10-Mbps and 100-Mbps Ethernet

10-Mbps Ethernet

10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, frame format, transmission process, and a basic design rule.

10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown in Figure (1 bit time at 10 Mbps = 100 nsec = 0.1 µsec = 1 ten-millionth of a second.)

10BASE5, 10BASE2, and 10BASE-T also have a common frame format.

The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. The Layer 2 frame data is converted from hex to binary. As the frame passes from the MAC sublayer to the physical layer, further processes occur prior to the bits being placed from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active:

• Within 4 to 8 microseconds following a normal transmission to indicate that the outbound frame was successfully transmitted
• Whenever there is a collision on the medium
• Whenever there is an improper signal on the medium. Improper signals might include jabber, or reflections that result from a cable short.
• Whenever a transmission has been interrupted

All 10 Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding. Line encoding describes how the bits are actually signaled on the wire. The simplest encodings have undesirable timing and electrical characteristics. So line codes have been designed to have desirable transmission properties. This form of encoding used in 10 Mbps systems is called “Manchester.”

Manchester encoding relies on the direction of the edge transition in the middle of the timing window to determine the binary value for that bit period. The top waveform has a falling edge, so it is interpreted as a binary 0. The second waveform shows a rising edge, which is interpreted as a binary 1. In the third waveform, there is an alternating binary sequence. With alternating binary data, there is no need to return to the previous voltage level. As can be seen from the third and fourth wave forms in the graphic, the binary bit values are indicated by the direction of change during any given bit period. The waveform voltage levels at the beginning or end of any bit period are not factors when determining binary values.

Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is of the utmost importance when implementing a mixed-media network. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on parameters such as:

• Cable length and its propagation delay
• Delay of repeaters
• Delay of transceivers
• Interframe gap shrinkage
• Delays within the station

10-Mbps Ethernet operates within the timing limits offered by a series of not more than five segments separated by no more than four repeaters. This is known as the 5-4-3 rule. No more than four repeaters may be connected in series between any two distant stations. There can also be no more than three populated segments between any two distant stations.

10BASE5

The original 1980 Ethernet product 10BASE5 transmitted 10 Mbps over a single thick coaxial cable bus. 10BASE5 is important because it was the first medium used for Ethernet. 10BASE5 was part of the original 802.3 standard. The primary benefit of 10BASE5 was length. Today it may be found in legacy installations, but would not be recommended for new installations. 10BASE5 systems are inexpensive and require no configuration, but basic components like NICs are very difficult to find as well as the fact that it is sensitive to signal reflections on the cable. 10BASE5 systems also represent a single point of failure. 10BASE5 uses Manchester encoding. It has a solid central conductor. Each of the maximum five segments of thick coax may be up to 500 m (1640.4 ft) in length. The cable is large, heavy, and difficult to install. However, the distance limitations were favorable and this prolonged its use in certain applications. Because the medium is a single coaxial cable, only one station can transmit at a time or else a collision will occur. Therefore, 10BASE5 only runs in half-duplex resulting in a maximum of 10 Mbps of data transfer. Figure illustrates one possible configuration for a maximum end-to-end collision domain. Between any two distant stations only three repeated segments are permitted to have stations connected to them, with the other two repeated segments used only as link segments to extend the network.

10BASE2

10BASE2 was introduced in 1985. Installation was easier because of its smaller size, lighter weight, and greater flexibility. It still exists in legacy networks. Like 10BASE5, it is not recommended for installations in networks today. It has a low cost and a lack of need for hubs. Again, NICs are also difficult to obtain for this medium. 10BASE2 also uses Manchester encoding. Computers on the LAN were linked together by an unbroken series of coaxial cable lengths. These lengths were attached by BNC connectors to a T-shaped connector on the NIC. 10BASE2 has a stranded central conductor. Each of the maximum five segments of thin coax may be up to 185 meters long and each station is connected directly to the BNC “T” connector on the coax. Only one station can transmit at a time or else a collision will occur. 10BASE2 also uses half-duplex. The maximum transmission rate of 10BASE2 is 10 Mbps. There may be up to 30 stations on any individual 10BASE2 segment. Out of the five consecutive segments in series between any two distant stations, only three may have stations attached

