Introduction Topologies, Virtual Networks and Wireless Networking Overview In this section we are going to start looking at Topologies, Virtual Networks and Wireless Networking. With each we are still at the link layer and in line with what I said last week if you think in terms of the properties of the media and transmission you will better understand these areas. To explore Topologies and Virtual Networks we will look at Ethernet. It has evolved over many years and has become the primary LAN technology. I want you to think back to the physical layer classes for a minute. If I tell you that most LANs use digital transmission and they have multiple machines all accessing the same media than you can make some guesses for how the LAN protocols will look. Knowing that you have multiple stations and digital transmission you should immediately know that LANs will have some sort of Time Division Multiplexing (TDM) mechanism. This is in fact the case and is the main design difference between different LAN Media Access Control protocols. Ethernets Media Access Control (MAC) method is CSMA/CD or Carrier Sense Multiple Access with Collision Detection. It is a relatively simple MAC where a station listens to the media when it wants to send a message and if it is free it does so. If it is busy it has to wait. Knowing that messages take some time (propagation delay) it has to listen to the media while it is sending a message because another station might have seen the free media and decided to send around the same time. While the stations are sending their message if another message they did not see in time comes down the wire both messages are destroyed. Both stations will recognize this happened and send a jam signal. Each station sets a random timer and backs off for that time to retry later. It is a non-deterministic system as a station does not get priority if it has had a collision and might keep colliding. The non-deterministic feature of Ethernets CSMA/CD is a problem for applications that are latency sensitive like streaming media. Initially, a major knock on Ethernet in its early form was that it was not suitable for running these type of applications. There were other LANs standards that came along like FDDI, ATM, and Token Ring that had priority mechanisms built in that made a case that they were better. Yet Ethernet has beaten back the competition and really today there is no other LAN standard. In Activity 1 we will explore how Ethernet is able to support these applications with CSMA/CD. To explore Virtual Networks I want you to consider that the Ethernet protocol has multiple stations with a many to many transmission system. Since there are many stations sharing the media they might be able to see others transmissions. This can be a problem for security. Activity 2 will talk about how an Ethernet LAN can be logically segmented on the physical hardware. With Activity 3 we will support Wireless networks. Wireless networking can be seen in multiple forms with two main ones being the Wireless Access Point(WAP) around the 802.11x protocols where a client accesses a fixed WAP and the other being cell wireless networks. This would be your smartphone technology. In our activity we will explore the WAPs.

Activity 1 Topologies Ethernet is by far the most popular LAN architecture. It has proven to be flexible and inexpensive. Lets explore the Media Access Control Protocol for Ethernet (CSMA/CD) and some of its effects on the standard. CSMA/CD is not an effective MAC for handling latency intolerant applications like streaming media. Yet Ethernet is used all the time for handling this type of traffic today? How can this be?

Activity 2 VLANs CSMA/CD is a simple and cost effective MAC. It works. But the problem is that all stations on the same Ethernet network may be able to see other stations traffic. This is definitely a bad thing when it comes to security. What mechanism was added to Ethernet to support segmentation without having a separate physical network? Could this work over the older non-switch wiring method (ie 10Base5)? Why or why not?

Activity 3 Wireless Wireless Access standards like 802.11x also have to share a media but unlike Ethernet they are sharing a frequency over the air. Because they are sharing a frequency they also need to have a Time Division Multiplexing method to arbitrate access to the media and they rely on CSMA/CA. This is like Ethernet CSMA/CD except with wireless they can only do collision avoidance. They cannot do collision detection. Recall that with Ethernet stations are all on the same wire and the standard makes sure that the cable is short enough that all transmissions can be seen. That is a key point as the CSMA/CD algorithm relies on all stations being able to see everyone else transmission. They must be able to hear all transmissions so that collisions can be detected. Can you think why a wireless standard like 802.11x has to use collision avoidance?