1. Ch. 15 Connecting LANs

2. 15.1 Connecting Device Five different categories of connecting devices
Passive hub, Repeater, Bridge, Router, Gateway

3. Passive Hubs and Repeaters
Connects wires
Can be considered as a part of transmission medium
Repeaters
Operates only in the physical layer
Connects “segments of a LAN” -- segments are considered as a single LAN
Forwards every frame by regenerating signals (not by amplifying)

4. Function of Repeater
Active hub = multiport repeater (often used to create a physical star topology)
Single LAN

5. Bridges Operate both in the physical and data link layer
Can check the physical (MAC) addresses
Decide if the received frame should be forwarded to the other side or dropped  Filtering

6. Transparent Bridges
Stations are completely unaware of the bridge’s existence
Frame must be forwarded  Forwarding
Forwarding table is automatically updated  Learning
Only one path exists between two stations  No looping

7. Learning

8. Loop Problem

9. Spanning Tree Loop-free graph that connects all stations
A LAN can be reached through one path only
Often, find a spanning tree that minimizes some cost (e.g., # hop distance, delay, bandwidth, etc)
Steps to find a spanning tree:
Elect a root station (e.g., station with smallest ID)
Find the shortest (= minimum cost) path from each station to the root
Based on the found paths, set forwarding ports and blocking ports
Can be found automatically by dynamic algorithm

10. Finding a Spanning Tree

11. Forwarding and Blocking Ports

12. Bridge An alternative way to prevent loop: Source routing
A sending station defines the bridges that the frame must visit
The addresses of all the bridges are included in the frame
Issues in bridging different LANs
Frame format
Frame size
Data rate
Bit order
Security
Multimedia support

13. Switches Two-layer switch (or Layer-two switch)
E.g., bridges
Three-layer switch (or Layer-three switch)
Often called “router”
Uses network layer info: logical address (IP)
Gateway
Some use gateway and router interchangeably
Gateway usually operates in “ALL layers” including application layer

14. 15.2 Backbone Networks A backbone network connects several LANs
Bus, or star backbone
Connecting remote LANs: use point-to-point links

15. 15.3 Virtual LANs Virtual local area network (VLAN) Advantages
A local area network configured by software (not by physical wiring)
VLANs group stations that connected to different switches, and create broadcast domain (or membership for grouping)
Stations can be re-grouped by software configuration
Advantages
Cost and time reduction
Creating virtual work groups
Security

16. Example

17. Homework
Exercise
12, 16, 20