Fiber Optic Communications for the Premises Environment

CHAPTER 4

EXPLOITING THE DELAY PROPERTIES OF FIBER OPTIC CABLE FOR LOCAL AREA NETWORK (LAN) EXTENSION


Fiber optic cable provides a way for extending reach of Local Area Networks (LANs). If you are well versed on the subject of LANs you are welcome to jump right into this subject and skip the next two subchapters. However, if you have not been initiated into LAN technology then you will find the subjects covered in these next two subchapters worthwhile reading.

4.1 Brief History of Local Area Networks

Two full generations ago, in the early days of the data revolution, each computer served only a single user. In the computer room (or at that time 'the building') of an installation there was 1 CPU, 1 keyboard, 1 card reader, (maybe) 1 magnetic tape reader, 1 printer, 1 keypunch machine etc. From a usage point of view this was highly inefficient. Most data processing managers were concerned that this highly expensive equipment spent most of the time waiting for users to employ it. Most data processing managers knew this looked bad to the Controllers of their organizations. This led to the pioneering development of time-sharing operating systems by MIT with Project MAC.

Time sharing opened up computational equipment to more than 1 user. Whole departments, companies, schools etc. began making use of the expensive computational equipment. A key element in time sharing systems concerned the keyboard. A computer terminal replaced it. The multiple terminals were connected to the CPU by data communications links. There was a marriage of computation and data communications. In particular, the data communications was mostly (though not exclusively) premises data communications.

Throughout the years time sharing led to distributed computation. The idea of distributed computation being that applications programs would reside on one central computer called the Server. Applications users would reside at PCs. When an applications user wanted to run a program a copy of it would be downloaded to him/her. In this way multiple users could work with the same program simultaneously. This was much more efficient than the original time sharing. Distributed computation required a data communications network to tie the Server to the PCs and peripherals. This network was called a Local Area Network (LAN). This network had to have high bandwidth. In fact, it had to accommodate speeds that were orders of magnitude greater than the original time sharing networks. Entire applications programs had to be downloaded to multiple users. Files, the results of running applications programs, had to be uploaded to be stored in central memory.

LANs first came on the scene in a noticeable sense in the late 1970's. From that time until the present many flavors of LANs have been offered in the marketplace. There are still a number of different flavors each with its group of advocates and cult following. However, some time around the late 1980's the market place began to recognize Ethernet as the flavor of choice. All of the discussion in the sequel will concern only Ethernet.

The Ethernet LAN architecture had its origins in work done at Xerox Palo Alto Research Center (PARC) by Robert Metcalf in the early 1970's. Metcalf later went on to become the founder of 3COM. Xerox was later joined by DEC and Intel in promoting Ethernet as the coming LAN standard. In the development of the Ethernet LAN architecture Metcalf built upon previous research funded by the Advanced Research Projects Agency (ARPA) at the University of Hawaii. This ARPA program was concerned with an asynchronous multiple access data communications technique called ALOHA.

The basic operation of an Ethernet LAN can be briefly explained with the aid of Figure 4-1. This illustration indicates various data equipment that all need to communicate with each other. The data equipment constitute the users of the LAN. Each is a Source and User within the context of the discussion of Chapter 1. The location on the LAN of each data equipment unit is termed the station.


Figure 4-1: Ethernet Bus architecture


The communication between the data equipment is accomplished by having all the data equipment tap onto a Transmission Medium. Each station taps onto the Transmission Medium. The Transmission Medium is typically some type of cable. As shown in Figure 4-1 it is labeled Broadcast Channel - The Ethernet Bus. The Bus Interface Units (BIUs) provide the essential interfacing at a station between the data equipment and the Broadcast Channel. That is, they provide the transmit/receive capability and all needed intelligence.

It is an essential feature of the Ethernet LAN architecture that any data equipment can transmit to any other data equipment and any data equipment can listen to all transmissions on the Broadcast Channel, whether intended for it or for some other data equipment user. Implicitly, the Ethernet architecture assumes that there is no coordination in the transmissions of the different data equipment. This is quite a bit different from the sharing of a Transmission Medium by TDM where coordination is essential. Transmitted data only goes in its assigned slot.

Now how does an Ethernet LAN operate? It operates by making use of three essential items. First, it employs a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol. Secondly, data to be communicated is enveloped in packets that have the addresses of the data equipment units communicating. The packet has the address of the equipment sending data (the origin) and the data equipment that is the intended recipient (the destination). Thirdly, the Ethernet Bus - the Transmission Medium - is taken as passive and supports broadcast type transmissions. The way in which the Ethernet LAN architecture uses these items is explained briefly below.

