High Speed LANs

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The IEEE 802.3 to 802.6 LANs having data transfer rates in the range of 10 Mbps to 16 Mbps have served the purpose very well for many years. But, with the availability of powerful computers at a low cost and emergence of new applications, particularly based on multimedia, there is a growing demand for higher network bandwidth. The combined effect of the growth in the number of users and increasing bandwidth requirement per user has led to the development of High Speed LANs with data transfer rate of 100 Mbps or more.

The high speed LANs that have emerged can be broadly categorized into three types based on token passing, successors of Ethernet and switching technology. These categories are:

1. FDDI and its variations, and high speed token ring
2. Fast Ethernet, Gigabit Ethernet and 100 VG-AnyLAN
3. ATM, Fiber channel and Ether-switches

We shall discuss some of the popular high speed LANs.

FDDI

Before we discuss FDDI, let us revisit the fiber optics network.

Fiber Optics

Computer technology is improving at a very fast pace. In the 1970s, a fast computer (e.g., CDC 6600) could execute an instruction in 100 nsec. Twenty years later, a fast Cray computer could execute an instruction in 1 nsec, a factor of 10 improvement per decade.

In the same period, data communication went from 56 kbps (the ARPANET) to 1 Gbps (modern optical communication), a gain of more than a factor of 100 per decade, while at the same time the error rate went down from 10^-5 per bit to almost zero.

Furthermore, single CPUs are beginning to approach physical limits, such as speed of light and heat dissipation problems. As a result, though, with current fiber technology, the achievable bandwidth is in excess of 50,000 Gbps (50 Tbps), scientists and researchers are still looking very hard for better materials to achieve this level. The current practical signaling limit of about 1 Gbps is due to our inability to convert between electrical and optical signals any faster. In the laboratory, 100 Gbps is feasible on short runs. A speed of 1 terabit/sec is only a few years down the road.

We will study fiber optics to see how that transmission technology works.

An optical transmission system has three components: the light source, the transmission medium, and the detector. Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a zero bit. The transmission medium is an ultra-thin fiber of glass. The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it to light pulses, and then reconverts the output to an electrical signal at the receiving end.

This transmission system would leak light and be useless in practice except for an interesting principle of physics. When a light ray passes from one medium to another, for example, from fused silica to air, the ray is refracted (bent) at the silica/air boundary as shown in Figure below.

Here we see a light ray incident on the boundary at an angle a₁ emerging at an angle b₁. The amount of refraction depends on the properties of the two media (in particular, their indices of refraction). For angles of incidence above a certain critical value, the light is refracted back into the silica; none of it escapes into the air. Thus a light ray incident at or above the critical angle is trapped inside the fiber, as shown in Fig.(b) above, and can propagate for many kilometers with virtually no loss.

The sketch of Fig.(b) above shows only one trapped ray, but since any light ray incident on the boundary above the critical angle will be reflected internally, many different rays will be bouncing around at different angles. Each ray is said to have a different mode so a fiber having this property is called a multimode fiber.

However, if the fiber’s diameter is reduced to a few wavelengths of light, the fiber acts like a wave guide, and the light can only propagate in a straight line, without bouncing, yielding a single-mode fiber. Single mode fibers are more expensive but can be used for longer distances. Currently available single-mode fibers can transmit data at several Gbps for 30 km. Even higher data rates have been achieved in the laboratory for shorter distances. Experiments have shown that powerful lasers can drive a fiber 100 km long without repeaters, although at lower speeds. Research on erbium-doped fibers promises even longer runs without repeaters.

Transmission of Light through Fiber

Optical fibers are made of glass, which in turn, is made from sand, an inexpensive raw material available in unlimited amounts. The glass used for modem optical fibers is so transparent that if the oceans were full of it instead of water, the seabed would be as visible from the surface as the ground is from an airplane on a clear day.

The attenuation of light through glass depends on the wavelength of the light. The near infrared part of the spectrum is used in practice in which case the attenuation is less. Visible light has slightly shorter wavelengths, from 0.4 to 0.7 microns.

