Friday, February 26, 2010
Saturday, February 6, 2010
Gigabit Ethernet architecture / 10-Gigabit Ethernet
7.2.4 This page will discuss the architecture of Gigabit Ethernet.
The distance limitations of full-duplex links are only limited by the medium, and not the round-trip delay. Since most Gigabit Ethernet is switched, the values in Figures and are the practical limits between devices. Daisy-chaining, star, and extended star topologies are all allowed. The issue then becomes one of logical topology and data flow, not timing or distance limitations.
A 1000BASE-T UTP cable is the same as 10BASE-T and 100BASE-TX cable, except that link performance must meet the higher quality Category 5e or ISO Class D (2000) requirements.
Modification of the architecture rules is strongly discouraged for 1000BASE-T. At 100 meters, 1000BASE-T is operating close to the edge of the ability of the hardware to recover the transmitted signal. Any cabling problems or environmental noise could render an otherwise compliant cable inoperable even at distances that are within the specification.
It is recommended that all links between a station and a hub or switch be configured for Auto-Negotiation to permit the highest common performance. This will avoid accidental misconfiguration of the other required parameters for proper Gigabit Ethernet operation.
The next page will discuss 10-Gigabit Ethernet.
7.2.5 This page will describe 10-Gigabit Ethernet and compare it to other versions of Ethernet.
IEEE 802.3ae was adapted to include 10 Gbps full-duplex transmission over fiber optic cable. The basic similarities between 802.3ae and 802.3, the original Ethernet are remarkable. This 10-Gigabit Ethernet (10GbE) is evolving for not only LANs, but also MANs, and WANs.
With the frame format and other Ethernet Layer 2 specifications compatible with previous standards, 10GbE can provide increased bandwidth needs that are interoperable with existing network infrastructure.
A major conceptual change for Ethernet is emerging with 10GbE. Ethernet is traditionally thought of as a LAN technology, but 10GbE physical layer standards allow both an extension in distance to 40 km over single-mode fiber and compatibility with synchronous optical network (SONET) and synchronous digital hierarchy (SDH) networks. Operation at 40 km distance makes 10GbE a viable MAN technology. Compatibility with SONET/SDH networks operating up to OC-192 speeds (9.584640 Gbps) make 10GbE a viable WAN technology. 10GbE may also compete with ATM for certain applications.
To summarize, how does 10GbE compare to other varieties of Ethernet?
• Frame format is the same, allowing interoperability between all varieties of legacy, fast, gigabit, and 10 gigabit, with no reframing or protocol conversions.
• Bit time is now 0.1 nanoseconds. All other time variables scale accordingly.
• Since only full-duplex fiber connections are used, CSMA/CD is not necessary.
• The IEEE 802.3 sublayers within OSI Layers 1 and 2 are mostly preserved, with a few additions to accommodate 40 km fiber links and interoperability with SONET/SDH technologies.
• Flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible.
• TCP/IP can run over LANs, MANs, and WANs with one Layer 2 transport method.
The basic standard governing CSMA/CD is IEEE 802.3. An IEEE 802.3 supplement, entitled 802.3ae, governs the 10GbE family. As is typical for new technologies, a variety of implementations are being considered, including:
• 10GBASE-SR – Intended for short distances over already-installed multimode fiber, supports a range between 26 m to 82 m
• 10GBASE-LX4 – Uses wavelength division multiplexing (WDM), supports 240 m to 300 m over already-installed multimode fiber and 10 km over single-mode fiber
• 10GBASE-LR and 10GBASE-ER – Support 10 km and 40 km over single-mode fiber
• 10GBASE-SW, 10GBASE-LW, and 10GBASE-EW – Known collectively as 10GBASE-W, intended to work with OC-192 synchronous transport module SONET/SDH WAN equipment
The IEEE 802.3ae Task force and the 10-Gigabit Ethernet Alliance (10 GEA) are working to standardize these emerging technologies.
10-Gbps Ethernet (IEEE 802.3ae) was standardized in June 2002. It is a full-duplex protocol that uses only optic fiber as a transmission medium. The maximum transmission distances depend on the type of fiber being used. When using single-mode fiber as the transmission medium, the maximum transmission distance is 40 kilometers (25 miles). Some discussions between IEEE members have begun that suggest the possibility of standards for 40, 80, and even 100-Gbps Ethernet.