Consider a specific data equipment unit at its station. This will be our data equipment unit, station and BIU of interest. For the sake of an example, suppose it is a PC wanting to communicate with the Computer with File Server at its station. Before attempting to transmit a data packet onto the Ethernet Bus our terminal's BIU first listens to determine if the Bus is idle. That is, it listens to determine if there are any other packets from other data equipment already on the Bus. It attempts to sense the presence of a communication signal representing a packet, a carrier, on the Bus. Our BIU and any BIU have circuitry to perform this Carrier Sensing. An active BIU transmits its packet on the Bus only if the Bus has been sensed as idle. In other words, it only transmits its packet if it has determined that no other packet is already on the Bus - carrier is absent. If the Bus is sensed as busy- carrier is present- then the BIU defers its transmission until the Bus is sensed as idle again. This procedure allows the various data equipment to operate asynchronously yet avoid interfering with one another's communications.

However, it may be that a carrier has not sensed an existing packet is already on the Bus. Transmission of a packet by the BIU of interest begins but there are still problems. There are propagation delays and carrier detection processing delays. Because of these, it may be that the packet from our PC's BIU still interferes with, or collides with, a packet transmitted by another equipment's BIU. This interfering packet is one that has not yet reached our BIU by the end of the interval in which it had performed the carrier sensing. A BIU monitors the transmission of the packet it is sending out to determine if it does collide with another packet. To do this it makes use of the broadcast nature of the transmission medium. A BIU can monitor what it has put on the Ethernet Bus and also any other traffic on the Ethernet Bus. Our BIU and any BIU has circuitry to perform Collision Detection. The BIU that transmitted the interfering packet also has circuitry to perform Collision Detection.

When both BIUs sense a collision they cease transmitting. Each BIU then waits a random amount of time before re-transmitting - that is sensing for carrier and transmitting the packet onto the bus. If another collision occurs then this random time wait is repeated but increased. In fact, it is increased at an exponential rate until the collision event disappears. This approach to getting out of collisions is called exponential back off.

4.2 Transmission Media Used To Implement An Ethernet LAN

Let us direct attention now to the Transmission Medium that is used to implement the Broadcast Channel, the Ethernet Bus.

Early implementations of Ethernet LANs employed thick coaxial cable. Actually, it was thick yellow coaxial cable - the original recipe Ethernet cable. The cable was defined by the 10Base-5 standard. This implementation was called Thicknet. It could deliver a BER of 10-8. It supported a data rate of 10 MBPS. The maximum LAN cable segment length was 500 meters. The segment length is the maximum distance between data terminal equipment stations. These are attractive features.

Unfortunately, the thick coaxial cable was difficult to work with. As a result, second wave implementations of Ethernet LANs employed thin coaxial cable. The cable was RG58 A/U coaxial cable - sometimes called Cheapernet. This cable was defined by the 10Base-2 standard. The implementation was called Thinnet. It supported a data rate of 10 MBPS. But, it had a BER somewhat degraded relative to Thicknet. The LAN cable segment length was reduced to the order of 185 meters.

Thinnet ultimately gave way to the replacement of coaxial cable with Unshielded Twisted Pair cable (UTP). This came about through an interesting merging of the Ethernet LAN architecture with another LAN flavor called StarLAN, an AT & T idea.

StarLAN was based upon what a Telco, a phone company, normally does for businesses that is, provide voice communications. The Transmission Medium a Telco uses within a facility for voice communications is Unshielded Twisted Pair cable (UTP). It provides voice communications within a facility and to the outside world by connecting all of the phones, the handsets, through a telephone closet or wiring closet. The distance from handset to telephone closet is relatively limited, maybe 250 meters. The StarLAN idea was to take this basic approach for voice and use it for a LAN. The LAN stations would be connected through a closet. The existing UTP cable present in a facility for voice would be used for the LAN data traffic. There would be no need to install a new and separate Transmission Medium. Installation costs would be contained. Unfortunately, StarLAN only supported 1 MBPS. It never got off the ground.

However, in 1990 aspects of StarLAN were taken and merged with the Ethernet LAN architecture. This resulted in a new Ethernet LAN based upon UTP and defined by the 10Base-T standard. It was with this UTP approach that Ethernet really took off in the market place.

