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Network Design

Network Design Criteria

Ethernets and Fast Ethernets have design rules that must be followed in order to function correctly. The maximum number of nodes, number of repeaters and maximum segment distances are defined by the electrical and mechanical design properties of each type of Ethernet media.

A network using repeaters, for instance, functions with the timing constraints of Ethernet. Although electrical signals on the Ethernet media travel near the speed of light, it still takes a finite amount of time for the signal to travel from one end of a large Ethernet to another. The Ethernet standard assumes it will take roughly 50 microseconds for a signal to reach its destination.

Ethernet is subject to the "5-4-3" rule of repeater placement: the network can only have five segments connected; it can only use four repeaters; and of the five segments, only three can have users attached to them; the other two must be inter-repeater links.

If the design of the network violates these repeater and placement rules, then timing guidelines will not be met and the sending station will resend that packet. This can lead to lost packets and excessive resent packets, which can slow network performance and create trouble for applications. New Ethernet standards (Fast Ethernet, GigE, and 10 GigE) have modified repeater rules, since the minimum packet size takes less time to transmit than regular Ethernet. The length of the network links allows for a fewer number of repeaters. In Fast Ethernet networks, there are two classes of repeaters. Class I repeaters have a latency of 0.7 microseconds or less and are limited to one repeater per network. Class II repeaters have a latency of 0.46 microseconds or less and are limited to two repeaters per network. The following are the distance (diameter) characteristics for these types of Fast Ethernet repeater combinations:

Fast Ethernet Copper Fiber
No Repeaters
One Class I Repeater
One Class II Repeater
Two Class II Repeaters
100m
200m
200m
205m
412m*
272m
272m
228m
* Full Duplex Mode 2 km

When conditions require greater distances or an increase in the number of nodes/repeaters, then a bridge, router or switch can be used to connect multiple networks together. These devices join two or more separate networks, allowing network design criteria to be restored. Switches allow network designers to build large networks that function well. The reduction in costs of bridges and switches reduces the impact of repeater rules on network design.

Each network connected via one of these devices is referred to as a separate collision domain in the overall network.

When and Why Ethernets Become Too Slow

As more users are added to a shared network or as applications requiring more data are added, performance deteriorates. This is because all users on a shared network are competitors for the Ethernet bus. On a moderately loaded 10Mbps Ethernet network that is shared by 30-50 users, that network will only sustain throughput in the neighborhood of 2.5Mbps after accounting for packet overhead, interpacket gaps and collisions.

Increasing the number of users (and therefore packet transmissions) creates a higher collision potential. Collisions occur when two or more nodes attempt to send information at the same time. When they realize that a collision has occurred, each node shuts off for a random time before attempting another transmission. With shared Ethernet, the likelihood of collision increases as more nodes are added to the shared collision domain of the shared Ethernet. One of the steps to alleviate this problem is to segment traffic with a bridge or switch. A switch can replace a hub and improve network performance. For example, an eight-port switch can support eight Ethernets, each running at a full 10 Mbps. Another option is to dedicate one or more of these switched ports to a high traffic device such as a file server.

Greater throughput is required to support multimedia and video applications. When added to the network, Ethernet switches provide a number of enhancements over shared networks that can support these applications. Foremost is the ability to divide networks into smaller and faster segments. Ethernet switches examine each packet, determine where that packet is destined and then forward that packet to only those ports to which the packet needs to go. Modern switches are able to do all these tasks at "wirespeed," that is, without delay.

Aside from deciding when to forward or when to filter the packet, Ethernet switches also completely regenerate the Ethernet packet. This regeneration and re-timing allows each port on a switch to be treated as a complete Ethernet segment, capable of supporting the full length of cable along with all of the repeater restrictions. The standard Ethernet slot time required in CSMA/CD half-duplex modes is not long enough for running over 100m copper, so Carrier Extension is used to guarantee a 512-bit slot time.

Additionally, bad packets are identified by Ethernet switches and immediately dropped from any future transmission. This "cleansing" activity keeps problems isolated to a single segment and keeps them from disrupting other network activity. This aspect of switching is extremely important in a network environment where hardware failures are to be anticipated. Full duplex doubles the bandwidth on a link, and is another method used to increase bandwidth to dedicated workstations or servers. Full duplex modes are available for standard Ethernet, Fast Ethernet, and Gigabit Ethernet. To use full duplex, special network interface cards are installed in the server or workstation, and the switch is programmed to support full duplex operation.