Three wavelength bands are used for communication. They are centered at 0.85, 1.30, and 1.55 microns respectively. The latter two have good attenuation properties (less than 5 percent loss per kilometer). The 0.85 micron band has higher attenuation, but the nice property that at that wavelength, the lasers and electronics can be made from the same material (gallium arsenide). All three bands are 25,000 to 30,000 GHz wide.

Light pulses sent down a fiber spread out in length as they propagate. This spreading is called dispersion. The amount of it is wavelength dependent. One way to keep these spread-out pulses from overlapping is to increase the distance between them. But this can only be done by reducing the signaling rate. Fortunately, it has been discovered that by making the pulses in a special shape related to the reciprocal of the hyperbolic cosine, all the dispersion effects cancel out, and it may be possible to send pulses for thousands of kilometers without appreciable shape distortion. These pulses are called solitons. A considerable amount of research is going on to take solitons out of the lab and into the field.

Fiber Cables

Fiber optic cables are similar to coax, except without the braid. The figure (a) below shows a single fiber viewed from the side. At the center is the glass core through which the light propagates. In multimode fibers, the core is 50 microns in diameter, about the thickness of a human hair. In single-mode fibers the core is 8 to 10 microns.

The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Next comes a thin plastic jacket to protect the cladding. Fibers are typically grouped together in bundles, protected by an outer sheath. Figure (b) above shows a sheath with three fibers.

Terrestrial fiber sheaths are normally laid in the ground within a meter of the surface, where they are occasionally subject to attacks by backhoes or gophers (a burrowing rat-like animal). Near the shore, transoceanic fiber sheaths are buried in trenches by a kind of seaplow. In deep water, they just lie on the bottom, where they can be snagged by fishing trawlers or eaten by sharks.

Fibers can be connected in three different ways. First, they can terminate in connectors and be plugged into fiber sockets. Connectors lose about 10 to 20 percent of the light, but they make it easy to reconfigure systems.

Second, they can be spliced mechanically. Mechanical splices just lay the two carefully cut ends next to each other in a special sleeve and clamp them in place. Alignment can be improved by passing light through the junction and then making small adjustments to maximize the signal. Mechanical splices take trained personnel about 5 minutes, and result in a 10 percent light loss.

Third, two pieces of fiber can be fused (melted) to form a solid connection. A fusion splice is almost as good as a single drawn fiber, but even here, a small amount of attenuation occurs. For all three kinds of splices, reflections can occur at the point of the splice, and the reflected energy can interfere with the signal.

Two kinds of light sources can be used to do the signaling, LEDs (Light Emitting Diodes) and semiconductor lasers. They have different properties, as shown in table below.

Item	LED	Semiconductor laser
Data rate	Low	High
Mode	Multimode	Multimode or single mode
Distance	Short	Long
Lifetime	Long life	Short life
Temperature sensitivity	Minor	Substantial
Cost	Low cost	Expensive

The receiving end of an optical fiber consists of a photodiode, which gives off an electrical pulse when struck by light. The typical response time of a photodiode is 1 nsec, which limits data rates to about I Gbps. Thermal noise is also an issue, so a pulse of light must carry enough energy to be detected. By making the pulses powerful enough, the error rate can be made arbitrarily small.

Fiber Optic Networks

Fiber optics can be used for LANs as well as for long-haul transmission, although tapping on to it is more complex than connecting to an Ethernet. One way around the problem is to realize that a ring network is really just a collection of point-to-point links, as shown in figure below.

The interface at each computer passes the light pulse stream through to the next link and also serves as a T junction to allow the computer to send and accept messages.

Two types of interfaces are used. A passive interface consists of two taps fused onto the main fiber. One tap has an LED or laser diode at the end of it (for transmitting), and the other has a photodiode (for receiving). The tap itself is completely passive and is thus extremely reliable because a broken LED or photodiode does not break the ring. It just takes one computer off-line.

