Volume 1 • Issue 6 May–June 2000 Introduction to Industrial Ethernet, Part 4. Part 3 was featured in Issue 5, the March–April 2000. If you would like a copy, please send your request to EXTENSION [email protected] A Technical Supplement to control NETWORK MULTI-SEGMENT ETHERNET NETWORKS Using Repeaters to Increase Network Diameter By George Thomas, Contemporary Controls INTRODUCTION that communication is half-duplex. operation or impacting media Although all nodes can send and arbitration. Repeaters are In our very first article on Ethernet receive, there cannot be any commonly viewed as a device that we discussed the basics of its simultaneous sending or receiving. restores the amplitude of the signal operation. We mentioned that This would result in collisions and in order to correct the effects of multi-segment Ethernet networks it is this detection of collisions that cable attenuation. However, can be constructed by using is used to arbitrate media access. Ethernet repeaters are required repeaters and hubs. A segment is Repeaters must not interfere with to do more. Repeaters must do defined as a length of cable this arbitration method by favoring the following: consisting of one or more cable one node over another. sections and associated connectors • Restore the amplitude with each end terminating in its of the signal characteristic impedance. For REPEATER REQUIREMENTS • Restore the symmetry example with 10BASE5, the of the signal segment represents the complete The requirements for repeaters are end to end length of thick coaxial stated in IEEE 802.3. The standard • Retime the signal cable even though several medium uses the term repeater set which attachment units (MAUs) are consists of a repeater with two or • Rebuild the preamble clamped onto the cable. The more attached MAUs. These MAUs • Enforce collisions on maximum length of a 10BASE5 may also have an AUI cable all segments segment is 500 m and this would connecting the repeater to its represent the network diameter of attached MAU but with modern • Extend fragments the Ethernet network if no repeaters this is not usually the repeaters were used. However, case. We will use the terms Ethernet can be expanded to a repeater and repeater set As a signal propagates down a larger network diameter by using interchangeably. A repeater is cable it suffers loss of signal repeaters as long as the network usually viewed as a two-port strength, symbol symmetry and diameter does not exceed the device, while a repeating hub has jitter is introduced due to effects collision domain of Ethernet. This more than two ports. Their identified as inter-symbol article will discuss those operation is the same. A valid interference. These effects must not restrictions. signal on one port is retransmitted accumulate through the use of to all other ports. Regardless if we repeaters. Repeaters must restore For this article, we will limit are using DIX V2.0 or IEEE 802.3 discussions to 10 Mbps, shared the integrity of the signals which frame format, the expansion issues includes retiming. Ethernet. With shared Ethernet, all are the same. Adding a repeater nodes participate in media should be transparent to the The preamble of an Ethernet frame arbitration and must reside within network by not causing any consists of 64 bits and it is possible one collision domain. Another disruption of Ethernet’s basic that all bits are not present due to characteristic of shared Ethernet is 1 transceiver startup delays. The contribute data latency due to their • When a transmission path repeater must count the bits in the electronics. The IEEE 802.3 consists of four repeaters and incoming preamble and insert bits standard does an exhaustive study five segments, up to three of the if any are missing. This means that on all contributors of data latency segments may be mixing and the the repeater must have a first-in- including cables, transceivers and remainder must be link first out (FIFO) buffer in order to the like. These values formulate segments. When five segments accomplish this. All repeated ports the rules that govern the number are present, each fiber optic link will have the proper 64-bit of repeaters that can be cascaded. segment shall not exceed 500m. preamble. A mixing segment is actually a bus Preamble regeneration should not segment such as 10BASE2 or be confused with packet store and APPROACH 1 10BASE5. A link segment consists forwarding. According to the There are two approaches that can of only two MAUs and is capable standard, repeaters are not allowed be used to calculate the number of of full-duplex operation (10BASE- to store and forward. Bridges and repeaters. Approach 1 is more