The next page will discuss the architecture of 10-Gigabit Ethernet.
1000BASE-T / 1000BASE-SX and LX
7.2.2 This page will describe 1000BASE-T.
As Fast Ethernet was installed to increase bandwidth to workstations, this began to create bottlenecks upstream in the network. The 1000BASE-T standard, which is IEEE 802.3ab, was developed to provide additional bandwidth to help alleviate these bottlenecks. It provided more throughput for devices such as intra-building backbones, inter-switch links, server farms, and other wiring closet applications as well as connections for high-end workstations. Fast Ethernet was designed to function over Category 5 copper cable that passes the Category 5e test. Most installed Category 5 cable can pass the Category 5e certification if properly terminated. It is important for the 1000BASE-T standard to be interoperable with 10BASE-T and 100BASE-TX.
Since Category 5e cable can reliably carry up to 125 Mbps of traffic, 1000 Mbps or 1 Gigabit of bandwidth was a design challenge. The first step to accomplish 1000BASE-T is to use all four pairs of wires instead of the traditional two pairs of wires used by 10BASE-T and 100BASE-TX. This requires complex circuitry that allows full-duplex transmissions on the same wire pair. This provides 250 Mbps per pair. With all four-wire pairs, this provides the desired 1000 Mbps. Since the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver.
The 1000BASE-T encoding with 4D-PAM5 line encoding is used on Category 5e, or better, UTP. That means the transmission and reception of data happens in both directions on the same wire at the same time. As might be expected, this results in a permanent collision on the wire pairs. These collisions result in complex voltage patterns. With the complex integrated circuits using techniques such as echo cancellation, Layer 1 Forward Error Correction (FEC), and prudent selection of voltage levels, the system achieves the 1-Gigabit throughput.
In idle periods there are nine voltage levels found on the cable, and during data transmission periods there are 17 voltage levels found on the cable. With this large number of states and the effects of noise, the signal on the wire looks more analog than digital. Like analog, the system is more susceptible to noise due to cable and termination problems.
The data from the sending station is carefully divided into four parallel streams, encoded, transmitted and detected in parallel, and then reassembled into one received bit stream. Figure represents the simultaneous full duplex on four-wire pairs. 1000BASE-T supports both half-duplex as well as full-duplex operation. The use of full-duplex 1000BASE-T is widespread.
The next page will introduce 1000BASE-SX and LX
7.2.3 This page will discuss single-mode and multimode optical fiber.
The IEEE 802.3 standard recommends that Gigabit Ethernet over fiber be the preferred backbone technology.
The timing, frame format, and transmission are common to all versions of 1000 Mbps. Two signal-encoding schemes are defined at the physical layer. The 8B/10B scheme is used for optical fiber and shielded copper media, and the pulse amplitude modulation 5 (PAM5) is used for UTP.
1000BASE-X uses 8B/10B encoding converted to non-return to zero (NRZ) line encoding. NRZ encoding relies on the signal level found in the timing window to determine the binary value for that bit period. Unlike most of the other encoding schemes described, this encoding system is level driven instead of edge driven. That is the determination of whether a bit is a zero or a one is made by the level of the signal rather than when the signal changes levels.
The NRZ signals are then pulsed into the fiber using either short-wavelength or long-wavelength light sources. The short-wavelength uses an 850 nm laser or LED source in multimode optical fiber (1000BASE-SX). It is the lower-cost of the options but has shorter distances. The long-wavelength 1310 nm laser source uses either single-mode or multimode optical fiber (1000BASE-LX). Laser sources used with single-mode fiber can achieve distances of up to 5000 meters. Because of the length of time to completely turn the LED or laser on and off each time, the light is pulsed using low and high power. A logic zero is represented by low power, and a logic one by high power.
The Media Access Control method treats the link as point-to-point. Since separate fibers are used for transmitting (Tx) and receiving (Rx) the connection is inherently full duplex. Gigabit Ethernet permits only a single repeater between two stations. Figure is a 1000BASE Ethernet media comparison chart.
The next page describes the architecture of Gigabit Ethernet
Fast Ethernet architecture / 1000-Mbps Ethernet
7.1.9 This page describes the architecture of Fast Ethernet.
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 (328 ft) UTP media distance limitation.
