1.0 Introduction

The Model 372 Fiber Optic Transceiver provides users with an inexpensive means to increase the distance between 100Base-TX compliant Local Area Networks (LAN) while utilizing network connections which are safe from electrical interference. It also frees the user from the problems associated with copper wiring pairs whose quality is not sufficient for error free transmission of 100 MBPS data. The Model 372 converts 100Base-TX compliant signals to a fiber optic format and vice versa. This enables users with 100Base-TX terminal equipment to connect via fiber optic link segments which can be extended to a distance of nearly 400 meters (for longer distance see section 1.3). The current 100Base-TX standard limits copper link segments to 100 meters without the use of repeaters. In addition to this length extension, a Model 372 equipped LAN gains the advantages of fiber optic communications by minimizing concern about ground loops, power surges, lightning strikes and electrical interference from nearby equipment.

The twisted-pair-to-fiber Fiber Optic Transceiver allows any two 100Base-TX compliant ports to be connected by fiber optic cable. A properly installed Fiber Optic Transceiver converts between electrical signals and optical signals, 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.

Because the 100Base-TX port of a Model 372 is compliant with IEEE 802.3, it may be attached to any other compliant 100Base-TX port. The Model 372 should be used in pairs to provide an end-to-end 100Base-TX compliant system. While the fiber port of a Model 372 is closely related to the 100Base-FX specification it is not completely compliant with the corresponding standard. This is of no concern as the Model 372 is used with itself as a link extender.

The Model 372 is offered in three versions as described below:

Model 372 With SC fiber connectors
Model 372ST With ST fiber connectors
Model 372LHST Long haul version, 2 Kms, with ST connectors.

1.1 Overview (Distance Limitation)
In order to understand the Model 372 Fiber Optic Transceiver, it is necessary to consider some of the features of the underlying 10Base and 100Base standards. Devices communicate with each other using a serial bit stream. Prior to beginning a transmission, the device needing to communicate monitors the link to ensure it is inactive. If it is not, it waits. There may be other devices that require to transmit at approximately the same time. If more than one device begins to transmit at the same time, a collision occurs. The 10Base and 100Base standards account for this event. Each transmitting device will continue to listen for a period of time. If the device senses activity during this period, it knows there was a collision and a recovery procedure is initiated.

Table 1: Network Specification Parameters

The 100Base standard was developed to closely emulate the 10Base standard, simplifying the transition between them. This is seen in table 1, which shows some standard operating parameters for 10Base and 100Base systems.

The Slot Time is the time after a device begins a transmission that it will listen for a collision. The Interframe Gap is the amount of time after the link becomes inactive that a device must wait before transmitting data on the link. As can be seen from the table, the Slot Time is defined by the units, Bit Times (BT) which represent the rate of transmission. For a 10 Megahertz 10BaseT system, the Bit Time is 100 nanoseconds. For a 100 Megahertz 100BaseT system, the Bit Time is 10 nanoseconds. Therefore, the Slot Time for a 10BaseT system is 51.2 microseconds and the Slot Time for a 100BaseT system is 5.12 microseconds.

The Slot Time is an important parameter in the design of a network. This is related to the amount of delay between the transmitting device and the furthermost receiving device. The following diagram illustrates this.


Figure 1: 10Base/100Base System

In Figure 1, assume that Unit A (which is a conforming Data Terminal) is just starting to transmit data over the attaching link to Unit B (another conforming Data Terminal). The transmitted data travels along the link in time T_A. Also assume that some time after Unit A begins to transmit, Unit B (assuming the line is not busy) also begins to transmit. The worst case occurs when Unit B begins to transmit just before the data from Unit A arrives. When the Unit A data arrives at Unit B, Unit B knows immediately that a collision has occurred and can begin recovery operations. However, Unit A will not know there has been a problem until the data from Unit B arrives (T_B). Therefore, Unit A has to wait at least T_A + T_B before it can be assured that no collision occurred during it's transmission. The standards recognize that some time is necessary to sense the collision, so some additional time (TC) is added. Therefore, the Slot Time is the sum of TA, TB, and TC. TA and TB represent the round trip delay of the cabling network and are equal. If the delay characteristics of the media used is known, the maximum cable length can be determined from the time delay, TA. For example, twisted pair CAT 5 cable has a delay of approximately 7-8 nsec/m.

