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The Background of IEEE Ethernet

In 1980, the Digital, Intel, and Xerox (DIX) consortium created the original Ethernet. Predictably, Ethernet_II followed and was released in 1984. The standards-setting organization Institute of Electrical and Electronics Engineers (IEEE) termed this the 802 project. The project was named for the start date of February 1980 or 802. The 802 project was initially divided into three groups:

The High Level Interface (HILI) became the 802.1 committee and was responsible for high-level internetworking protocols and management.

The Logical Link Control (LLC) group became the 802.2 committee and focused on end-to-end link connectivity and the interface between the higher layers and the medium-access-dependant layers.

The Data Link and Medium Access Control (DLMAC) group became responsible for the medium-access protocols. The DLMAC ended up splitting into three different committees:
DEC, Intel, and Xerox pushed Ethernet, while Burroughs, Concord Data Systems, Honeywell, Western Digital, and later, General Motors and Boeing, pushed 802.4. IBM took on 802.5.

The IEEE then created the 802.3 subcommittee to come up with an Ethernet standard that happens to be almost identical to the Ethernet_II version of Ethernet. The two differ only in their descriptions of the Data Link layer.

Ethernet_II has a Type field, whereas 802.3 has a Length field. Even so, they’re both common in their Physical layer specifications, MAC addressing, and understanding of the LLC layer’s responsibilities.

Ethernet_II and 802.3 both define a bus-topology LAN at 10Mbps, and the cabling defined in these standards are identical:
Switched Ethernet
Ethernet is the most popular type of network in the world and will continue to be so. It is important to understand how hubs and switches work within an Ethernet internetwork. By using switched Ethernet in layer 2 of your network, you no longer have to share bandwidth with the different departments in the corporation. With hubs, all devices have to share the same bandwidth, which can cause havoc in today’s networks.

Remember that layer 2 switches break up collision domains, but the network is still one large broadcast domain. Switched Ethernet has replaced shared hubs in the networking world because each connection from a host to the switch is its own collision domain. Remember that, with shared hubs, the network was one large collision domain and one large broadcast domain, whereas layer 2 switches break up collision domains on each port, but all ports are still considered, by default, to be in one large broadcast domain.

Only virtual LANs break up broadcast domains in a layer 2 switched network. Switched Ethernet is a good way to dynamically allocate dedicated 10, 100, and 1000Mbps connections to each user. By also running full-duplex Ethernet, you can theoretically double the throughput on each link. In the next sections, we’ll discuss how Ethernet is used in your internetwork, the differences between the Ethernet types, and half- and full-duplex.

Using Ethernet Media in Your Internetwork
In this section, you’ll learn the difference between the Ethernet media types and how to use them in your internetworks. We’ll cover the following Ethernet types:
10BaseT stands for 10 million bits per second (Mbps), baseband technology, twisted-pair. This Ethernet technology has the highest install base of any network in the world. It runs the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol and, if correctly installed, is an efficient network. However, if it gets too large and the network is not segmented correctly, problems occur. It is important to understand collision and broadcast domains and how to correctly design the network with switches and routers.

Use 10BaseT at the Access Layer
10BaseT Ethernet is typically used only at the access layer, and even then, FastEthernet (100BaseT) is quickly replacing it as the prices for 100BaseT continue to drop. It would be poor design to place 10BaseT at the distribution or core layers. You need transits that are much faster than 10BaseT at these layers.

The distance that 10BaseT can run and be within specification is 100 meters (330 feet). The 100 meters includes the following:
This doesn’t mean that you can’t really run more then 100 meters on a cable run; it just is not guaranteed to work.

FastEthernet is 10 times faster than 10Mbps Ethernet. The great thing about FastEthernet is that, like 10BaseT, it is still based on the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) signaling. What this means is that you can run 10BaseT and 100BaseT on the same network without any problems. What a nice upgrade path this type of network can give you. You can put all your servers on 10BaseT and upgrade only the clients to 100BaseT if you need to. However, you can’t really even buy a PC that doesn’t have a 10/100 Ethernet card in it anymore, so you really don’t need to worry about compatibility and speed issues from the user’s perspective.

Use FastEthernet at All Three Layers
FastEthernet works great at all layers of the hierarchical model. It can be used to give high performance to PCs and other hosts at the access layer, provide connectivity from the access layer to the distribution layer switches, and connect the distribution layer switches to the core network. Connecting a server block to the core layer would need, at a minimum, FastEthernet or maybe even Gigabit Ethernet.

