1.3 10BaseT

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The 10BaseT is the IEEE standard for providing 10-Mbps Ethernet performance and functionality over ubiquitously available unshielded twisted-pair (UTP) wiring. This standard is noteworthy in that it specifies a star topology, unlike traditional 10Base2 and 10Base5, which use coaxial cabling arranged in a bus (single cable with closed ends) or ring topology. The star topology, whereby individual segments of cable are attached to a hub, permits centralized network monitoring, which enhances fault isolation and bandwidth management. For new installations, twisted-pair wire is substantially less expensive as well as easier to install and maintain than Ethernet’s original “thick” coaxial cable (10Base5) or the “thin” coaxial alternative (10Base2).

1.3.1 Performance

Traditionally, Ethernet has relied on coaxial cable with multi-drop connections to a LAN backbone. Repeaters between the segments keep the signal strength at a consistent level across the Ethernet. The key disadvantage of this bus topology is that any disturbance to the continuity of the cable at any point renders the entire LAN inoperable.

In contrast, the star topology relies on a hub or switch to support a dedicated link to each user. If an individual link, port, or workstation were to fail, there would be no impact on the rest of the LAN. This is precisely the benefit promised by 10BaseT: Because the network can tolerate a malfunction of any end-user device or its physical link, the rest of the network will not be affected, resulting in improved network availability. This is made possible by the link test and auto-partition logic inherent in the port-level circuitry of a 10BaseT hub or switch.

A network management system can make problems even easier to identify. When a predefined error threshold is exceeded, for example, the network management system will alert the LAN administrator, enabling a technician to be dispatched before the user even realizes that a problem exists. Alarm notification is usually by a screen message or visual indicator; some systems can even notify the LAN administrator via pager.

The 10BaseT and traditional bus Ethernet LANs have different distance limitations: 10BaseT operates reliably over cable segments not exceeding 100 meters (330 feet), whereas traditional Ethernet LANs using thick coaxial cable operate reliably over segments of up to 500 meters (1,650 feet) in length, and 10Base2 Ethernets are effective at 200 meters (660 feet) using thin coax. The cable limitation of 10BaseT aside, its bit error rate performance is at least as good as 10Base2 and 10Base5 systems. The 10BaseT specification allows for bit error rates of no more than 1 in 100 million bits. The data-encoding scheme used for 10BaseT systems is the same as that used for coaxial-based Ethernets—self-clocking Manchester encoding.

The 10BaseT LANs are designed to support the same applications as traditional Ethernet LANs. The relatively short distance over which 10BaseT LANs operate is rarely a factor in its ability to support these applications. During the standardization process, a survey by AT&T revealed that over 99% of employee desktops are located within 100 meters of a telephone wire closet where all the connections meet at a patch panel.

1.3.2 Media

Most installed telephone wiring is of the type known as 24 American wire gauge (AWG). Even if existing telephone wiring follows the required star configuration and is within the roughly 100-meter distance limitation, it may still not be suitable to handle the 10-Mbps data rate. This is because no particular wire gauge or type is specified in the 10BaseT standard, although 24 AWG is what most equipment vendors have used in conformance tests to confirm that their products transmit reliably at up to 100 meters. Generally, the inside wiring installed in the last 20 years for telephone connections meets the 10BaseT specifications at cabling runs of up to 100 meters. Older wiring may not meet the standards, in which case, poor or erratic LAN performance could result and transmission distances could be considerably less than 100m.

The 10BaseT standard was designed to eliminate the requirement for shielded wiring. It relies on the twists in twisted-pair wire to hold down frequency loss, which, in turn, improves the integrity of the signal. The twists minimize the effects of this loss by canceling high-frequency signal energy to prevent signals from radiating to and corrupting the signal being carried over nearby twisted pairs (cross-talk). Even the individual wire pairs in 25-pair telephone cable are twisted for this purpose. This cable is widely used in large installations; it enables multiple 10BaseT segments to be neatly carried, separated, and patched to a panel for distribution to the hub.

1.3.3 Media Connections

Adapter cards, also known as network interface cards (NICs), are boards that insert into an expansion slot of PCs and servers. The adapter card connects the device’s bus directly to the LAN segment, eliminating the need for a separate transceiver. Depending on the type of adapter, it permits connections to thick or thin coaxial cable, as well as to unshielded twisted pairs. An adapter with an attachment unit interface (AUI) port will permit connection to a transceiver, allowing the card to be used with thick and thin coaxial cables and fiber-optic cables.