Introduction to Ethernet, Fast Ethernet and Gigabit Ethernet

It is nearly impossible to discuss networking without the mention of Ethernet, Fast Ethernet and Gigabit Ethernet. But, in order to determine which form is needed for your application, it’s important to first understand what each provides and how they work together.

A good starting point is to explain what Ethernet is. Simply, Ethernet is a very common method of networking computers in a LAN using copper cabling. Capable of providing fast and constant connections, Ethernet can handle about 10,000,000 bits per second and can be used with almost any kind of computer.

Implementing Fast or Gigabit Ethernet to increase performance is the next logical step when Ethernet becomes too slow to meet user needs. Higher traffic devices can be connected to switches or each other via Fast Ethernet or Gigabit Ethernet, providing a great increase in bandwidth. Many switches are designed with this in mind, and have Fast Ethernet uplinks available for connection to a file server or other switches. Eventually, Fast Ethernet can be deployed to user desktops by equipping all computers with Fast Ethernet network interface cards and using Fast Ethernet switches and repeaters.

While that may sound fast to those less familiar with networking, there is a very strong demand for even higher transmission speeds, which has been realized by the Fast Ethernet and Gigabit Ethernet specifications (IEEE 802.3u and IEEE 802.3z respectively). These LAN (local area network) standards have raised the Ethernet speed limit from 10 megabits per second (Mbps) to 100Mbps for Fast Ethernet and 1000Mbps for Gigabit Ethernet with only minimal changes made to the existing cable structure.

The building blocks of today's networks call out for a mixture of legacy 10BASE-T Ethernet networks and the new protocols. Typically, 10Mbps networks utilize Ethernet switches to improve the overall efficiency of the Ethernet network. Between Ethernet switches, Fast Ethernet repeaters are used to connect a group of switches together at the higher 100 Mbps rate.

However, with an increasing number of users running 100Mbps at the desktop, servers and aggregation points such as switch stacks may require even greater bandwidth. In this case, a Fast Ethernet backbone switch can be upgraded to a Gigabit Ethernet switch which supports multiple 100/1000 Mbps switches. High performance servers can be connected directly to the backbone once it has been upgraded.

Integrating Fast Ethernet and Gigabit Ethernet

Many client/server networks suffer from too many clients trying to access the same server, which creates a bottleneck where the server attaches to the LAN. Fast Ethernet, in combination with switched Ethernet, can create an optimal cost-effective solution for avoiding slow networks since most 10/100Mbps components cost about the same as 10Mbps-only devices.

When integrating 100BASE-T into a 10BASE-T network, the only change required from a wiring standpoint is that the corporate premise distributed wiring system must now include Category 5 (CAT5) rated twisted pair cable in the areas running 100BASE-T. Once rewiring is completed, gigabit speeds can also be deployed even more widely throughout the network using standard CAT5 cabling.

The Fast Ethernet specification calls for two types of transmission schemes over various wire media. The first is 100BASE-TX, which, from a cabling perspective, is very similar to 10BASE-T. It uses CAT5-rated twisted pair copper cable to connect various hubs, switches and end-nodes. It also uses an RJ45 jack just like 10BASE-T and the wiring at the connector is identical. These similarities make 100BASE-TX easier to install and therefore the most popular form of the Fast Ethernet specification.

The second variation is 100Base-FX which is used primarily to connect hubs and switches together either between wiring closets or between buildings. 100BASE-FX uses multimode fiber-optic cable to transport Fast Ethernet traffic.

Gigabit Ethernet specification calls for three types of transmission schemes over various wire media. Gigabit Ethernet was originally designed as a switched technology and used fiber for uplinks and connections between buildings. Because of this, in June 1998 the IEEE approved the Gigabit Ethernet standard over fiber: 1000BASE-LX and 1000BASE-SX.

The next Gigabit Ethernet standardization to come was 1000BASE-T, which is Gigabit Ethernet over copper. This standard allows one gigabit per second (Gbps) speeds to be transmitted over CAT5 cable and has made Gigabit Ethernet migration easier and more cost-effective than ever before.

Rules of the Road

The basic building block for the Fast Ethernet LAN is the Fast Ethernet repeater. The two types of Fast Ethernet repeaters offered on the market today are:

Class I Repeater -- The Class 1 repeater operates by translating line signals on the incoming port to a digital signal. This allows the translation between different types of Fast Ethernet such as 100BASE-TX and 100BASE-FX. A Class I repeater introduces delays when performing this conversion such that only one repeater can be put in a single Fast Ethernet LAN segment.