Table 2: 100BaseT Maximum Cabling Diameter Specifications

* The diameter is equivalent to the maximum end-to-end distance.
1.2 The Model 372 Fiber Optic Transceiver vs. the Two-Port Repeater
A two-port repeater enables a user to extend the operational distance of a 100Base-TX segment. However, the total time delay indicated by the Slot Time (section 1.1) is still in effect. Therefore, the distance allowed may not be extended indefinitely without arriving at either a Data Terminal port or a Bridge Port as was shown in Table 2. Without a repeater, the segment length is limited to 100 meters because of degenerative effects on the signal. With the addition of a single repeater, the segment length can be effectively extended to 200 meters. However, further extension of the effective segment length is not possible.

The reason for the severe limitation on extension of the effective segment length using a repeater is due to the functions it must perform. A repeater is responsible for signal restoration of both amplitude and timing components of the incoming signal. This involves synchronizing to the signal after restoring the amplitude and regenerating the signal for further transmission. It is also responsible for detecting collisions and taking action when they are discovered. As such, each repeater added to a network performs a useful service, but requires time to perform the stated responsibilities. For the 100Base standard, there are two types of repeaters defined: CLASS I and CLASS II. CLASS I repeaters are intended to be used to switch between different transmission formats (100Base-TX, 100Base-FX and 100Base-T4). The allowed time delay for a CLASS I repeater is 168 Bit Times (1.68 microseconds). The allowed time delay for a CLASS II repeater is 92 Bit Times (0.92 microseconds). These added time delays effectively remove the ability for the extension of the effective length segment beyond 200 meters for a 100Base-TX network using repeaters.

The Model 372 is not a 100Base repeater. It does not perform timing restoration nor does it sense collisions. The Model 372 converts the 100Base-TX signal to a fiber signal which may be sent over a longer distance than is allowed by either a direct 100Base-TX connection or one using repeaters. A Model 372 adds much less time delay than a repeater because the repeater functions are not included. A Model 372 requires 5 bit times to convert the twisted pair signal into fiber and 5 bit times to convert the fiber signal into twisted pair. Therefore, a Model 372 requires 10 bit times to transmit form one twisted pair to the other twisted pair. Since a Model 372 must be used in pairs, the total round trip added time delay is approximately 20 Bit Times. Because a Model 372 does not perform timing restoration, it should not be used with long 100Base-TX segments. However, it can be used with long fiber segments.

1.3 Full Duplex Considerations
The transmission medium in Figure 1 was shown as a single bi-directional segment. In practice, the 100Base-TX standard (and the 100Base-FX standard) are really two parallel single direction segments. This architecture can therefore accommodate full duplex operation where collisions would not occur. The current standard does not define how this full duplex feature would be utilized. In a full duplex network, there is no need to consider the time limitations imposed by collision considerations.

A Model 372 Fiber Optic Transceiver can take advantage of full duplex operation to allow much longer fiber segments. The standard Model 372 may be used in full duplex networks with fiber segments of up to 500 meters. An optional fiber transmitter in the Model 372LHST allows the fiber segments to be extended to up to 2000 meters. Additional full duplex considerations are beyond the scope of this manual.

1.4 Features
The Model 372 Fiber Optic Transceiver has the following features:

Translates signal activity information between twisted-pair and fiber, including collision information
Three diagnostic LEDs provide information about device operation and network status
Compatible with IEEE 802.3 specification for 100Base-TX
Maintains the full duplex nature of the 100Base-TX architecture
Allows the extension of the distance between DTE ports from 100 meters to approximately 400 meters in collision sensing systems and extends the distance to 500 meters for Full Duplex systems
SC or ST fiber connectors
Long Haul version available for 2,000 meters in full duplex systems
Easy to install and operate

2.0 Installation

2.1 Cabling Considerations

Table 3: Pertinent Factors in 100Base-TX Network Design

Section 1 of this manual gave background useful in determining the cabling lengths in a network system. When using a Model 372, it is important to remember that 400 meters of fiber (approximately two microseconds of delay in one direction) is allowed in a collision sensing system with no repeaters. If repeaters are added to the network, they will detract from the total cable length allowed. Table 3 shows the pertinent quantities involved in determining the cable length in a network.

