Introduction
Full motion video for video conferencing requires, typically, at least 25 Mb/sec. That means that a legacy Ethernet, at 10 Mb/sec, can only deliver poor quality real-time video. With 100 Mb/sec, however, you can be watching a broadcast presentation in one window while you're in conference with three people in three other windows (for a total of 100 megabits of bandwidth).
Consider a file server that requires 0.6 Mb/sec (6 million bits per second; 60% utilization on a 10 Mb/sec Ethernet). With a 100 Mb/sec Ethernet this server can now utilize interface hardware that can pump data down the pipe at a greatly increased rate.
It seems clear that the evolution of the industry is moving away from 10 Mb/sec Ethernet and towards the 100 Mb/sec (or higher) rates of data transfer. This section of the compendium discusses 100 Mb/sec Ethernet technology
Virtually everyone who uses Ethernet has wished from time to time that their network had a higher bandwidth. When Ethernet was being designed in the late 1970s, 10Mbps seemed immense. With today's bandwidth-intensive multimedia applications, or even with just the departmental server, that number sometimes is barely adequate. Yes, faster network technologies were available, but they were complicated and expensive. Then came Fast Ethernet.
Anyone who understands classic Ethernet already understands much about Fast Ethernet. Fast Ethernet uses the same cabling and access method as 10Base-T. With certain exceptions, Fast Ethernet is simply regular Ethernet - just ten times faster! Whenever possible, the same numbers used in the design of 10Base-T were used in Fast Ethernet, just multiplied or divided by ten. Fast Ethernet is defined for three different physical implementations.
The Implementations of Fast Ethernet:
- 100BASE-TX: Category 5
- 100BASE-FX: Multimode fibre
- 100BASE-T4: Category 3
Probably the most popular form of Fast Ethernet is 100BASE-TX. 100BASE-TX runs on EIA/TIA 568 Category 5 unshielded twisted pair, sometimes called UTP-5. It uses the same pair and pin configurations as 10Base-T, and is topologically similar in running from a number of stations to a central hub.
As an upgrade to 10Mbps Ethernet over multimode fibre (10Base-F), 100BASE-FX is Fast Ethernet over fibre. Single duplex runs are supported up to 400m and full duplex runs are supported for up to 2km.
Fast Ethernet is possible on Category 3 UTP with 100BASE-T4. There is a popular misconception that Fast Ethernet will only run on Category 5 cable. That is true only for 100BASE-TX. If you have Category 3 cable with all four pairs (8 wires) connected between station and hub, you can still use it for Fast Ethernet by running 100BASE-T4. 100BASE-T4 sends 100Mbps over the relatively slow UTP-3 wire by fanning out the signal to three pairs of wire.
This "demultiplexing" slows down each byte enough that the signal won't overrun the cable. Category 3 cable has four pairs of wire, eight wires total, running from point to point. 10Base-T only uses four wires, two pairs. Some cables only have these two pairs connected in the RJ-45 plug. If the category 3 cabling at your site has all four pairs between hub and workstation, you can use Fast Ethernet by running 100BASE-T4.
Please select on of the following sections:
Differences Between Classic Ethernet And Fast Ethernet
Introduction
The two primary areas for concern when upgrading the network from 10Mbps to 100Mbps are cabling and hubs. As discussed on the Fast Ethernet Introduction page, in Fast Ethernet twisted pair cabling needs either to be category 5 or to be category 3 with proper twist on all four pairs.
The problem with hubs is the number of hubs allowed in a single collision domain. Classic Ethernet allows hubs to be cascaded up to four deep between any two stations. In Fast Ethernet, the number of hubs allowed in a collision domain is drastically reduced - to a single hub. Sometimes it may be possible to have more than one hub in a collision domain, but it will probably be easier in the long term to design a Fast Ethernet network assuming that only one hub is allowed.