A Class I repeater may introduce up to 140 bit-times latency. Any repeater that changes between one Ethernet implementation and another is a Class I repeater. A Class II repeater is restricted to smaller timing delays, 92 bit times, because it immediately repeats the incoming signal to all other ports without a translation process. To achieve a smaller timing delay, Class II repeaters can only connect to segment types that use the same signaling technique.
As with 10-Mbps versions, it is possible to modify some of the architecture rules for 100-Mbps versions. Modification of the architecture rules is strongly discouraged for 100BASE-TX. 100BASE-TX cable between Class II repeaters may not exceed 5 m (16 ft). Links that operate in half duplex are not uncommon 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. Switches have made this distance limitation less important. Most Fast Ethernet implementations are switched.
This page concludes this lesson. The next lesson will discuss Gigabit and 10-Gigabit Ethernet. The first page describes 1000-Mbps Ethernet standards.
7.2.1 This page covers the 1000-Mbps Ethernet or Gigabit Ethernet standards. These standards specify both fiber and copper media for data transmissions. The 1000BASE-T standard, IEEE 802.3ab, uses Category 5, or higher, balanced copper cabling. The 1000BASE-X standard, IEEE 802.3z, specifies 1 Gbps full duplex over optical fiber.
1000BASE-TX, 1000BASE-SX, and 1000BASE-LX use the same timing parameters, as shown in Figure . They use a 1 ns, 0.000000001 of a second, or 1 billionth of a second bit time. The Gigabit Ethernet frame has the same format as is used for 10 and 100-Mbps Ethernet. Some implementations of Gigabit Ethernet may use different processes to convert frames to bits on the cable. Figure shows the Ethernet frame fields.
The differences between standard Ethernet, Fast Ethernet and Gigabit Ethernet occur at the physical layer. Due to the increased speeds of these newer standards, the shorter duration bit times require special considerations. Since the bits are introduced on the medium for a shorter duration and more often, timing is critical. This high-speed transmission requires higher frequencies. This causes the bits to be more susceptible to noise on copper media.
These issues require Gigabit Ethernet to use two separate encoding steps. Data transmission is more efficient when codes are used to represent the binary bit stream. The encoded data provides synchronization, efficient usage of bandwidth, and improved signal-to-noise ratio characteristics.
At the physical layer, the bit patterns from the MAC layer are converted into symbols. The symbols may also be control information such as start frame, end frame, and idle conditions on a link. The frame is coded into control symbols and data symbols to increase in network throughput.
Fiber-based Gigabit Ethernet, or 1000BASE-X, uses 8B/10B encoding, which is similar to the 4B/5B concept. This is followed by the simple nonreturn to zero (NRZ) line encoding of light on optical fiber. This encoding process is possible because the fiber medium can carry higher bandwidth signals.
The next page will discuss the 1000BASE-T standard.
100BASE-TX / 100BASE-FX
7.1.7 This page will describe 100BASE-TX.
In 1995, 100BASE-TX was the standard, using Category 5 UTP cable, which became commercially successful.
The original coaxial Ethernet used half-duplex transmission so only one device could transmit at a time. 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 replaced hubs in many networks. These switches had full-duplex capabilities and could handle Ethernet frames quickly.
100BASE-TX uses 4B/5B encoding, which is then scrambled and converted to Multi-Level Transmit (MLT-3) encoding. Figure shows four waveform examples. The top waveform has no transition in the center of the timing window. No transition indicates a binary zero. The second waveform shows a transition in the center of the timing window. A transition represents a binary one. The third waveform shows an alternating binary sequence. The fourth wavelength shows that signal changes indicate ones and horizontal lines indicate zeros.
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 more important as Ethernet speeds increase.
7.1.8 This page covers 100BASE-FX.
When copper-based Fast Ethernet was introduced, a fiber version was also desired. A fiber version could be used for backbone applications, connections between floors, 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 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 the same in both versions of 100-Mbps Fast Ethernet. In Figure , the top waveform has no transition, which indicates a binary 0. In the second waveform, the transition in the center of the timing window indicates a binary 1. In the third waveform, there is an alternating binary sequence. In the third and fourth waveforms it is more obvious that no transition indicates a binary zero and the presence of a transition is a binary one.
Figure summarizes a 100BASE-FX link and pinouts. A fiber pair with either ST or SC connectors is most commonly used.
The separate Transmit (Tx) and Receive (Rx) paths in 100BASE-FX optical fiber allow for 200-Mbps transmission.
