tutorials: FAQ2

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Ethernet Frequently Asked Questions:

Cabling Basics

Is it safe to run Unshield Twisted Pair next to power cable?
Can I connect the 10BaseT interface of two devices directly together, without using a hub?
Does my Ethernet coax have to be grounded? How?
What is a "segment"?
What is a "subnet"?
What does "AUI" mean?
What exactly do 10Base5, 10BaseT, 10Base2, and 10Broad36, etc mean?
What does FOIRL mean?
What is coax cable?
What does UTP, STP cabling mean?
Are there any restrictions on how Ethernet is cabled?
Can I mix 10Base2 and 10Base5 cabling on a single segment?
What about wireless Ethernet? Are there any?
When should I choose 10BaseT, when 10Base2 (or others)?
What are the advantages/disadvantages of a star like cabling?

Q: Is it safe to run Unshield Twisted Pair next to power cable?

A: According to EIA/TIA-569, the standard wiring practices for running data cabling and companion to the above referenced EIA/TIA-568, you should not run data cable parallel to power cables. However, in reality, this should not be a problem with networks such as 10BaseT. 10BaseT uses differential signaling to pick the data signals off the wire. Since any interference from nearby power lines will usually affect all pairs equally, anything that is not canceled-out by the twists in the UTP should be ignored by the receiving network interface.

Q: Can I connect the 10BaseT interface of two devices directly together, without using a hub?

A: Yes, but not more than 2 devices, and you also need a special jumper cable between the two 10BaseT ports: RJ45 pin RJ45 pin ======== ======== 1 <--[TX+]--------[RX+]--> 3 2 <--[TX-]--------[RX-]--> 6 3 <--[RX+]--------[TX+]--> 1 6 <--[RX-]--------[TX-]--> 2

Q: Does my Ethernet coax have to be grounded? How?

A: Yes and no. The 10Base2 spec says the coax MAY be grounded at one and only one point, while the 10Base5 spec says the coax SHALL be grounded at one and only one point. Grounding your coax is generally a good idea; it allows static electricity to bleed off and, supposedly, makes for a safer installation. Further, many local electrical codes will require your network cabling to be grounded at some point. However, I have personally seen many Ethernet networks work with absolutely NO ground on the segment, and even a few unreliable segments become reliable when the one and only ground was removed. I’m not saying you should not ground your networks—you should absolutely install cabling according to your electrical codes. On the other hand, if you do ground your cable, make sure you do so only at one point. Multiple grounds on an Ethernet segment will not only cause network errors, but also risk damage to equipment and injury to people. If you have a repeater on one end of the segment, this will usually automatically ground that end of the segment (you may want to check the repeater documentation and configuration to assure this is the case—most repeaters can be set-up to NOT ground). If you don’t have a repeater, you can get terminating resistors with ground straps attached.

Q: What is a "segment"?

A: A piece of network wire bounded by bridges, routers, repeaters or terminators.

Q: What is a "subnet"?

A: Another overloaded term. It can mean, depending on the usage, a segment, a set of machines grouped together by a specific protocol feature (note that these machines do not have to be on the same segment, but they could be) or a big nylon thing used to capture enemy subs.

Q: What does "AUI" mean?

A: Attachment Unit Interface, an IEEE term for the connection between a controller and the transceiver

Q: What exactly do 10Base5, 10BaseT, 10Base2, and 10Broad36, etc mean?

A: These are the IEEE names for the different physical types of Ethernet. The "10" stands for signaling speed: 10MHz. "Base" means Baseband; "broad" means broadband. Initially, the last section as intended to indicate the maximum length of an unrepeated cable segment in hundreds of meters. This convention was modified with the introduction of 10BaseT, where the T means twisted pair, and 10BaseF where the F means fiber (see the following Q&A for specifics). This actually comes from the IEEE committee number for that media. In actual practice: 10Base2 Is 10MHz Ethernet running over thin, 50-Ohm baseband coaxial cable. 10Base2 is also commonly referred to as thin-Ethernet or Cheapernet. 10Base5 Is 10MHz Ethernet running over standard (thick) 50 Ohm baseband coaxial cabling. 10BaseF Is 10MHz Ethernet running over fiber-optic cabling. 10BaseT Is 10MHz Ethernet running over unshielded, twisted- pair cabling. 10Broad36 Is 10MHz Ethernet running through a broadband cable.

Q: What does FOIRL mean?