10BASE-T

10BASE-T was introduced in 1990. 10BASE-T used cheaper and easier to install Category 3 unshielded twisted pair (UTP) copper cable rather than coax cable. The cable plugged into a central connection device that contained the shared bus. This device was a hub. It was at the center of a set of cables that radiated out to the PCs like the spokes on a wheel. This is referred to as a star topology. The distances the cables could extend from the hub and the way in which the UTP was installed increasingly used stars made up of stars, referred to as an extended star topology. Originally 10BASE-T was a half-duplex protocol, but full-duplex features were added later. The explosion in the popularity of Ethernet in the mid-to-late 1990s was when Ethernet came to dominate LAN technology.

10BASE-T also uses Manchester encoding. A 10BASE-T UTP cable has a solid conductor for each wire in the maximum 90 meter horizontal cable. UTP cable uses eight-pin RJ-45 connectors. Though Category 3 cable is adequate for use on 10BASE-T networks, it is strongly recommended that any new cable installations be made with Category 5e or better. All four pairs of wires should be used either with the T568-A or T568-B cable pinout arrangement. With this type of cable installation, supports the use of multiple protocols without rewiring. Figure shows the pinout arrangement for a 10BASE-T connection. The transmitting pair on the receiving side are connected to the receiving pair on the attached device.

Half duplex or full duplex is a configuration choice. 10BASE-T carries 10 Mbps of traffic in half-duplex mode and 20 Mbps in full-duplex mode
10BASE-T wiring and architecture

10BASE-T links generally consist of a connection between the station and a hub or switch. Hubs are multi-port repeaters and count toward the limit on repeaters between distant stations. Hubs do not divide network segments into separate collision domains. Because hubs or repeaters merely extend the length of a network segment within a single collision domain, there is a limit on how many hubs may be used in that segment. Bridges and switches divide a segment into separate collision domains, only leaving the media limitations to determine the distance between the switches. 10BASE-T limits the distance between switches to 100 m (328 ft). Although hubs may be linked, it is best to avoid this arrangement. This is to prevent exceeding the limit for maximum delay between distant stations. When multiple hubs are required, it is best to arrange them in hierarchical order as to create a tree structure. Performance will be improved if fewer repeaters separate stations. An architectural example is shown in Figure . All distances between stations are acceptable. However, the total distance from one end of the network to the other, places the architecture at its limit. The most important aspect to consider is how to keep the delay between distant stations to a minimum, regardless of the architecture and media types involved. A shorter maximum delay will provide better overall performance. 10BASE-T links can have unrepeated distances up to 100 m. While this may seem like a long distance, it is typically “used up” when wiring an actual building. Hubs can solve the distance issue but will allow collisions to propagate. The widespread introduction of switches has made the distance limitation less important. As long as workstations are located within 100 m of a switch, the 100 m distance starts over at the switch.

100-Mbps Ethernet

100-Mbps Ethernet is also known as Fast Ethernet. The two technologies that have become important are 100BASE-TX, which is a copper UTP medium and 100BASE-FX, which is a multimode optical fiber medium.

Three characteristics common to 100BASE-TX and 100BASE-FX are the timing parameters, the frame format, and parts of the transmission process. 100BASE-TX and 100-BASE-FX both share timing parameters. Note that one bit time in 100-Mbps Ethernet is 10nsec = .01 microseconds = 1 100-millionth of a second.

The 100-Mbps frame format is the same as the 10-Mbps frame.

Fast Ethernet represents a 10-fold increase in speed over 10BASE-T. Because of the increase in speed, extra care must be taken because the bits being sent are getting shorter in duration and occurring more frequently. These higher frequency signals are more susceptible to noise. In response to these issues, two separate encoding steps are used by 100-Mbps Ethernet. The first part of the encoding uses a technique called 4B/5B, the second part of the encoding is the actual line encoding specific to copper or fiber.