What the IEEE 802.3 spec does not explicitly state is that this limitation only applies to shared 100BASE-T, not to switched 100BASE-T. Since switches act like bridges in defining a separate collision domain, installing Fast Ethernet switches will allow you to work around the single-hub problem. Even if it is not necessary to deliver dedicated switched Fast Ethernet to each desktop, Fast Ethernet hubs can be connected to switches. Connecting a number of repeaters to a switch will provide shared Fast Ethernet and allow you to maintain the size of your network.
Intergrating Fast Ethernet into 10MB Ethernet Networks
Introduction
Now that Fast Ethernet is here, the question becomes, "How do I start using it ?" Integrating Fast Ethernet into existing networks need not be done all at once.
Here are some aspects of 100Mbps implementation that should be considered:
- Implementing Switching
- Eliminating Bottlenecks
- Expand The Topology Outwards and Downwards
Implementing Switching
Implement switching in high-traffic areas to concentrate the bottlenecks on the network. Since Fast Ethernet provides higher throughput of bits, it makes sense to figure out which network connections need the most relief. Which segments consistently attempt to pump the most bytes? Which segments consistently demonstrate the highest average percent bandwidth usage according to your protocol analyzer?
Installing switches will help you figure out which network segments are moving the most information due to the effect switches have on your network. Installing switches is like moving from traffic lights to limited-access highways. The idea works extremely well in isolating cross-town traffic, e.g. peer-to-peer networking, but doesn't necessarily help when all of the traffic slows down at particular locations, e.g. an enterprise-wide server or the network Internet firewall. Because there are other ways of isolating network bottlenecks, implementing switches is primarily useful when installing 10/100 switches in preparation for 100Mbps Ethernet.
Installing switches also gives the added benefit of segmenting collision domains. In classic Ethernet, there can be up to four hubs or repeaters between any two stations, but in Fast Ethernet that number is only one or two. Installing switches in place of repeaters spares you having to segment your network at a later point, allowing the cost of the transition to be spread over a longer period of time.
Eliminating Bottlenecks
Once bottlenecks have been identified, upgrade those network connections to 100 Mbps. The primary difficulty in this step is verifying that the existing cabling will be sufficient for Fast Ethernet. On UTP, the cable either needs to meet Category 5 specifications or have four pairs with proper twist maintained on Category 3. If you're planning on using 100BASE-TX, your wiring closet will also need to be certified for a higher speed. There are many devices available such as wire pair scanners, which will make this job much easier.
Installing the initial Fast Ethernet connections is much easier if the switches installed earlier are 10/100, capable of operating at either classic Ethernet speeds or Fast Ethernet speeds. If the switches installed were only 10Mbps switches, they could be used as "hand-me-downs," replacing hubs in segments where users require more bandwidth.
Expand The Topology Outwards and Downwards
Gradually work the Fast Ethernet out into the rest of the network, as far out and down as desired. Note that the price of 10/100 cards is not substantially higher than that of 10Mbps cards, so it may be a wise idea to plan ahead by installing 10/100 cards when installing new machines.
If there comes a point in the future when 100Mbps Ethernet needs to be implemented on that machine, all that will need to be changed is the connection on the other end. On the other hand, upgrading a machine from a 10Mbps card to a 100Mbps card will require reconfiguring the user's machine, installing a new driver, etc. A short-term expenditure can greatly offset the cost in man-hours and down-time later on.
Upgrading And Migrating From Ethernet To Fast Ethernet
Introduction
Here we are going to analyse the following aspects of upgrading/migrating from 10Mbit Ethernet to 100Mbit Ethernet.
- Cabling
- Incompatible Implementations
- Repeaters In Fast Ethernet
- Replacement Of Illegal Byte
- Codes Data Translation
- Error Handling And Partitioning
Cabling
There are two methods of running Fast Ethernet over UTP and one method of running it over fibre.