IEEE Specifications for FastEthernet
There are two different specifications for FastEthernet, but the IEEE 802.3u is the most popular. The 802.3u specification is 100Mbps over category 3 or 5, twisted-pair (typically just category 5 or 5 plus is used for FastEthernet). The second Ethernet specification, called 802.12, used a different signaling technique, which was more efficient than the CSMA/CD access method. The IEEE passed both methods in June 1995, but because 802.3 Ethernet had such a strong name in the industry, 802.12, also called Demand Priority.

Access Method (DPAM), has virtually disappeared from the market. As with the Macintosh and NetWare operating systems, it doesn’t mean anything if you have a better product; it only matters how you market it.

The IEEE 802.3u committee can be summarized as follows:
Media Independent Interface (MII)
FastEthernet requires a different interface than 10BaseT Ethernet. 10Mbps Ethernet used the Attachment Unit Interface (AUI) to connect Ethernet segments together. This provided a decoupling of the MAC layer from the different requirements of the various Physical layer topologies, which allowed the MAC to remain constant but meant the Physical layer could support any existing and new technologies. However, the AUI interface could not support 100Mbps Ethernet because of the high frequencies involved. 100BaseT needed a new interface, and the MII provides it. 100BaseT actually created a new subinterface between the Physical layer and the Data Link layer called the Reconciliation Sublayer (RS). The RS maps the 1s and 0s to the MII interface. The MII uses a nibble, which is defined as 4 bits. AUI used only 1 bit at a time. Data transfers across the MII at one nibble per clock cycle, which is 25MHz. 10Mbps uses a 2.5MHz clock.

Full-Duplex Ethernet and FastEthernet
Full-duplex Ethernet can both transmit and receive simultaneously and uses point-to-point connections. It is typically referred to as collision free because it doesn’t share bandwidth with any other devices. Frames sent by two nodes cannot collide because there are physically separate transmit and receive circuits between the nodes.

Use Full-Duplex Ethernet in the Distribution Layer
Because users typically use client/server applications using read/write asymmetrical traffic, the best performance for full-duplex would be in the distribution layer, not necessarily in the access layer. Full-Duplex with Flow Control was created to avoid packets being dropped if the buffers on an interface fill up before all packets can be processed. However, some vendors might not interoperate, and the buffering might have to be handled by upper-layer protocols instead.

Auto-negotiation is a process that allows clients and switches to agree on a link capability. This is used to determine the link speed as well as the duplex being used. The auto-negotiation process uses priorities to set the link configuration. Obviously, if both a client and switch port can use 100Mbps, full-duplex connectivity, that would be the highest-priority ranking, whereas half-duplex, 10Mbps Ethernet is the lowest ranking.

You need to understand that the auto-negotiation protocols do not work that well and you would be better off to configure the switch and NICs to run in a dedicated mode instead of letting the clients and switches auto-negotiate.

FastEthernet does have some drawbacks. It uses the same singing techniques as 10Mbps Ethernet, so it has the same distance constraints. In addition, 10Mbps Ethernet can use up to four repeaters, whereas FastEthernet can use only one or two, depending on the type of repeater. The table below shows a comparison of FastEthernet technologies.

Comparison of FastEthernet Technologies

Technology Wiring Category Distance
100BaseTX Category 5 UTP wiring, category6 and 7 is now available. Category 6 is sometimes referred to as Cat 5 plus. Two-pair wiring 100 meters
100BaseT4 Four-pair wiring, using UTP category 3, 4, or 5 100 meters
100BaseFX Multi-Mode Fiber (MMF) with 62.5-micron fiber-optic core with a 125-micron outer cladding (62.5/125) 400 meters

Gigabit Ethernet
In the corporate market, Gigabit Ethernet is the new hot thing. What is so great about Gigabit is that it can use the same network that your 10 and 100Mbps Ethernet now use. You certainly do have to worry about distance constraints, but what a difference it can make in just a server farm alone! Just think how nice it would be to have all your servers connected to Ethernet switches with Gigabit Ethernet and all your users using 100BaseT switched connections. Of course, all your switches would connect with Gigabit links as well. Add the hot xDSL to connect to the Internet and you have more bandwidth than you ever could have imagined just a few years ago. Will it be enough bandwidth a few years from now? Probably not. If you have the bandwidth, users will find a way to use it.