The 10BaseT adapters vary considerably in their capabilities and features for reporting link status. Some adapters notify the user of a miswired connection to the LAN. Many have light-emitting diodes (LEDs) that indicate link status after the connection is made to the hub, such as link, collision, transmit, and receive. Others provide minimal information, such as if the link is correctly wired and working, while some light only when something is wrong. Still other cards have no LEDs to display link information. Some cards feature a menu-driven diagnostic program to help the user isolate problems with the cards, such as finding out if the adapter is capable of responding to commands from the software.

Transceivers connect PCs and peripherals already equipped with legacy Ethernet-cards to 10BaseT wiring. Typically, it consists of a small external box with an RJ45 jack at one end for connecting to the UTP LAN segment and an AUI port at the other end for connecting to the Ethernet adapter card.

A medium access unit (MAU) terminates each end of the 10BaseT link. As such, it accommodates two wire pairs: one pair for transmitting the Ethernet signal and the other pair for receiving the signal. The 10BaseT standard describes seven basic functions performed by the MAU. Transmit, receive, collision detection, and loop-back functions direct data transfer through the MAU. The jabber detect, signal quality error test, and link integrity functions define ancillary services provided by the MAU.

The jabber function removes equipment from the network whenever it continuously transmits for periods significantly longer than required for a maximum-length packet, indicating a possible problem with the NIC. The signal quality error test detects silent failures in the circuitry, while the link integrity signal detects breaks in the wire pairs. Both assist in fault isolation.

1.3.4 Hubs

All 10BaseT stations are connected to the hub via two twisted-pair wire pairs—one two-wire path for transmitting and the other two-wire path for receiving—over a point-to-point link. In essence, the hub acts as a multiport repeater. It contains the circuitry to retime and regenerate the signal received from any of the wire segments that connect at the hub to each of the other segments. However, a 10BaseT hub is more than a simple repeater; it serves as an active filter that rejects damaged packets.

The multiport repeater provides packet steering, fragment extension, and automatic partitioning. The packet steering function broadcasts copies of packets received at one repeater port to all of its other ports. Fragment extension ensures that partially filled packets are sent to their proper destination. Automatic partitioning isolates a faulty or misconnected 10BaseT link to prevent it from disrupting traffic on the rest of the network.

There are several types of 10BaseT hubs, the most common being chassis-based solutions and stackable 10BaseT hubs that can be cascaded with appropriate cable connections. At the low end, hubs come in managed or unmanaged versions.

The chassis models are high-end devices that provide a variety of connections to the WAN, support multiple LAN topologies and media, and offer advanced network management, with Simple Network Management Protocol (SNMP) typically included as well. This type of 10BaseT hub is also the most expensive. Aside from enhancing their products with more interfaces and network management features to further differentiate them from competitive offerings, vendors are continually increasing the port capacity of these devices to bring down the price-per-port, making them more appealing to larger users.

Depending on port capacity, stackable hubs are used to link members of a workgroup or department. With a pass-through or crossover cable, additional units can be added in a cascading arrangement to meet growth requirements or to connect to a high-end hub.

1.3.5 Management

The promise of 10BaseT networks was not only that Ethernet would run over economical UTP wiring and offer unprecedented configuration flexibility, but that it would offer a superior approach to LAN management. Some of the most important management capabilities available through the hub include:

• Support for the IEEE 802.3 repeater management standard and the Internet’s repeater management information base (MIB);

• Remote site manageability from a central management station;

• Provision of performance statistics, not only at the port level where such information has been traditionally available, but at the module and hub level as well;

• Autopartitioning, which entails the ability to automatically remove disruptive ports from the network;

• The ability to set performance thresholds that notify managers of a problem or automatically take action to address the problem;

• Port address association features that connect the Ethernet media access control address of a device with the port to which it is attached;

• Source/destination address information, which aids network redesign, traffic redistribution, troubleshooting, and security.

Overall, 10BaseT LANs offer a sound technical solution for most routine applications. In the office environment, packet throughput and error rates over twisted-pair wiring are the same as with coaxial systems, but the former is limited to shorter distances. The standard offers protection against equipment and media faults that can potentially disrupt the network, and the signaling method used is reasonably immune from most sources of electromagnetic interference commonly found in the office environment. As organizations have added more applications, more users, and more workstations, 10BaseT has been displaced by 100 Mbps (100BaseT) and 1,000 Mbps (1000BaseT) Ethernet LANs. To ease the migration, vendors support 10/100BaseT from the same network interface card, and some support 10/100/1,000BaseT from the same card.

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