IMPLEMENTATION ..........CABLE TYPE............... NUMBER OF PAIRS
..100BASE-TX ................ Category 5 .........................2
..100BASE-T4..................Category 3 or 5................. ..4
..100BASE-FX.................. Fiber....................... (Not Applicable)
Category 3 cabling is not rated to carry the fast signaling of 100BASE-TX, so 100BASE-T4 must be used. 100BASE-T4 may also be used on Category 5 cabling, but 100BASE-TX is probably a better choice.
Incompatible Implementations
Fast Ethernet brings a new urgency to an old problem. Many network technologies use RJ-45 connectors. In the past, it was usually not difficult to figure out whether a jack was Ethernet or token ring: even at a site where both were in use they seldom were found in the same vicinity, so the network administrator could make an "educated guess". Today, with Fast and classic Ethernet interspersed and 10/100 cards common, some mechanism is needed to allow quick identification of the signal that is running across the wire.
Autonegotiation works by having each end of the connection send a series of pulses down the wire to the other end. These pulses are the same signals used in 10Base-T to test link integrity and cause the link indicator light to turn on. If a station receives a single pulse, referred to as a Normal Link Pulse (NLP), it recognizes that the other end is only capable of 10Base-T.
If autonegotiation is being used, a station will transmit a series of these pulses spaced closely together, referred to as a Fast Link Pulse (FLP). An FLP consists of 17 "clocking" pulses interspersed with up to 16 "signal" pulses to form a 16-bit code word. If a signal pulse occurs between two clocking pulses, that bit is a one. Absence of a signal pulse is a zero.
By comparing the 16-bit code words received in the FLP, a station and hub will agree on what implementation of Ethernet to use. The 16-bit code word describes what implementations of Ethernet are supported. Both station and hub will compare what it supports to what the other end supports, then choose which implementation to use for that link according to following priorities, defined by IEEE 802.3 clause 28B.3:
100BASE-TX full duplex
100BASE-T4
100BASE-TX 1
10BASE-T full duplex
10BASE-T
If the station supports 100BASE-T4, 100BASE-TX, and 10BASE-T and the hub supports full duplex 100BASE-TX, single-duplex 100BASE-TX, and 10BASE-T, they will each discover that the Ethernet implementations they have in common are 100BASE-TX and 10BASE-T. Since 100BASE-TX is defined to have a higher priority that 10BASE-T, the station and hub will use 100BASE-TX. This decision takes place independently on each side of the link, but since each side uses the same decision-making process and priorities, the same decision is reached on each end. Because each end of the connection agrees on what implementation of Ethernet is being used, the potential problem of incompatible signaling is averted.
Repeaters In Fast Ethernet
In Fast Ethernet the number of repeaters allowed per network segment is only 1 or 2. Whether one or two repeaters may be used is determined by what class of repeater will be used on the segment. Two classes of Fast Ethernet repeater are defined, Class I and Class II. Only one Class I repeater can be used in a single collision domain. Two Class II repeaters are allowed in a single collision domain, with up to a 5 metre inter-repeater link between them. The only technical difference between Class I and Class II repeaters is that Class II repeaters are faster than Class I repeaters. This allows Class I repeaters to provide other services besides simple repeating, such as translating between 100BASE-TX and 100BASE-T4. Class II repeaters are primarily used to link two hubs supporting only a single implementation of Fast Ethernet.
However, with the trade-off in fewer repeaters comes greater intelligence in each repeater. In addition to implementing the functionality of 10Mbps repeaters, 100Mbps repeaters are responsible for the following:
Replacement Of Illegal Byte
Unlike classic Ethernet, Fast Ethernet does not send a straightforward representation of the actual bits across the physical layer. A different representation of the information is sent instead. As a result, there are possible patterns on the wire which are not defined for use in Fast Ethernet. If a repeater detects an illegal pattern on the wire, it may replace that pattern (and every remaining pattern in the frame) with a special symbol identifying that the frame is corrupt.