The total time allowed in a network is 512 Bit Times (5.12 microseconds). You must allow for the time required to sense a collision (sensing time) so the amount of time remaining for your total network delay is approximately 400 Bit Times (4 microseconds). The following examples may be helpful in designing your network connection.

EXAMPLE #1 (Two data terminals and one interconnecting fiber segment using a pair of Model 372's)

Figure 2a - Configuration for EXAMPLE #1

A 100Base-TX repeater is added to the first example. This repeater is connected with an additional 1 meter category 5 cable. The total delay excluding the fiber segment is 1.35 microseconds (from above) plus 15 nanoseconds for the additional category 5 cable delay plus the repeater delay. The repeater delay in the standard already accounts for round trip considerations. The repeater delay is therefore, 0.95 microseconds. Therefore, the total delay excluding the fiber segment is 2.315 microseconds. Subtracting from the maximum system delay, the total remaining delay is 2.805 (5.12 - 2.315) microseconds. This delay represents a fiber cable of 280.5 meters. For this example, the total network diameter is 283.5 meters.

EXAMPLE #3 (Two data terminals and two CLASS II repeaters using a pair of Model 372's)

Figure 2c - Configuration for EXAMPLE #3

Another 100Base-TX repeater is added to example #2. Assuming the same length category 5 cable added along with the repeater, the additional delay excluding the fiber length is 0.965 (0.95 + 0.015) microseconds. This would be subtracted from the 2.805 microseconds of remaining delay above to yield 1.84 microseconds. This delay represents a fiber segment of 184 meters. For this example, the total network length is 188 meters. This example shows the combination of repeaters and Fiber Optic Transceivers is actually worse than repeaters alone. This would not be an example system where a Model 372 would provide additional network diameter. It would, however, allow the user to benefit from a fiber optic connection as described in Section 1.0.

2.2 Location
The Model 372 should be located close enough to an AC power source to permit the use of the supplied power module. In addition, it should be placed in an area that is well ventilated and away from electrically noisy equipment. Unshielded twisted-pair connection should be routed such that the effects of interference from other power or data connections are minimized.

2.3 Connections
2.3.1 Power
The power module supplied with the Model 372 provides 12 Volts DC at 500 mA to the device through the power connector located next to the RJ-45 connector. The module plugs into a standard 115V, 60 Hz AC power source. An optional 220 Volt 50 Hz power module is available.
There is no ON/OFF switch on the device. When the power is connected to the device, the device is ON. This will be indicated by the power LED being illuminated.
2.3.2 Fiber Optic Connector
The Model 372 is available with duplex SC-Type connectors or with two ST-type connectors as the Model 372ST, depending on the interface to the fiber optic link segment. In addition, the Model 372 may be ordered as the Model 372LHST for those full duplex installations requiring link extension to 2 Kms. Do not remove the covers on the fiber connectors until you are ready to connect the fiber cables. Power should be connected before attaching the fiber optic cables. The fiber optic cables must be terminated with the correct connectors. It is important when dealing with fiber optic cables to insure that the TX on one end of the link is connected to the RX at the other end of the link. Some duplex fiber optic cables are coded to help monitor the direction of data travel. If the fibers are not coded, special attention must be paid to ensure a proper connection.
2.3.3 Twisted Pair Connector
The Model 372 provides an RJ-45 connector to interface to the twisted pair link. It is important to note that THE MODEL 372 DOES NOT HAVE AN INTERNAL CROSSOVER. This means that on the MODEL 372, pins 1 and 2 are outputs, and pins 3 and 6 are inputs. Thus, if connecting to a DTE, a crossover cable will probably be necessary. However, if connected to a repeater, a straight through cable will probably be necessary, since most repeaters have an internal crossover. The user should check the cable specifications for the device to be attached to the other end of the twisted-pair link to ensure proper operation. The following chart and figure describes the Model 372 pin requirements.