100BASE-TX

In 1995, 100BASE-TX was the standard, using Cat 5 UTP cable, which became commercially successful. The original coaxial Ethernet used half-duplex transmission so only one device could transmit at a time. However, in 1997, Ethernet was expanded to include a full duplex capability that allowed more than one PC on a network to transmit at the same time. Switches increasingly replaced hubs. These switches had the capability of full duplex and rapid handling of Ethernet frames. 100BASE-TX uses 4B/5B encoding, which is then scrambled and converted to multi-level transmit-3 levels or MLT-3. In the example, the highlighted window shows four waveform examples. The top waveform has no transition in the center of the timing window. No transition indicates that a binary 0 is present. The second waveform shows a transition in the center of the timing window. A binary 1 is represented by a transition. The third waveform shows an alternating binary sequence. The absence of binary transition indicates a binary 0, and the presence of a transition indicates a binary 1. Rising or falling edges indicate 1s. Very steep signal changes indicate 1s. Any noticeable horizontal line in the signal indicates a 0. Figure shows the pinout for a 100BASE-TX connection. Notice that the two separate transmit-receive paths exist. This is identical to the 10BASE-T configuration. 100BASE-TX carries 100 Mbps of traffic in half-duplex mode. In full-duplex mode, 100BASE-TX can exchange 200 Mbps of traffic. The concept of full duplex will become increasingly important as Ethernet speeds increase.

100BASE-FX

At the time copper-based Fast Ethernet was introduced, a fiber version was also desired. A fiber vervsion could be used for backbone applications, connections between floors and buildings where copper is less desirable, and also in high noise environments. 100BASE-FX was introduced to satisfy this desire. However, 100BASE-FX was never adopted successfully. This was due to the timely introduction of Gigabit Ethernet copper and fiber standards. Gigabit Ethernet standards are now the dominant technology for backbone installations, high-speed cross-connects, and general infrastructure needs. The timing, frame format, and transmission are all common to both versions of 100 Mbps Fast Ethernet. 100BASE-FX also uses 4B/5B encoding. In Figure notice the highlighted waveform in the example. The top waveform has no transition, which indicates that a binary 0 is present. In the second waveform, a transition is in the center of the timing window. A binary 1 is represented by a transition. In the third waveform, there is an alternating binary sequence. In this example it is more obvious that no transition indicates a binary 0 and the presence of a transition is a binary 1. Figure summarizes a 100BASE-FX link and pinouts. Fiber pair with either ST or SC connectors is most commonly used. 200 Mbps transmission is possible because of the separate Transmit and Receive paths in 100BASE-FX optical fiber.

Fast Ethernet architecture

Fast Ethernet links generally consist of a connection between a station and a hub or switch. Hubs are considered multi-port repeaters and switches are considered multi-port bridges. These are subject to the 100 m UTP media distance limitation. A Class I repeater may introduce up to 140 bit-times of latency. Any repeater that changes between one Ethernet implementation and another is a Class I repeater. A Class II repeater may only introduce a maximum of 92 bit-times latency. Because of the reduced latency it is possible to have two Class II repeaters in series, but only if the cable between them is very short. As with 10 Mbps versions, it is possible to modify some of the architecture rules for 100 Mbps versions. However there is virtually no allowance for additional delay. Modification of the architecture rules is strongly discouraged for 100BASE-TX. 100BASE-TX cable between Class II repeaters may not exceed 5 meters. Links operating in half duplex are not uncommon to find in Fast Ethernet. However, half duplex is undesirable because the signaling scheme is inherently full duplex. Figure shows architecture configuration cable distances. 100BASE-TX links can have unrepeated distances up to 100 m. The widespread introduction of switches has made this distance limitation less important. If workstations are located within 100 m of a switch, the 100 m distance starts over at the switch. Since most Fast Ethernet is switched, these are the practical limits between devices. Cisco Systems, Inc.