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10BASE2

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10BASE2

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 Ethernet cable, and the popularity of 802.11 wireless networks, both 10BASE2 and 10BASE5 have become increasingly obsolete, though they still exist in some locations. As of 2011, IEEE 802.3 has deprecated this standard for new installations.

Contents

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 (millions of bits per second). 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. This allows the clock to be recovered from the signal. However, the additional transitions 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 with a minimum distance of 50 cm. 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 though it does not technically have to be. 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. 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, however many people never understood how to properly ground cabling and thus grounded the terminators at both ends rather than just one end. This caused many of the grounding loop problems during that era which caused network outages and/or data corruption when swells of electricity traversed the coaxial cabling's outer shield on its path to the ground with the least resistance.

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. An alternative, more reliable connection was established by the introduction of EAD sockets.

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

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. 10BASE2 uses BNC T-connectors with tranceivers integrated into the NIC instead of the 10BASE5 "vampire tap" system with external transceivers connected via AUI.

References

10BASE2 Wikipedia


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