The other interface type, shown in the figure above, is the active repeater. The incoming light is converted to an electrical signal, regenerated to full strength if it has been weakened, and retransmitted as light. The interface with the computer is an ordinary copper wire that comes into the signal regenerator. Purely optical repeaters are now being used, too. These devices do not require the optical to electrical to optical conversions, which means they can operate at extremely high bandwidths.

If an active repeater fails, the ring is broken and the network goes down. On the other hand, since the signal is regenerated at each interface, the individual computer-to-computer links can be kilometers long, with virtually no limit on the total size of the ring. The passive interfaces lose light at each junction, so the number of computers and total ring length are greatly restricted.

A ring topology is not the only way to build a LAN using fiber optics. It is also possible to have a passive star construction.

Comparison of Fiber Optics and Copper Wire

Fiber has many advantages. To start with, it can handle much higher bandwidths than copper. This alone would require its use in high-end networks. Due to the low attenuation, repeaters are needed only about every 30 km on long lines, versus about every 5 km for copper, a substantial cost saving. Fiber also has the advantage of not being affected by power surges, electromagnetic interference, or power failures. Nor is it affected by corrosive chemicals in the air, making it ideal for harsh factory environments.

Oddly enough, telephone companies like fiber for a different reason - it is thin and lightweight. Many existing cable ducts are completely full, so there is no room to add new capacity. Removing all the copper and replacing it by fibers empties up the ducts, and the copper has excellent resale value to copper refiners who see it as very high grade ore. Also fiber is lighter than copper. One thousand twisted pairs 1 km long weigh 80,000 kg. Two fibers have more capacity and weigh only 100 kg, which greatly reduces the need for expensive mechanical support systems that must be maintained. For new routes, fiber wins hands down due to its much lower installation cost.

Finally, fibers do not leak light and are quite difficult to tap. This gives them excellent security against potential wiretappers.

On the downside, fiber is an unfamiliar technology requiring skills most engineers do not have. Since optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber.

Finally, fiber interfaces cost more than electrical interfaces. Nevertheless, the future of all fixed data communication for distances of more than a few meters is clearly with fiber.

FDDI Ring Standard

The Fiber Distributed Data Interface (FDDI), developed by ANSI, is a standard for a high-speed token passing ring LAN. It is a high performance fiber optic token ring LAN running at 100 Mbps over distances up to 200km with up to 1000 stations connected.

Like the IEEE 802.5 standard, FDDI employs the token ring algorithm. There are, however, several differences that are intended to allow FDDI to take advantage of the high speed (100 Mbps) of its ring and maximize efficiency and reliability.

In FDDI, a station emits a new token immediately following the frame, where as in IEEE token ring, a station emits a new token only after the leading edge of its transmitted frame returns. The FDDI scheme is thus more efficient, especially in large rings.

Another difference between the IEEE token ring algorithm and that of FDDI is in the area of capacity allocation. The FDDI scheme is designed to be efficient and flexible in meeting a wide range of high-speed requirements. Specifically, FDDI provides support for a mixture of stream and bursty traffic.

The key features of FDDI are:

Medium

The standard physical medium is multi-mode 62.5/125 micron optical fiber cable using light emitting diode (LED) transmitting at 1300 nanometers, as the light source. FDDI can support up to 500 stations with a maximum distance of 2 km between stations and maximum ring circumference of 200 km.

Single-mode 8-10/125 micron optical fiber cable has also been included in the standard for connecting a pair of stations separated by a distance in excess of 20 km.

The standard has also been extended to include copper media – Shielded Twisted Pair (STP) and some categories of Unshielded Twisted Pair (UTP) with a maximum distance of 100 m between stations.

Topology

The basic topology for FDDI is dual counter rotating rings – one transmitting clockwise and the other transmitting counter clockwise, as illustrated below.

One is known as primary ring and the other secondary ring. Although, theoretically, both the rings can be used to achieve a data transfer rate of 200 Mbps, the standard recommends to use the primary ring for data transmission and secondary ring as a backup. In case of failure of a node or a fiber link, the ring is restored by wrapping the primary ring to the secondary ring as shown in figures below.