Use Gigabit Ethernet in the Switch, Core, and Server Blocks
Gigabit Ethernet can work in the switch block, the core block, and your server blocks:

Switch block - You can use Gigabit Ethernet between the access layer switches and the distribution layer switches. Gigabit is not typically connected to end users, but that can change quickly.

Core block - You can use Gigabit Ethernet to connect distribution layer switches in each building to the core switches.

Server block - By placing a Gigabit switch in the server block, you can effectively connect your high-performance servers to the network with gigabit speeds. However, remember that, unless the server is tremendously fast, you might not notice a difference in speeds from FastEthernet because the server processing can become the bottleneck. Time to throw out your Pentium 90 servers.

Protocol Architecture
Gigabit Ethernet became an IEEE 802.3 standard in the summer of 1998. The standard was called 802.3z. Gigabit uses Ethernet framing the same way 10BaseT and FastEthernet does. This means that, not only is it fast, it can run on the same network as older Ethernet technology, which provides a nice migration plan. The goal of the IEEE 802.3z was to maintain compatibility to the 10Mb/s and 100Mb/s existing Ethernet network. They needed to provide a seamless operation to forward frames between segments running at different speeds. The committee kept the minimum and maximum frame lengths the same. However, they needed to change the CSMA/CD for half-duplex operation from its 512-bit times to help the distance that Gigabit Ethernet could run.

Will Gigabit ever run to the desktop? Maybe. People said that FastEthernet would never run to the desktop when it came out, but it’s now common. If Gigabit is run to the desktop, however, it’s hard to imagine what we’ll need to run the backbone with. 1000BaseT to the rescue! Yes, 10 Gigabit Ethernet is just around the corner!

Comparing 10BaseT, FastEthernet, and Gigabit Ethernet
There are some major differences between FastEthernet and Gigabit Ethernet.

FastEthernet uses the Media Independent Interface (MII), and Gigabit uses the Gigabit Media Independent Interface (GMII). 10BaseT used the Attachment Unit Interface, or AUI. A new interface was designed to help FastEthernet scale to 100Mbps, and this interface was redesigned for Gigabit Ethernet. The GMII uses an 8-bit data path instead of the 4-bit path that FastEthernet MII uses. The clocking must operate at 125MHz to achieve the 1Gb/s data rate.

Time Slots
Because Ethernet networks are sensitive to the round-trip-delay constraint of CSMA/CD, time slots are extremely important. Remember that in 10BaseT and 100BaseT, the time slots were 512-bit times. However, this is not feasible for Gigabit because the time slot would be only 20 meters in length. To make Gigabit useable on a network, the time slots were extended to 512 bytes (4096-bit times!). However, the operation of full-duplex Ethernet was not changed at all. The table below compares the new Gigabit Ethernet technologies.

Comparison of Gigabit Ethernet Technologies

Technology Wiring Category Distance
100Base CX Copper shielded twisted-pair 25 meters
100BaseT Copper category 5, four-pair wiring, UTP 100 meters
100BaseSX MMF using 62.5 and 50-micron core, uses a 780-nanometer laser 260 meters
1000BaseLX Single-mode fiber that uses a 9- micron core, 1300-nanometer laser 3 km up to 10 km

Broadcast and Multicast Frames
Remember that layer 2 switches forward all broadcasts by default. The forwarding/filtering decision is not used in a broadcast situation because broadcast and multicast frames do not have a destination hardware address specified. The source address will always be the hardware address of the device transmitting the frame, and the destination address either will be all 1s (broadcast), or it will be a combination of the network or subnet address specified and all 1s for the host address (multicast). For example, a broadcast and multicast in binary would be as shown as:

  Binary Decimal
Broadcast 11111111.11111111.11111111.11111111
Multicast 10101100.00010000.11111111.11111111

Even though we have given you an example of a multicast address, the term multicast is most commonly used with regard to multicasting groups using the class D IP address space.

Notice that the broadcast is all 1s and the multicast is not. They are both a type of broadcast, except that multicasts send the frame to only a certain network or subnet and all hosts within that network or subnet, whereas a broadcast of all 1s sends the frame to all networks and all hosts. When a switch receives these types of frames, the frames are then quickly flooded out all active ports of the switch by default. To have broadcasts and multicasts forwarded out only a limited amount of administratively assigned ports, you create virtual LANs (VLANs).