Ethernet under the 10Base-T standard has a hub and spoke architecture. This is illustrated in Figure 4-2. The various data equipment units, the stations, are all connected to a central point called a Multipoint Repeater or Hub. The connections are by UTP cable. This architecture does support the Broadcast Channel-Ethernet Bus. This occurs because all data equipment units can broadcast to all other data equipment units through the Hub. Likewise, all data equipment units can listen to the transmissions from all other data equipment units as they are received via the UTP cable connection to the Hub. The Hub takes the place of the telephone closet. The Hub may be strictly passive or it may perform signal restoration functions.


Figure 4-2: 10Base-T hub-and-spoke architecture


The illustration Figure 4-3 indicates how the 10Base-T topology may actually look in an office set-up at some facility. Here the data equipment units are all PCs. One serves as the file server. The illustration shows what is usually referred to as a 10Base-T Work Group. It may serve one specific department in a company. By connecting together these work groups Ethernet LANs may be extended. This is accomplished by connecting hubs using LAN network elements called bridges, routers and switches. A description of their operation is beyond the focus of the present discussion.


Figure 4-3: Ethernet operating as a 10Base-T work group


But, let us get back to 10Base-T. It supports a data rate of 10 MBPS. It has a BER comparable to Thinnet. However, the LAN segment length is reduced even further. With 10Base-T the LAN segment length is only 100 m - a short distance but a distance that is tolerable for many data equipment stations in a typical business. However, it may be too short for others. This is a place where fiber optic cable can come to the rescue.

For the LAN market place 10Base-T was far from the last word. It led to the development of 100Base-T - Fast Ethernet. It is also based on using UTP cable for transmission medium. However, it supports a data rate of 100 MBPS over cable segments of 100 m.

Fast Ethernet, itself, is not the end of the road. Vendors are starting to promote Giga Bit Ethernet which is capable of supporting 1 GBPS. However, we will stop at Fast Ethernet and the problem that both it and 10Base-T have - the short cable segment of 100 m.

Before continuing it will be worthwhile to define two terms that come up in discussing Ethernet characteristics. These are 1) Network Diameter and 2) Slot Time.

The Network Diameter is simply the maximum end-to-end distance between data equipment users, stations, in an Ethernet network. It is really what has been referred to above as the cable segment. The Network Diameter is the same for both 10Base-T and 100Base-T, 100 m.

After a BIU has begun the transmission of a packet the Slot Time is the time interval that a BIU listens for the presence of a collision with an interfering packet. The Slot Time cannot be infinite. It is set for both the 10Base-T and 100Base-T Ethernet architectures. It is defined for both standards as the time duration of 512 bits. With a 10Base-T Ethernet network operating at 10 MBPS the Slot Time translates to 51.2msec. With a 100Base-T Ethernet network operating at 100 MBPS the Slot Time translates to 5.12msec.

4.3 Examining the Distance Constraint

The distance constraint of an Ethernet LAN is the Network Diameter. As noted above this is 100 m for both the 10Base-T and 100Base-T implementations. This may not be enough for all potential users of an Ethernet LAN. Now how do you support LAN users that are separated by more than this 100-m constraint? To deal with this question it is important to understand where this constraint comes from and what is driving it.

Many people believe that the Network Diameter is set strictly by the attenuation properties of the UTP copper cable connecting data equipment to the Hub. This is erroneous. Attenuation does affect the Network Diameter, but it is not the dominant influence. However, if it were, you would be able to see the immediate possibilities of improving it by using fiber optic cable rather than UTP copper cable. The significantly less attenuation of fiber optic cable would boost the Network Diameter. No, it is not attenuation but instead the Slot Time that really sets the Network Diameter.

The Slot Time is related to the amount of time delay between a transmitting BIU and the furthermost receiving BIU. The diagram showed in Figure 4-3 illustrates the Slot Time issues to be discussed now. Here we show two data equipment users of an Ethernet LAN - either 10Base-T or 100Base-T - it doesn't matter. These are labeled as Data Terminal Equipment Unit A and Data Terminal Equipment Unit B. For brevity they will be referred to as Unit A and Unit B. The BIU's are taken as subsumed in the ovals.


Figure 4-4: 2 Stations communicating on an Ethernet Bus. Delays shown.