The next page will explain the Fast Ethernet architecture.
10BASE-T wiring and architecture / 100-Mbps Ethernet
7.1.5 This page explains the wiring and architecture of 10BASE-T.
A 10BASE-T link generally connects a station to 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. Bridges and switches divide segments into separate collision domains. The maximum distance between bridges and switches is based on media limitations.
Although hubs may be linked, it is best to avoid this arrangement. A network with linked hubs may exceed the limit for maximum delay between stations. Multiple hubs should be arranged in hierarchical order like a tree structure. Performance is better if fewer repeaters are used between stations.
An architectural example is shown in Figure . The 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 of up to 100 m (328 ft). While this may seem like a long distance, it is typically maximized 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. If workstations are located within 100 m (328 ft) of a switch, the 100-m distance starts over at the switch.
The next page will describe Fast Ethernet.
7.1.6 This page will discuss 100-Mbps Ethernet, which 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 100BASE-FX both share timing parameters. Note that one bit time at 100-Mbps = 10 ns = .01 microseconds = 1 100-millionth of a second.
The 100-Mbps frame format is the same as the 10-Mbps frame.
Fast Ethernet is ten times faster than 10BASE-T. The bits that are sent are shorter in duration and occur 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.
The next page will discuss the 100BASE-TX standard.
10BASE2
10BASE2
7.1.3 This page covers 10BASE2, which was introduced in 1985.
Installation was easier because of its smaller size, lighter weight, and greater flexibility. 10BASE2 still exists in legacy networks. Like 10BASE5, it is no longer recommended for network installations. It has a low cost and does not require hubs.
10BASE2 also uses Manchester encoding. Computers on a 10BASE2 LAN are linked together by an unbroken series of coaxial cable lengths. These lengths are attached to a T-shaped connector on the NIC with BNC connectors.
10BASE2 has a stranded central conductor. Each of the maximum five segments of thin coaxial cable may be up to 185 m (607 ft) long and each station is connected directly to the BNC T-shaped connector on the coaxial cable.
Only one station can transmit at a time or 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 a 10BASE2 segment. Only three out of five consecutive segments between any two stations can be populated.
The next page will discuss 10BASE-T.
10BASE-T
7.1.4 This page covers 10BASE-T, which was introduced in 1990.
10BASE-T used cheaper and easier to install Category 3 UTP copper cable instead of 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. As additional stars were added and the cable distances grew, this formed 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. The maximum cable length is 90 m (295 ft). UTP cable uses eight-pin RJ-45 connectors. Though Category 3 cable is adequate for 10BASE-T networks, new cable installations should 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. This type of cable installation supports the use of multiple protocols without the need to rewire. Figure shows the pinout arrangement for a 10BASE-T connection. The pair that transmits data on one device is connected to the pair that receives data on the other 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.
The next page describes the wiring and architecture of 10BASE-T.
7.1.3 This page covers 10BASE2, which was introduced in 1985.
Installation was easier because of its smaller size, lighter weight, and greater flexibility. 10BASE2 still exists in legacy networks. Like 10BASE5, it is no longer recommended for network installations. It has a low cost and does not require hubs.
10BASE2 also uses Manchester encoding. Computers on a 10BASE2 LAN are linked together by an unbroken series of coaxial cable lengths. These lengths are attached to a T-shaped connector on the NIC with BNC connectors.
10BASE2 has a stranded central conductor. Each of the maximum five segments of thin coaxial cable may be up to 185 m (607 ft) long and each station is connected directly to the BNC T-shaped connector on the coaxial cable.
Only one station can transmit at a time or 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 a 10BASE2 segment. Only three out of five consecutive segments between any two stations can be populated.
The next page will discuss 10BASE-T.
10BASE-T
7.1.4 This page covers 10BASE-T, which was introduced in 1990.
10BASE-T used cheaper and easier to install Category 3 UTP copper cable instead of 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. As additional stars were added and the cable distances grew, this formed 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. The maximum cable length is 90 m (295 ft). UTP cable uses eight-pin RJ-45 connectors. Though Category 3 cable is adequate for 10BASE-T networks, new cable installations should 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. This type of cable installation supports the use of multiple protocols without the need to rewire. Figure shows the pinout arrangement for a 10BASE-T connection. The pair that transmits data on one device is connected to the pair that receives data on the other 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.
The next page describes the wiring and architecture of 10BASE-T.
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