of FL and 10BASE-T qualify). Notice routers provide this functionality, the “cookbook” approach while that although 10BASE-FL is capable not repeaters. approach 2 is the more analytical. of achieving a 2km segment It would be nice to have simple length, it is limited to 500m under Ethernet relies upon collision the above conditions. Figure 1 sensing as it arbitrates access to the cabling rules for expanding an Ethernet network but unfortunately shows this situation. Notice that cable. Repeaters must reinforce the the maximum segment length for detection of a collision by asserting that is not the case. Here are the rules for approach 1: 10BASE2, 10BASE5 and 10BASE-T the same collision signal on all can be achieved. Only the 10BASE- ports. It does this by sending out a • The transmission path permitted FL segment length is restricted. 32-bit jam signal. If the collision between any two DTEs may This rule says that you cannot have was sensed during the 64-bit consist of up to five segments, an all coaxial system when using preamble, the preamble is still four repeater sets (including four repeaters; however, an all repeated but a 32-bit jam signal is optional AUIs), two MAUs, and fiber or all twisted-pair network is appended in order that all ports two AUIs. possible using four repeaters. see a minimum of 96 bits for proper collision detection by A DTE is data terminal equipment • When a transmission path devices connected to the ports. which is either the source or consists of three repeater sets This is called fragment extension. destination of the traffic. A repeater and four segments, the following set is actually a repeater with two restrictions apply: attached medium attachment units (MAUs). An AUI is an attachment – The inter-repeater fiber NETWORK DIAMETER segment can now be 1000m. LIMITATION unit interface which is required if external MAUs are being used. – The end fiber segments Repeaters can be connected in With this rule the two MAUs and (connected to DTEs) can series (cascaded) in order to the two AUIs are reserved for the be 400m. increase the network diameter but DTEs. The repeater sets, by there are restrictions. As mentioned definition, have their own MAUs. – All segments can be mixing. before, repeaters must reinforce collision detection but if the Repeater Set Repeater Set Repeater Set Repeater Set network diameter exceeds a single collision domain, unreliable operation will result. The DTE DTE maximum collision diameter is MAU MAU determined by the round-trip time of a signal propagating between the two furthest nodes. This time AUI 10BASE-T Coax Seg Coax Seg Coax Seg 10BASE-T AUI cannot exceed 575 bits (57.5µs at Link Seg 500 m 10BASE5 500 m 10BASE5 500 m 10BASE5 Link Seg 10 Mbps). 100 m 185 m 10BASE2 185 m 10BASE2 185 m 10BASE2 100 m or FOIRL/ or FOIRL/ 10BASE-FL 10BASE-FL Repeaters impact the maximum Link Seg Link Seg collision diameter since they 500 m 500 m Figure 1. Good example of 5-4-3 rule. Notice the distance limitation on the fiber segments. 2 LEFT END MID-SEGMENT(S) RIGHT END As an example let us assume an all twisted-pair network consisting of DTE MAU MAURPTR MAU MAU RPTR MAU MAU DTE six segments and five repeaters. Since both ends are 10BASE-T segments, there is no significance LEGEND: to left end and right end. If the AUI CABLE MAU—Medium attachment unit DTE—Data terminal equipment end segments are indeed different, MEDIUM RPTR—Repeater you would need to do the Figure 2. Approach 2 uses this path model. calculation twice since the left and right end delays are different. Use An all coaxial network can be the more generalized approach the worst case calculation. created when using three number 1. For our example, we want to use repeaters, and it appears that an The model used consists of two the maximum allowable segment all fiber system can extend to length of 100 meters. Therefore, 2800 meters. DTEs interconnected by repeaters as shown in figure 2. There is a from the chart select 26.55 and left end DTE and a right end DTE. 176.3 for the ends and 53.3 for the The middle segments are for the four mid-segments. Adding them 5-4-3 Rule repeaters. The round-trip bit times all up yields 416.05 which is less than the 572 limit. There seems to The above rules have lead to a for all devices can be found in the table. This calculation is done first. be much margin. If you do not use simplified procedure for creating the maximum length of the multi-segment Ethernet networks The total round trip delay of all devices or components cannot segments in the calculation, you called the 5-4-3 rule. In the 5-4-3 will need to calculate the actual rule, a total of five segments can exceed 575 bit times.