A: Fiber Optic Inter Repeater Link. A "IEEE 802 standard" worked out between many vendors some time ago for carrying Ethernet signals across long distances via fiber optic cable. It has since been adapted to other applications besides connecting segments via repeaters (you can get FOIRL cards for PCs). The larger 10BaseF standard have superseded it.

Q: What is coax cable?

A: Coaxial cable (coax) is a metallic electrical cable used for RF (radio frequency) and certain data communications transmission. The cable is constructed with a single solid or stranded center conductor that is surrounded by the dielectric layer, an insulating material of constant thickness and high resistance. A conducting layer of aluminum foil, metallic braid or a combination of the two encompass the dielectric and act as both a shield against interference (to or from the center conductor) and as the return ground for the cable. Finally, an overall-insulating layer forms the outer jacket of the cable. Coaxial cable is generally superior in high-frequency applications such as networking. However, for shorter distances (up to 100 meters), UTP or STP cable is generally just as reliable when using differential modulation techniques (such as with 10BaseT). There are three types of RG-58 cable, as far as I can tell. There are probably other subtle differences, but for 10BASE2, impedance and velocity of propagation are the important ones. The table below summarizes: Cable Impedance Velocity ---------- ---------- -------------- RG-58A/U 50 ohms .66 or .78 RG-58C/U 50 ohms .66 RG-58/U 53.5 ohms .66 or .695

Q: What does UTP, STP cabling mean?

A: Twisted pair cables. UTP is for Unshielded, twisted pair, while STP is for SHIELDED twisted pair. UTP is what’s typically installed by phone companies (though this is often not of high enough quality for high- speed network use) and is what 10BaseT Ethernet runs over. UTP is graded according to its data carrying ability (e.g., Level 3, Level 4, and Level 5). 10BaseT Ethernet requires at least Level 3 cable. Many sites now install only Level-5 UTP, even though level 4 is more than sufficient for 10BaseT, because of the greater likelihood that emerging high-speed standards will require cable with better bandwidth capabilities. STP is typically used for Token-Ring networks, where it is commonly referred to IBM Type 1 (or 2, 3, 6, 8, etc); however there are several manufacturers of Ethernet equipment and interfaces that support Ethernet over STP. Nevertheless, Ethernet over STP is not officially defined in any standards. While there is a good level of interoperability with Ethernet over STP, (Lattisnet, developed by Synoptics, is the recognized de facto standard in this area), one should consider the long-term availability and cost of this non-standard scheme before planning new networks around it.

Q: Are there any restrictions on how Ethernet is cabled?

A: Yes, there are many, and they vary according to the media used. First of all, there are distance limitations: 10Base2 limited to 185 meters (607 ft) per unrepeated cable segment. 10Base5 limited to 500 meters (1,640 ft) per unrepeated cable segment. 10BaseF depends on the signaling technology and medium used but can go up to 2KM. 10BaseT generally accepted to have a maximum run of 100-150M, but is really based on signal loss in dB’s (11.5db maximum loss source to destination). 10Broad36 limited to 3,600 meters (almost 2.25 miles). Then there are limitations on the number of repeaters and cable segments allowed between any two stations on the network. There are two different ways of looking at the same rules: 1. The Ethernet way: A remote repeater pair (with an intermediate point-to-point link) is counted as a single repeater (IEEE calls it two repeaters). You cannot put any stations on the point to point link (by definition!), and there can be two repeaters in the path between any pair of stations. This seems simpler to me than the IEEE terminology, and is equivalent. 2. The IEEE way: There may be no more than five (5) repeated segments, nor more than four (4) repeaters between any two Ethernet stations; and of the five cable segments, only three (3) may be populated. This is referred to as the "5-4-3" rule (5 segments, 4 repeaters, 3 populated segments). It can really get messy when you start cascading through 10BaseT hubs, which are repeaters unto themselves. Just try to remember, that any possible path between two network devices on an unbridged/unrouted network cannot pass through more than 4 repeaters or hubs, nor more than 3 populated cable segments. Finally, 10Base2 is limited to a maximum of 30 network devices per unrepeated network segment with a minimum distance of 0.5m (1.5ft) between T-connectors. 10Base5 is limited to a maximum of 100 network devices per unrepeated segment, with a minimum distance of 2.5m (8.2ft) between taps/T’s (usually indicated by a marker stamped on the cable itself every 2.5m). 10BaseT and 10BaseF are star-wired, so there is no minimum distance requirement between devices, since devices cannot be connected serially. You can install up to the Ethernet maximum of 1024 stations per network with both 10BaseT and 10BaseF.