Suppose Unit A transmits a data packet over the Ethernet Bus to Unit B. The transmitted data packet travels along the Ethernet Bus. It takes a time interval of TA seconds to reach Unit B. In the meantime, Unit B has performed carrier sensing and has determined, from its perspective, that the Ethernet Bus is not busy and so it also begins to transmit a data packet. From a collision detection point of view the worst case occurs when Unit B begins to transmit its data packet just before the data packet from Unit A arrives in front of it. Why is this worst case? When the Unit A data packet arrives at Unit B, Unit B immediately knows that a collision has occurred and can begin recovery operations. However, Unit A will not know that there has been any collision problem until the data packet from Unit B arrives in front of it. This packet from Unit B takes a time interval of TB seconds to arrive at Unit A. Putting this together Unit A has to wait at least TA + TB seconds before it can detect the presence/absence of a collision. There is some additional time needed to sense the presence/absence of a collision at both Unit A and Unit B. The collision detection processing time is denoted as TC. For 100Base-T networks a typical value for this is 1.12 msec. The Slot Time is the sum TA + TB + TC.

TA and TB usually can be taken as equal and denoted as t. Putting these together brings:

t = (Slot Time- TC) / 2


The one way delay, t, is equal to the distance between Unit A and Unit B divided by the velocity of transmission between Units A and B. The maximum distance is of course the Network Diameter. The velocity of transmission will be denoted by 'V.' This is the speed of an electromagnetic wave on the Ethernet Bus. Applying these brings:

Network Diameter = (V/2)(Slot Time- TC)


The Slot Time is fixed by the 10Base-T and 100Base-T Ethernet standards. TC is a function of BIU design. It is evident then that it is the value of V that really drives the Network Diameter. In characterizing the Ethernet Bus you usually deal with the inverse of V. For UTP copper cable V-1 is approximately 8 nsec/m. Consider a 100Base-T Ethernet LAN. Applying this value for V-1 above brings a value of 250 m for the Network Diameter. On the face of it this is quite a bit better than the 100-m allotted for the Network Diameter by the standard. The difference is accounted for by a number of delay items that were excluded from the example. These were excluded in order to bring out the principle point - the dependence of Network Diameter on V-1. This difference is taken up by margin allotted for other processing functions. These functions include the delay through the Hub. They include processing delays in software at the interface between the data equipment and its BIU. The margin is also allotted for deleterious properties of cable.

However, the essential point remains. The achievable Network Diameter is determined by the delay through the transmission medium. The speed of V-1 through UTP copper cable results in a Network Diameter of 100 m.

Consider a fiber optic cable. Typically, the value of V-1 is 5 nsec/m for multi-mode fiber optic cable. This is almost 50% lower than for UTP copper cable. Applying this value in the above example would bring a Network Diameter of 400 m, quite a bit more than 250 m.

By using a fiber optic cable you can connect data equipment stations to the LAN that are much further apart than the 100 m distance allowed for by the assumed UTP copper cable in 10Base-T or 100Base-T LANs. You can do this because the velocity of light through a fiber optic cable is much faster than the group velocity of electromagnetic waves in copper cable- the speed of current in copper cable. You can do this because the transmission delay, V-1, of a packet traversing a fiber optic cable is about 50% lower than it is for UTP copper cable.

How would you do it? How would you exploit a fiber optic cable to bring distant users into a UTP copper cable based Ethernet LAN? How would you accommodate really distant stations to a 10Base-T or 100Base-T Ethernet LAN, stations much further than the Network Diameter?

In order to do this you need to connect them to the Hub using a fiber optic cable. This may be either a multi-mode or single-mode fiber optic cable. However, neither the Ethernet Hub nor the BIU at the distant data equipment user knows anything about signaling on a fiber optic cable Transmission Medium. So, at the Hub you need some type of equipment that will take the 10Base-T or 100Base-T packets, in their electrical format, and convert it to light to propagate down a fiber optic cable. You need the same equipment at the distant data equipment's BIU for transmission toward the Hub. Similarly, you need this device to be able to take the light wave representations of a packet coming out of the fiber optic cable and convert it to an electrical format recognizable by the Hub or the BIU. This is called a LAN Extender.

By using a LAN Extender you get a distance benefit. In addition, on the particular LAN link you get the other benefits available with fiber optic cable. These include protection from ground loops, power surges and lightning.

4.4 Examples of LAN Extenders Shown In Typical Applications

Telebyte offers a variety of LAN Extenders. These are now described.

Model 373 10Base-T to Multimode Fiber Optic Transceiver
This unit is pictured in Figure 4-5. It extends the distance of a 10Base-T Ethernet LAN to over 2 km. The Model 373 10Base-T to Multi-Mode Fiber Optic Transceiver takes 10Base-T Ethernet signals and converts them to/from optical signals that are transmitted/received from multi-mode fiber optic cable.


