CIS131 Chapter 06
From Cis131
Contents |
Summary - Reducing Congestion and Driving Faster on the Local (Network) Roadway
In this chapter we will look at a switch and the advantages of it over a hub. We will look at full duplex verses half duplex and list the different speeds that Ethernet supports.
Reducing Congestion by Opening Up More Lanes on Each LAN
LAN switches overcome the performance problems with hubs. We will look at how a switch is able to overcome problems associated with a hub.
Hubs: A One Lane Road
In Chapter 5 we learned that a hub is a device that repeats an electrical signal out all ports besides the one the signal was received on. If we combine this with CSMA/CD's requirement that a sending network device wait until the line is free before it transmits , this means that network devices have to wait until no one is talking on the network before they transmit data.
Switches: How to Create Dozens of Lanes on the LAN
Switches work differently then a hub. Instead of sending the electrical signal out all ports, it sends it out the port to which the destination network device is attached. This is called a forwarding decision where the switch looks at the destination MAC address in the frame header, compares the address to a switching table, or MAC address table that tells it which port the recipient network device can be located. It then sends the data out that port and only that port. This way if two network devices are communicating a third device isn't receiving the signal, so it is free to transmit.
The Perfect Roadway: No Wrecks Allowed
What happens if more then one network device is try to communicate with the same network device? This is common when you have a server, or network printer, that multiple access throughout the day. In this case the switch detects the possible problem and will store message in a buffer until it can reach the destination. A buffer is memory in the switch that is used to store frames temporally. This method prevents all collisions. You are not supposed to have collisions on a switch at all. Each port becomes its own collision domain.
Using Full Duplex: Making the Streets Two Way
We know from Chapter 4 that we have two cables used to send data and two cable to receive data. Now that we don't have to worry about collisions when we send data we don't have to worry about sending data when we are receiving data. But CSMA/CD says we have to wait, we can turn this off by enabling full duplex which allows the computer to send data while it is receiving data. When CSMA/CD is in use it is referred to as half duplex.
Switches: The Rest of the Story
Where does the MAC address table come from? How is it created and what happens if it receives a frame for a network device that isn't in its table? We will fill in those gaps now.
Painting the Road Signs: Learning MAC Addresses
The MAC address table is a dynamic list of MAC addresses and there corresponding ports. The switch learns where the network devices are located through a process called learning. When the switch receives a frame it looks at the source address, if the address isn't in the table it adds it.
The Forward Versus Filter Decision
Forward decision is when the switch forwards the frames only out of the port that contains the destination network device. It makes the decision to forward it out a port. It can also be viewed as it decided not to forward the frames out the other ports. This is referred to as filter decision.
What to Do When the Road Sign (Address Table) Doesn't List Your Destination
If a switch receives a frame where the destination MAC address is not in the MAC address table it performs a function called flooding. It send the frame out all ports besides the one it was received on. The destination responds and is added to the MAC address table.
How to Go Everywhere at the Same Time
When data is sent between computers the MAC addressed in the source and destination fields of the header are referred to as unicast MAC addresses. Each address represents an individual network device. If you wanted to send data to all network devices on a network you would use the broadcast address. The broadcast MAC address is FFFF.FFFF.FFFF. When a switch receives a frame addressed to the broadcast address it repeats it out all ports except the one it was received on.
I Feel the Need, the Need for Speed
When the IEEE took over the development of Ethernet, a 10 Mbps transmission rate was used. Over the years this speed has increased.
A New, Improved, and Faster Ethernet - Let's Call it Fast Ethernet
When they bumped the speed up to 100 Mbps they called it Fast Ethernet and was named 802.3u. It was a change in the encoding standard. All the headers and other information was the same, so the adoption of Fast Ethernet was easy for vendors to do. The old standard for twisted pair cabling was 10BaseT, the new one is 100BaseT.
If Fast Ethernet is Good, Even Faster Is Better: Gigabit Ethernet
Gigabit Ethernet (GigE) runs at 1 Gbps and uses two working groups, 802.3z and 802.ab. 802.3z defines how data is encoded at gigabit speeds over fiber, and 802.3ab defines how data is encoded at gigabit speeds using copper.
Ultra Super-Fast Fast Ethernet: 10 Gigabit Ethernet
10 Gbps Ethernet, or 10 GigE is defined as 802.3ae. 802.3ae tells how to encode the data at 10 GigE over fiber.
A Switch for All Speeds
Since the only difference between the different speed Ethernet standards is the encoding standard, you can use a single switch to connect devices at different speeds. A process called autonegotiation takes place that determines the speed that both devices can communicate. It can also detect if full duplex can be used. Man NIC's today are 10/100 cards so you can use them on a newer switch, or an older hub.
