Lecture 12 Chapter 5 Access Network Design

1. Lecture 12 Chapter 5Access Network Design

2. Overview • A Backbone network connects major sites. • Access networks connect “small” sites to the backbone network. • Access networks are the “ends” and “tails” of networks that connect the smallest sites to the network. • Access networks only function if they are attached to a backbone.

3. Access Network Example Visit to Grandma • You live in Scarsdale, NY outside of New York City • Grandma lives in Framingham, MA, outside of Boston • The trip involves 3 segments: • Travel from Scarsdale to the interstate by local access roads • Then traverse the Interstate backbone to the location closest to Framingham • Then you travel on local roads again to get to Grandma’s house.

4. Access Network Example cont’d • Backbone/access division is efficient for cars and it is efficient for telecommunications networks. • Access networks collect traffic from small sites into the high speed backbone network. • Sharing high speed links, yields the benefit of economy of scale. • Local access may represent most of the total network cost • Examples of expensive local access networks • Lottery network • ATM network • Insurance companies.

5. A Simple Access Design Problem • A problem with 6 access locations and 1 backbone site. Traffic is Symmetric and shown at right. • Using Leased Lines cost=$400 plus $3.00/km/monthfor the first 300km and a cost of $1.75/km/month after 300km

6. A Simple Access Design Problem Cost Matrix for 56Kbps Lines

7. Star • Cost=$9650; Maximum Utilization=23.2%

8. One Concentrator • N2 serves as a concentrator for N6 and N7. • Shorter less expensive links are used. • Cost=$8660; Maximum Utilization=46.4%

9. Two Concentrators • N2 for N6 and N7; N4 for N3. • Cost=$8158; Maximum Utilization=46.4%

10. Final Design • Choose N7 as concentrator instead of N2. Is also MST • Cost=$7659; Maximum Utilization=46.4%

11. MSTs Are Not Always Optimal Access Designs • When traffic grows 50%, MST costs $10,616 and the links to concentrators N4 and N7 must have two links to keep utilization below 50%.

12. Backbone Sites • Definition 5.1: Given a set of sites Ni and traffic matrix T(i,j), weight(Ni)=Sj(T(i,j)+T(j,i)). • Design Principle 5.2 Compute the weight of all the nodes to determine if there are natural traffic centers or if the network is flat. • Design Principle 5.3: It is acceptable for small nodes to route their traffic via big nodes, but generally we do not want to route the traffic between big nodes via the small nodes.

13. OR OR Problem: Connecting sites to one backbone node, all links with the same capacity One-Speed One-Center Design

14. One-Speed One-Center Example • Problem: Connect a large number of sites to a hub • 19 nodes that are to be connected to a hub • N14 is the hub location • 4 sites can share a line • Traffic to and from each node Ni is 1200bps • Capacity of the links is 9600bps • Limit the utilization to %50

15. SPT(Star) • Cost= $26358 • Very low link utilization and expensive

16. MST • Cost= $18,730 • More cost effective but may have higher delays

17. Prim-Dijkstra with a=0.3 Cost= $15,930.

18. Exhaustive Search • Cayley’s Theorem: Given n nodes, there are nn-2 different spanning trees. • For 20 nodes, there are 2018=2.621*1023 different trees.