Q: Can I mix 10Base2 and 10Base5 cabling on a single segment?

A: It is not "legal", but the network police will not read you your rights and drag you away. Ideally, you should use a repeater (or bridge, router, etc..) between the different cabling types. However, in reality, it will work fine, as long as none of the other network parameters (lengths, numbers of stations, repeaters, etc) are near the limit of the specification.

Q: What about wireless Ethernet? Are there any?

A: Yes, and no. Many vendors offer equipment for Ethernet across a variety of unbounded, or wireless, connections using lasers, microwaves, and spread-spectrum radio transmissions. However, none of these methods are organized by any standards body, so it is unlikely to find equipment from any two different manufacturers that work together.

Q: When should I choose 10BaseT, when 10Base2 (or others)?

A: The specific environment and application must be considered when selecting your media type. However, there are some general rules-of-thumb that you can consider. Avoid using copper between buildings. The electrical disturbances caused by lightning, as well as naturally occurring differences in ground potential over distance can very quickly and easily cause considerable damage to equipment and people. The use of fiber-optic cabling between buildings eliminates network cabling as a safety risk. There are also various wireless media available for inter-building links, such as laser, spread-spectrum RF and microwave. However, wireless media is much more expensive and less reliable than fiber-optic, and should only be considered when it is impossible to get right-of-way for fiber-optic cable. 10Base2 (thin Ethernet or Cheapernet) is the least expensive way to cable an Ethernet network. However, the price difference between 10Base2 and 10BaseT (Ethernet over UTP) is rapidly diminishing. Still, for small, budget-conscious installations, 10Base2 is the most economical topology. The disadvantages of 10Base2 is that any break in the cable or poor connection will bring the entire network down, and you need repeaters if you have more than 30 devices connected to the network or the cable length exceeds 185 meters (607 feet). 10Base5 is generally used as a low-cost alternative to fiber-optic media for use as a backbone segment within a single building. It’s extended length (500m or 1640ft), higher attached device count (100) and better noise resistance make 10Base5 well suited for use as a network trunk for one or more floors in a building. However, the high cost of connecting each device (in addition to the interface, you also need an external transceiver, or MAU, and an AUI cable) makes 10Base5 too expensive for most LAN installations, and like 10Base2, a single break or bad connection in the cable can bring the entire network down. 10BaseT is the most flexible topology for LANs, and is generally the best choice for most network installations. 10BaseT hubs, or multi-hub concentrators, are typically installed in a central location to the user community, and inexpensive UTP cabling is run to each network device (which may be 100m, or 330ft, from the hub). The signaling technology is very reliable, even in somewhat noisy environments, and 10BaseT hubs will usually detect many network error conditions and automatically shut-down the offending port(s) without affecting the rest of the network (unless, of course, the offending port was your server, shared printer, or router to the rest of the world). While the hardware is more expensive than 10Base2, the cabling is cheaper and requires less skill to install, making 10BaseT installation costs only slightly higher than 10Base2. The flexibility and reliability more than offset the marginally higher price. 10BaseF, and its predecessor, FOIRL, are the only recommended topologies for inter-building links. However, they need not be limited to this role. 10BaseF can also be run to the desktop, though the cost is prohibitively high in all but the most specialized environments (generally, extremely noisy manufacturing facilities, or very security-conscious installations). More commonly, FOIRL (and now, 10BaseF) is used inside buildings to form backbone networks and to connect wiring closets together.

Q: What are the advantages/disadvantages of a star like cabling?

A: Old style Ethernet bus wiring (i.e., taking the cable from one and quickly consumes machine to the next, and then to the next, etc) is prone to cable failure allowed distances due to aestheticnetwork (segment) fails - wiring needs. If the wiring connection is broken at any point, the entire and the much greater number offhand, it’s pretty easy to connections increases the probability of a failure or break. On the other do for a layman and may involvepoint of failure of a less actual wiring for small segments. Star wiring eliminates the single commonfails it usually doesn’t wire. A central hub has many connections that radiate out to hosts, if one of these hosts connections affect the others. Obviously, however, the hub becomes a central quality hub is less likely point of failure itself, but studies show a to fail before a heavily used strand of coax.this is the biggie.