The redundancy in the ring design provides a degree of fault tolerance not found in other network standards.

Medium Access Control

The FDDI medium access control protocol is responsible for the following services.

1. Fair and equal access to the ring by using a timed token protocol. To transmit on the ring, a station must first acquire the token. A station holds the token until it has transmitted all of its frames or until the transmission time for the appropriate service is over. Synchronous traffic is given a guaranteed bandwidth by ensuring that token rotation time does not exceed a preset value. FDDI implements this using three timers, Token holding Timer (THT), which determines how long a station may continue once it has captured a token. Token Rotation Timer (TRT) is reset every time a token is seen. When timer expires, it indicates that the token is lost and recovery is started. The Valid Transmission Timer (VTT) is used to time out and recover from some transient (lasting for a short time) ring errors.

2. Construction of frames and tokens are done as per the format shown below. The frame status (FS) byte is set by the destination and checked by the source station which removes its frame from the ring and generates another token.

   

   PA = Preamble	SD = Starting Delimiter
   FC = Frame Control	DA = Destination Address
   SA = Source Address	FCS = Frame Check Sequence
   ED = Ending Delimiter	FS = Frame Status

3. Transmitting, receiving, repeating and stripping (draining out of ring) frames and tokens from the ring, unlike IEEE 802.5, is possible for several frames on the ring simultaneously. Thus a station will transmit a token immediately after the completion of its frame transmission. A station further down the ring is allowed to insert its own frame. This improves the potential throughput of the system. When the frame returns to the sending station, that station removes the frame from the ring by a process called stripping.

4. It also does ring initialization, fault isolation and various error detection mechanisms in a similar way as discussed for IEEE 802.5.

Due to its high bandwidth, FDDI is quite commonly used as a backbone to connect copper LANs, as shown below.

The FDDI has been successfully used as a backbone LAN in an enterprise network or in a campus network.

FDDI was supposed to be the next generation LAN, but it never really caught on much beyond the backbone market. The station management was too complicated, which led to complex chips and high prices. The substantial cost of FDDI chips made workstation manufacturers unwilling to make FDDI the standard network. So FDDI could never break trough the market.

Comparison of the important features of the FDDI with the two popular IEEE 802 LAN standards is given in the following table.

	FDDI	IEEE 802.3 (Ethernet)	IEEE 802.5 (Token Ring)
BANDWITH	100Mb/s	10Mb/s	4 or 16Mb/s
NUMBER OF STATIONS in a single network	500	1024	250
MAX. DISTANCE BETWEEN STATIONS	2 Km (MMF) 20 Km (SMF)	2.5 Km	300m (4Mb/s) 100m (recommended)
MAX. NETWORK EXTENT	100Km	2.5 Km	VARIED WITH CONFIGURATION
LOGICAL TOPOLOGY	DUAL RING, DUAL RING OF TREES	BUS	SINGLE RING
PHYSICAL TOPOLOGY	RING, STAR HIERARCHICAL STAR	BUS, STAR	RING, BUS, HIERARCHICAL STAR
MEDIA	OPTICAL FIBER	OPTICAL FIBER, TWISTED-WIRE, COAXIAL CABLE	TWISTED-WIRE OPTICAL FIBER
ACCESS METHOD	TIMED-TOKEN PASSING	CSMA/CD	TOKEN PASSING
TOKEN ACQUISITION	CAPTURES THE TOKEN		BY SETTING A STATUS BIT
TOKEN RELEASE	AFTER TRANSMIT		AFTER STRIPPING (in case of 4 Mbps) OR AFTER TRANSMIT (in case of 16 Mbps)
FRAMES ON LAN	MULTIPLE	SINGLE	SINGLE
FRAMES TRANSMITTED PER ACCESS	MULTIPLE	SINGLE	SINGLE
MAX. FRAME SIZE	4500 BYTES	1518 BYTES	4500 BYTES (4Mb/s) 17,800 BYTES (16Mb/s)

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