Codes Data Translation
For repeaters that implement more than one implementation of Ethernet, the repeater will change the data encoding to be appropriate to the outgoing ports. 100BASE-T4 and 100BASE-TX use very different representations when sending data across a network. A Class I repeater which implements both 100BASE-TX and 100BASE-T4 needs to ensure that the signal going across the wire is the appropriate representation for the Ethernet implementation.
Error handling and partitioning
A Fast Ethernet repeater will monitor the state of each port in order to protect the network from any faults that might interrupt the flow of information.
If 60 consecutive collisions are detected from any particular port, the repeater will partition that port: it will stop forwarding information from that port to the rest of the network, but will still continue to repeat all frames from the network to the port. If the station on that port has broken so that it no longer is obeying the rules of CSMA/CD, then it needs to be separated from the network to allow traffic to flow.
However, it is possible that there could be 60 consecutive collisions on an extremely busy segment, so the repeater still forwards information to that port. If the repeater detects between 450 and 560 bits of information from that port without a collision occurring, the repeater will re-activate that port. A legal frame is received from the partitioned port, so we know that the hardware is working.
If between 40000 and 75000 consecutive bits are received from a port, the device at the other end of that cable is assumed to be "jabbering", sending an endless stream of bits, so the output from the port is cut off from the rest of the network. Such a "jabbering" device could prevent any traffic from flowing on a network, since there would never be a break for the other stations to transmit. If the station stops "jabbering", then the repeater will once again activate the port.
In 100BASE-TX and 100BASE-FX, a repeater will further monitor traffic to ensure that only frames with a valid preamble are passed. If two consecutive "false carrier events" occur, or a "false carrier event" lasts for 450-500 bits, the repeater will declare that link to be "unstable" and stop sending information to that port. As a result, faulty links are isolated from the rest of the network, resulting in improved overall network reliability. The link will be reactivated if between 24814 and 37586 bit-times have passed without any information having been received, or if a valid carrier is received after between 64 and 86 bit-times of idle have occurred.
802.3 Fast Ethernet (100 Mb/Sec) Model
Introduction
Here we see a logical drawing of the Fast Ethernet Data Link Layer sublayers. Data is passed down from the upper layers (such as TCP/IP or Novell Netware) to the LLC sublayer. From there it is passed to the MAC sublayer and then, depending on whether this is a 100BASE-T4 or 100BASE-TX environment, either down the right or left-hand path to the wire.
We will intentionally avoid a detailed discussion of exactly what goes on at each of these layers here. Some of the layers' functions, such as 8B6T encoding, Fan-out and NRZI signaling are labeled and will be discussed in this essay.
In 10Mbps Ethernet, the data is handed directly from the MAC layer to the PMA (Physical Medium Attachment) sublayer and onto the wire. The Reconciliation, PCS and PMD sublayers do not exist in 10Mbps Ethernet.
Troubleshooting techniques for Fast Ethernet
Introduction
This page will primarily discuss problems unique to Fast Ethernet.
- The Collision Domain
- Incompatible Ethernet Jabber
- Auto-negotiation Priorities And Alternatives
- Incompatible Cabling Specifications
The Collision Domain
The single biggest change in network design in Fast Ethernet is the smaller collision domain. Technically, the size of a collision domain in all flavors of Ethernet is exactly the same - 256 bits. On the wire, ten times as many 100Mbps bits can occupy the same space as an equal number of 10Mbps bits, so the collision domain in 100Mbps Ethernet can be only physically one tenth the size of a 10Mbps collision domain.
Effectively this means that whereas up to four hubs can legally be cascaded in 10Base-T between any two stations, only one (or two) hubs can be used in a single segment in 100BASE-T without going through an interconnect device that provides link segmentation; such as a store-and-forward bridge, switch or bridge, or a router. A separate section of the Compendium discusses INTERCONNECT DEVICES in detail. If you see signs of corruption on your network that correspond to propagation delay, check to make sure that you're not cascading too many hubs.
