10BASE2

10BASE2 cable showing the BNC connector end.
10BASE2 cable with a BNC T-connector.
10BASE2 cable end terminator.
EAD outlet
Different types of T-connectors, with AAUIs (an AUI variant specific to Apple computers)

10BASE2 (also known as cheapernet, thin Ethernet, thinnet, and thinwire) is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors. During the mid to late 1980s this was the dominant 10 Mbit/s Ethernet standard, but due to the immense demand for high speed networking, the low cost of Category 5 cable, and the popularity of 802.11 wireless networks, both 10BASE2 and 10BASE5 have become increasingly obsolete, though devices still exist in some locations.[1] As of 2011, IEEE 802.3 has deprecated this standard for new installations.[2]

Name origination

The name 10BASE2 is derived from several characteristics of the physical medium. The 10 comes from the transmission speed of 10 Mbit/s. The BASE stands for baseband signalling, and the 2 for a maximum segment length approaching 200 m (the actual maximum length is 185 m).

Signal encoding

10 Mbit/s Ethernet uses Manchester coding. A binary zero is indicated by a low-to-high transition in the middle of the bit period and a binary one is indicated by a high-to-low transition in the middle of the bit period. Manchester coding allows the clock to be recovered from the signal. However, the additional transitions associated with it double the signal bandwidth.

Network design

10BASE2 coax cables have a maximum length of 185 metres (607 ft). The maximum practical number of nodes that can be connected to a 10BASE2 segment is limited to 30[3] with a minimum distance of 50 cm between devices.[4] In a 10BASE2 network, each stretch of cable is connected to the transceiver (which is usually built into the network adaptor) using a BNC T-connector, with one stretch connected to each female connector of the T. The T-connector must be plugged directly into the network adaptor with no cable in between.

As is the case with most other high-speed buses, Ethernet segments have to be terminated with a resistor at each end. Each end of the cable has a 50 ohm (Ω) resistor attached. Typically this resistor is built into a male BNC and attached to the last device on the bus. This is most commonly connected directly to the T-connector on a workstation.[lower-alpha 1] If termination is missing, or if there is a break in the cable, the AC signal on the bus is reflected, rather than dissipated, when it reaches the end. This reflected signal is indistinguishable from a collision, so no communication can take place.

Some terminators have a metallic chain attached to them for grounding purposes. The cable should be grounded at one end. Grounding the terminators at both may produce a ground loop and can cause cause network outages or data corruption when swells of electricity traverse the coaxial cabling's outer shield.

When wiring a 10BASE2 network, special care has to be taken to ensure that cables are properly connected to all T-connectors, and appropriate terminators are installed. One, and only one, terminator must be connected to ground via a ground wire. Bad contacts or shorts are especially difficult to diagnose, though a time-domain reflectometer will find most problems quickly. A failure at any point of the network cabling tends to prevent all communications. For this reason, 10BASE2 networks can be difficult to maintain and were often replaced by 10BASE-T networks, which (provided category 5 cable or better was used) also provided a good upgrade path to 100BASE-TX. Other connectors such as EAD sockets were promoted as a less error-prone alternative to BNC connectors.

Comparisons to 10BASE-T

10BASE2 networks cannot generally be extended without breaking service temporarily for existing users and the presence of many joints in the cable also makes them very vulnerable to accidental or malicious disruption. There were proprietary wallport/cable systems that claimed to avoid these problems (e.g. SaferTap) but these never became widespread, possibly due to a lack of standardization.

10BASE2 systems do have a number of advantages over 10BASE-T. No hub is required as with 10BASE-T, so the hardware cost is minimal, and wiring can be particularly easy since only a single wire run is needed, which can be sourced from the nearest computer. These characteristics mean that 10BASE2 is ideal for a small network of two or three machines, perhaps in a home where easily concealed wiring may be an advantage. For a larger complex office network, the difficulties of tracing poor connections make it impractical. Unfortunately for 10BASE2, by the time multiple home computer networks became common, the format had already been practically superseded. It is very difficult to find 10BASE2-compatible network cards as distinct pieces of equipment, and integrated LAN controllers on motherboards don't have the connector.

Comparisons to 10BASE5, use of AUI

10BASE2 uses RG-58A/U or similar for a maximum segment length of 185 m as opposed to the thicker RG-8-like cable used in 10BASE5 networks with a maximum length of 500 m. The RG-58 type wire used by 10BASE2 in addition to being smaller and much more flexible than the specialized RG-8 variant, was also inexpensive and readily available as RG-58 is the same wire used for Citizens band radio (also known as CB radio) which was conveniently very popular during the early ‘80s when 10BASE2 was introduced to the marketplace.

An Ethernet network interface controller (NIC) may include the 10BASE2 transceivers and thus directly provide a 10BASE2 BNC connector (that the T-connector plugs into), or it offers an AUI connector that external transceivers (see Medium Attachment Unit) can connect to. These can be transceivers for 10BASE2, but also for 10BASE5 ("vampire tap" system) and 10BASE-T (with twisted-pair cable). Some NICs offer both (10BASE2-)BNC and AUI connectors, or other combinations like BNC and 10BASE-T, with only one connector to be used at the same time.

See also

Notes

  1. ↑ A few devices such as Digital's DEMPR and DESPR have a built-in terminator and so can only be used at one physical end of the cable run.

References

  1. ↑ "L-com Introduces Commercial-Grade Thinnet (10Base-2) and Thicknet (10Base-5) Converters for Legacy Installs". L-com, Inc. 2012-06-11. Retrieved 2012-07-01.
  2. ↑ IEEE 802.3-2012 10. Medium attachment unit and baseband medium specifications, type 10BASE2
  3. ↑ IEEE 802.3 10.7.2.2 MAU placement
  4. ↑ IEEE 802.3 10.7.2.1 Cable sectioning

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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