minimum distance to root • X=rootid

11. Initially each bridge thinks itself to be the root and sends----(myid, 0, myid) • Update and resend (incrementing d) if • Receive a message with a smaller rootid • Receive a message with a smaller distance to root • Receive a message with same rootid and distance but smaller sender id

12. Do not forward traffic from Port A to Port B if a message arrives on Port A with a smaller distance to the root than Port B. (remember ties are broken using smallest ID)

13. Spanning signaling-an example

14. Limits of Extended LANs • Spanning tree becomes inoperable for more than 10’s of LANs • Broadcast traffic becomes a burden • VLANs can be used to break up an extended LAN into logical domains • Security

15. Cell Switching (ATM) • Fixed Size Cells-53 bytes • 48 bytes of data • 5 bytes of header • Address formats-Network Service Access Point (SAP) or E.164 formats

16. Fixed Cells- Facilitate hardware switches Facilitate parallel switching-scalable Small Cells Reduced delay for unloaded switches Finer queue control-QOS Variable Cells Minimize overhead Large Cells Minimize overhead ATM Design Considerations

17. ATM and Telephone Traffic • 8 bit sampling at 8KHz • 125msec/ 1 Byte sample • Time to accumulate a N byte cell is N*125 msec • 48 byte cell takes 6 msec

18. ATM Header • Generic Flow Control • Virtual Circuit Identifier • VPI –Virtual Path Identifier • VCI-Virtual Channel Identifier • Type • 1 yy network signaling • 0 xz • x=EFCI-congestion • z=user signaling • Cell Loss Priority • Header Error Correction

19. Segmentation and Reassembly (SAR)—Application Adaptation Layer (AAL)

20. Segmentation and Encapsulation for AAL 3/4

21. ATM Adaptation Layer 3/4 Packet Format • Convergence Sublayer Protocol Data Unit (CS-PDU) • ATM Cells