The Model 372 is supplied with a crossover cable.

Table 4 : Twisted Pair Connector Pin Definitions

Definitions:
A straight through cable uses a "straight connection" on both ends of the cable attaching a Model 372 to a TX compatible unit. A crossover cable provides "crossover connection" between the ends of the cable.

3.0 Operation

3.1 Status LEDs
Power - This LED will be lit to indicate that external power is connected and the internal power converter is functioning properly and supplying power to the internal circuitry.

Fiber Link Monitor - This LED will be lit when the input power at the RX port exceeds a certain minimum level. If the input power falls below the minimum level, the LED will go off indicating that there is no valid link to the RX fiber port.

Twisted-pair Link Monitor - This LED will be lit when the input power at the RX port exceeds a certain minimum level. If the input power falls below the minimum level, the LED will go off indicating that there is no valid link to the RX twisted pair port.

3.2 Starting Up the Model 372
Plug the power adapter into the wall outlet, then attach the power connector to the media converter to turn the device on. Connect the twisted-pair cable and fiber optic cables. Put the Data Terminals in an operational condition. Check the Model 372 status LEDs for proper operation.

3.3 Diagnostic Checks
3.3.1 Power Connection
When the power connector is inserted into the receptacle in the Model 372, the power LED should be lit. If it is not, check to see that the power module is supplying 12 VDC at 500 mA minimum.
3.3.2 Fiber Connection
When the fiber optic link is connected properly, the fiber link monitor LED should be lit. If it is not, check to see that the RX on one end of the link is connected to the TX on the other end, and vice versa. Ensure that the connecting Model 372 unit is powered. If the link is still not operating, check the continuity of the fiber, and ensure that the fiber connectors are clean.
3.3.3 Twisted-Pair Connection
When the twisted pair link is connected properly, the twisted-pair link monitor LED should be lit. If it is not, check to see that the twisted-pair cable is such that the transmit pins (1,2) for the Model 372 are connected to the receive circuitry on the DTE port. Then check that the receive pins (3,6) for the Model 372 are connected to the transmit circuitry on the DTE port. In other words, check the specifications of the DTE to determine if a crossover cable is necessary. Ensure the DTE equipment is operational.

4.0 Model 372 Device Specifications

5.0 Help

If any assistance is required call Bomara Associates at (800)-5BOMARA or (978)-452-2299.
Technical Support E-Mail: bobr@bomara.com
or use our Online Contact Form

Document No. 0315-0238
Rev. -

Warranty

Telebyte warrants the equipment to be free from defects in material and workmanship, under normal and proper use and in its unmodified condition, for 12 months, starting on the date it is delivered for use. TELEBYTE's sole obligation under this warranty shall be to furnish parts and labor for the repair or replacement of products found by TELEBYTE to be defective in material or workmanship during the warranty period. Warranty repairs will be performed at the point of manufacture. Equipment approved for return for warranty service shall be returned F.O.B. TELEBYTE factory and will be redelivered by TELEBYTE freight prepaid, except for non- continental U.S.A. locations. These deliveries will be sent COD freight and import/export charges.

THE ABOVE WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, STATUTORY OR OTHERWISE, INCLUDING ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. TELEBYTE SHALL NOT BE LIABLE FOR ANY DAMAGES SUSTAINED BY RESELLER OR ANY OTHER PARTY ARISING FROM OR RELATING TO ANY EQUIPMENT FAILURE, INCLUDING, BUT NOT LIMITED TO CONSEQUENTIAL DAMAGES NOR SHALL TELEBYTE HAVE ANY LIABILITY FOR DELAYS IN REPLACEMENT OR REPAIR OF EQUIPMENT.

Out of warranty equipment may be returned to the Greenlawn, NY customer service facility prepaid as described above. Return shipping charges will be billed to the customer. The repaired unit will have a 90 day warranty. In those cases where "NO TROUBLE" is found, a reduced charge will be billed to cover handling, testing and packaging.