You can make some generalizations regarding the structure of corrupted data frames (as discussed in the 10 Mbps Ethernet FRAME CORRUPTION section) but remember that these corruption patterns may be quite misleading, since you have a hub or switch in the network.
Note that many hub vendors sell stackable hubs. Hubs in a single stack connected via a common backplane are usually considered to be a single hub in terms of propagation delay, but multiple stacks cascaded externally via 100BASE-TX, 100BASE-T4, or 100BASE-FX could definitely cause problems. These 100BASE standards are discussed in the INTRODUCTION to this Fast Ethernet section.
Incompatible Ethernet Jabber
Another potential problem in 100Mbps Ethernet is the use of RJ-45 jacks for more than one flavor of Ethernet. Since 100BASE-TX and 100BASE-T4 both use RJ-45 jacks, as do 10Base-T and many other network technologies, the IEEE 802.3 specified an auto-negotiation protocol to allow stations to figure out the networking technology to use.
Unfortunately, they made its implementation optional. If you're using equipment that does not implement IEEE-spec auto-negotiation, the incompatible Ethernet signals could prevent one of your stations from connecting to your network, or even simulate "jabber" by constantly transmitting a TX idle stream and bringing down the network.
The possibility for this jabber is uncertain, considering that the flavors of Ethernet use different signal formats in transmission. Even if data is not exchanged, it is still possible that incompatible Ethernet flavors could assume that they have a proper connection. Ethernets using RJ-45 connections to a hub use a Link Test Pulse to verify link integrity. This pulse is the same in all flavors of Ethernet if auto-negotiation is not used. The auto-negotiation protocol itself uses a modified form of these pulses to negotiate a common Ethernet implementation.
If Ethernet incompatibility jabber were to occur between 100BASE-TX and another flavor of Ethernet, the results could be catastrophic, as 100BASE-TX transmits a continuous idle signal between frames. Although transparent to 100BASE-TX, this idle signal would completely busy out a 10Base-T or 100BASE-T4 segment. On the other hand, the 802.3 specifies that a Fast Ethernet repeater should implement jabber control, automatically partitioning off any port that is streaming information for more than 40000 to 75000 bits. If the repeater were to partition off the "jabbering" port, the symptom would be reduced to inability to connect the 100BASE-TX station to the network.
Auto-negotiation Priorities And Alternatives
If the station and repeater both support 100BASE-TX and 100BASE-T4 and 802.3 auto-negotiation, the link will autonegotiate to 100BASE-T4 instead of 100BASE-TX. Since 100BASE-TX requires Category 5 cabling but 100BASE-T4 requires only Category 3, 100BASE-T4 is assumed to be a better default.
If the cabling is known to be UTP-5, then it is probably more efficient to turn off auto-negotiation and use 100BASE-TX wherever possible. 100BASE-T4 requires more overhead than TX because it multiplexes and demultiplexes the data stream over three wire pairs. There is also significantly less overhead in translating between 100BASE-TX and 100BASE-FX than between 100BASE-T4, as TX and FX both use 4B5B encoding instead of T4's 8B6T. 100BASE-TX and 100BASE-FX also leave open the possibility of Full Duplex communication, although full duplex is not yet part of the 802.3 spec.
On the other hand, 100BASE-TX sends an idle signal whenever it is not transmitting data. The 802.3 spec implies that it may very well be preferable to use 100BASE-T4 for battery-powered operation, since the card would only be transmitting when there is actual information to be moved.
Incompatible Cabling Specifications
One final problem with the advent of Fast Ethernet is the different cabling specifications. In classic Ethernet it was difficult to mistake 10Base-2 for 10Base-5. With Fast Ethernet, special care must be taken to verify that the entire connection between station and concentrator either supports TX's 31.25MHz signal or maintains T4's four pairs with proper twist. There are a number of good cable testers and pair scanners available to assist you in determining this for your network.
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