Figure 4-5: Model 373 10Base-T to Multi-Mode Fiber Optic Converter


The Model 373 has a group of five LED's. These indicate the presence of the fiber optic link, traffic going back and forth in both directions, the presence of a collision and power. The unit even includes a Link Test switch. This assures compatibility between older and newer Ethernet adapters. It allows the enabling/disabling of the Link Test heart beat option.

The Model 373 uses ST connectors for the fiber optic cable. It is designed for transmission/reception over 62.5/125 multi-mode fiber optic cable. On the 10Base-T port side, it is in full compliance with the IEEE 802.3 specification. The Model 373 is also in full compliance with the Ethernet 10Base-FL standard. This is the standard for using multi-mode fiber optic cable to extend the Network Diameter of a 10Base-T Ethernet LAN.

The Model 373 is illustrated in a typical application in Figure 4-6. This shows the stations of a 10Base-T Ethernet LAN in a typical business environment. Most of the stations of the LAN are located near one another in the same building. This is Building A. All of the stations in Building A are within 100 m of one another. For purposes of this example, these people at these stations may all be in the company's Accounting Department. They can all be connected to the LAN through the Hub located in Building using the UTP copper cable - the ordinary building block of a 10Base-T LAN. They are all within the 100-m Network Diameter for a UTP copper cable based 10Base-T network. However, there is one remote station of this LAN that is not in Building A. This may be the station of the manufacturing manager. His office is in Building B- the production facility. Building B is located some distance away from the front office of Building A. In fact, Building B is about 1 km away from Building A. The manufacturing manager needs to be tied into the Accounting Department LAN so that he can update the Controller with inventory and purchasing information.

As Figure 4-6 indicates the manufacturing manager in Building B can easily be tied into the LAN. This is accomplished by placing a Model 373 at the Hub in Building A. A multi-mode fiber optic cable to another Model 373 in Building B then connects the Model 373. The second Model 373 is connected to the manufacturing manager's work station. The pair of Model 373's and the fiber optic cable will be completely transparent to all stations of the LAN, both the Accounting Department stations in Building A and the remote station of the manufacturing manager in Building B.


Figure 4-6: Model 373 shown in a typical application


Model 374 10Base-T to Single Mode Fiber Optic Transceiver
This unit is pictured in Figure 4-7. It is the almost the same as the Model 373 except that its fiber optic components are adapted for single-mode transmission. Because single mode fiber optic cable has much lower attenuation this allows a significant extension of distance. In fact, the Model 374 10Base-T to Single-Mode Fiber Optic Transceiver extends the distance of a 10Base-T Ethernet LAN to over 14 km.

The ability to achieve the extended distance is due to full duplex transmission. Full-Duplex* has one important advantage. Since there are separate transmit and receive paths, DTE's can transmit and receive at the same time. Collisions are therefore eliminated. Full Duplex Ethernet is a collision free environment.

For single-mode fiber optic cable transmission there is no standard comparable to 10Base-FL.

*Duplex operation - Transmission on a data link in both directions. Half duplex refers to such transmission, but in a time-shared mode- only one direction can transmit at a time. With full duplex there can be transmission in both direction simultaneously.


Figure 4-7: Model 374 10Base-T to Single Mode Fiber Optic Transceiver.


The application illustrated in Figure 4-6 also applies to the Model 374. However, now our manufacturing manager can be located in a building as far as 14 km away from the Accounting Department and still be tied into their 10Base-T Ethernet LAN.

The Model 375 100Base-T to Multimode Fiber Optic Transceiver allows any two 100Base-TX compliant ports to be connected by multimode, 62.5/125 micron fiber optic cable, while assuring that collision information is preserved and translated from one segment to the other. The operation of the device is transparent to the network and is offered in two versions. The Model 375ST is equipped with ST fiber connectors and offers 2 Km performance.

The 100 BASE-T adapters allow full duplex, simultaneous transfer of data with a minimum of collisions. The Model 375 extends this full duplex capability using dual fibers, while offering flawless data transmission at 100 MBPS. The Model 375 incorporates three LED's that report if the 100 Base-T and fiber are active and powered. The fiber optic connector is a duplex as ordered, designed for operation at 100 MBPS for FDDI, ATM or Fast Ethernet. Power for the Model 375 is via a supplied power pack.
 

Figure 4-8: Model 375 100Base-T to Fiber Transceiver for Fast Ethernet


The application illustrated in Figure 4-6 applies to the Model 375. You merely have to substitute a Fast Ethernet, 100Base-T Ethernet LAN, for the 10Base-T Ethernet LAN and substitute a Model 375 for the Model 373.

 


 

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