Class II Repeater -- The Class II repeater immediately repeats the signal on an incoming port to all the ports on the repeater. Very little delay is introduced by this quick movement of data across the repeater; thus two Class II repeaters are allowed per Fast Ethernet segment. 
Network managers understand the 100 meter distance limitation of 10BASE-T and 100BASE-T Ethernet and make allowances for working within these limitations. At the higher operating speeds, Fast Ethernet and 1000BASE-T are limited to 100 meters over CAT5-rated cable. The EIA/TIA cabling standard recommends using no more than 90 meters between the equipment in the wiring closet and the wall connector. This allows another 10 meters for patch cables between the wall and the desktop computer.

In contrast, a Fast Ethernet network using the 100BASE-FX standard is designed to allow LAN segments up to 412 meters in length. Even though fiber-optic cable can actually transmit data greater distances (i.e. 2 Kilometers in FDDI), the 412 meter limit for Fast Ethernet was created to allow for the round trip times of packet transmission. Typical 100BASE-FX cable specifications call for multimode fiber-optic cable with a 62.5 micron fiber-optic core and a 125 micron cladding around the outside. This is the most popular fiber optic cable type used by many of the LAN standards today. Connectors for 100BASE-FX Fast Ethernet are typically ST connectors (which look like Ethernet BNC connectors).

Many Fast Ethernet vendors are migrating to the newer SC connectors used for ATM over fiber. A rough implementation guideline to use when determining the maximum distances in a Fast Ethernet network is the equation: 400 - (r x 95) where r is the number of repeaters. Network managers need to take into account the distance between the repeaters and the distance between each node from the repeater. For example, in Figure 1 two repeaters are connected to two Fast Ethernet switches and a few servers.

Figure 1: Fast Ethernet Distance Calculations with Two Repeaters

Maximum Distance Between End nodes: 
400-(rx95) where r = 2 (for 2 repeaters) 
400-(2x95) = 400-190 = 210 feet, thus
A + B + C = 210 Feet

There is yet another variation of Ethernet called full-duplex Ethernet. Full-duplex Ethernet enables the connection speed to be doubled by simply adding another pair of wires and removing collision detection; the Fast Ethernet standard allowed full-duplex Ethernet. Until then all Ethernet worked in half-duplex mode which meant if there were only two stations on a segment, both could not transmit simultaneously. With full-duplex operation, this was now possible. In the terms of Fast Ethernet, essentially 200Mbps of throughput is the theoretical maximum per full-duplex Fast Ethernet connection. This type of connection is limited to a node-to-node connection and is typically used to link two Ethernet switches together.

A Gigabit Ethernet network using the 1000BASE-LX long wavelength option supports duplex links of up to 550 meters of 62.5 millimeters or 50 millimeters multimode fiber. 1000BASE-LX can also support up to 5 Kilometers of 10 millimeter single-mode fiber. Its wavelengths range from 1270 millimeters to 1355 millimeters. The 1000BASE-SX is a short wavelength option that supports duplex links of up to 275 meters using 62.5 millimeters at multimode or up to 550 meters using 55 millimeters of multimode fiber. Typical wavelengths for this option are in the range of 770 to 860 nanometers.

Maintaining a Quality Network

The CAT5 cable specification is rated up to 100 megahertz (MHz) and meets the requirement for high speed LAN technologies like Fast Ethernet and Gigabit Ethernet. The EIA/TIA (Electronics industry Association/Telecommunications Industry Association) formed this cable standard which describes performance the LAN manager can expect from a strand of twisted pair copper cable. Along with this specification, the committee formed the EIA/TIA-568 standard named the “Commercial Building Telecommunications Cabling Standard” to help network managers install a cabling system that would operate using common LAN types (like Fast Ethernet). The specification defines Near End Crosstalk (NEXT) and attenuation limits between connectors in a wall plate to the equipment in the closet. Cable analyzers can be used to ensure accordance with this specification and thus guarantee a functional Fast Ethernet or Gigabit Ethernet network.

The basic strategy of cabling Fast Ethernet systems is to minimize the re-transmission of packets caused by high bit-error rates. This ratio is calculated using NEXT, ambient noise and attenuation of the cable.  

Fast Ethernet Migration

Most network managers have already migrated from 10BASE-T or other Ethernet 10Mbps variations to higher bandwidth networks. Fast Ethernet ports on Ethernet switches are used to provide even greater bandwidth between the workgroups at 100Mbps speeds. New backbone switches have been created to offer support for 1000Mbps Gigabit Ethernet uplinks to handle network traffic. Equipment like Fast Ethernet repeaters will be used in common areas to group Ethernet switches together with server farms into large 100Mbps pipes. This is currently the most cost effective method of growing networks within the average enterprise.