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The Impact of Bit Error Rate on LAN Throughput

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The Impact of Bit Error Rate on LAN Throughput The Impact of Bit Error Rate on LAN Throughput Synopsis Claims that a low level of re-transmissions could have a devastating affect on LAN throughput prompted us to examine the mechanisms that might cause this. The following analysis challenges the claim that just 1% packet re-transmission can reduce throughput by as much as 80% but illustrates that such claims are not without foundation. We will attempt to answer a number of related questions using basic technical assumptions but also by adopting a worst case scientific approach. Question 1: What is the correlation between Bit Error Rate (BER) and packet loss? Let’s consider a fully-loaded switched Ethernet, where frames (or packets) are being transferred back-to- back, separated by the usual 96-bit Inter-Frame Gap (IFG). With a switched LAN we can load each connection to almost 100% - it is only the IFG that prevents this. It’s well known that smaller packets are less susceptible to interference as they are statistically more likely to miss noise caused by internal or external sources (e.g. cabling crosstalk or electromagnetic interference). We will therefore examine both small and large packet scenarios. It’s easy to calculate the relationship between BER at the physical transmission level and packet loss. For a minimum sized Ethernet frame of 64 octets, the BER required to corrupt a single bit in every frame is: Ethernet min_frame + IFG = 64 x 8 bits + 96 bits = 1 bit in every 608 bits For a maximum sized Ethernet frame of 1518 octets, the BER required to corrupt a single bit in every frame is: Ethernet max_frame + IFG = 1518 x 8 bits + 96 bits = 1 bit in every 12,240 bits The above scenarios equate to 100% packet loss or zero throughput, based on the assumption that bit errors are equally spaced. Of course, this is not the case, as bit errors will be statistically distributed. We will return to this important assumption later. 50% packet loss occurs when the BER is halved. 25% packet loss occurs when it is halved again, and so on. The result is illustrated. Packet Throughput % 1% loss 1% lolosss 1% loss 100 90 W 80 70 h 60 i t 50 e 40 Ethernet Ethernet 16x Ethernet min frames max frames max frames P 30 [64 octets] [1518 octets] [big packet] a 20 p 10 e 0 r 10E2 10E3 10E4 10E5 10E6 10E7 10E8 Bit Error Rate [1 in x] viewfield industrial estate • glenrothes • fife • united kingdom • KY6 2RS T: +44 (0) 1592 772124 F: +44 (0) 1592 775314 E: [email protected] W: www.brand-rex.com The Impact of Bit Error Rate on LAN Throughput From this straightforward analysis, 1% packet loss corresponds to: • a BER of 1 in 67,000 bits (approximately) for Ethernet min-frames • a BER of 1 in 1,250,000 bits (approximately) for Ethernet max-frames At this point, it is tempting to remind ourselves that these figures are a factor of 80-times worse that the maximum BER specified for a LAN physical layer. In which case, the packet loss would be more like 0.0125% for max_frames, or 1 errored frame in every 8,000 frames sent. But this is not the end of the story. Some protocols, such as TCP/IP, may send big packets across the LAN. How big? Well, it’s quite usual to see 8kbytes and, sometimes, even 24kbytes are seen. These big packets are sent as a series of Ethernet max_frames and are error-managed on an end-to-end basis at the transport layer. In other words, up to 16 Ethernet max_frames will be sent to transfer the bigger packet and, if any of the frames are damaged in transit, then the whole sequence will be re-transmitted. Admittedly, this is unusual, however let’s continue with our pessimistic analysis. If we treat 16 Ethernet max_frames as a single large packet, then 1% packet loss now corresponds to a BER of 1 in 20,000,000 bits, as shown in the above graph. This is now only a factor of 5-times worse than the maximum BER specified by 10BASE-T (10-8) and is equivalent to a packet loss of 0.2%, or 1 errored frame in every 500 frames sent. The situation now starts getting a little less comfortable. Question 2: Does LAN speed impact the relationship between BER and packet loss? Good question, to which the short answer is – yes. It is easy to calculate the average frame error frequency vs BER for a particular data transfer rate. This is shown together with maximum BER performance specified for the main twisted-pair LAN technologies. Note that the data transfer rate is the actual number of bits sent per second – not the operational bit rate of the LAN. This is an interesting graph, as it illustrates quite clearly the relationship between BER, data transfer rate and frame error rate. Within the range of error rates shown, there is no distinction between small and large packets. The graph shows average times between errored frames corresponding to the maximum BER specified for 10BASE-T, 100BASE-TX and 1000BASE-T. For example, a 100BASE-TX LAN carrying 10 Mbit/s (10% load) has an average period between errored frames of 100 seconds. It is worth noting at this point that optical fibre LAN technologies generally specify maximum BERs 100-times better than their copper counterparts (for the same speed). This is a major technological advantage in high capacity transmission systems. Bit Error Rate [1 in y] 10E12 1000 Mbit/s W 10E11 100 Mbit/s h 1000BASE-T max BER i 10E10 t 10 Mbit/s e 100BASE-TX max BER 10E9 1 Mbit/s P 10BASE-T max BER a 10E8 p 10E7 e r 10E6 0.1 1 10 100 1000 Average Frame Error Frequency [seconds] viewfield industrial estate • glenrothes • fife • united kingdom • KY6 2RS T: +44 (0) 1592 772124 F: +44 (0) 1592 775314 E: [email protected] W: www.brand-rex.com The Impact of Bit Error Rate on LAN Throughput Question 3: Are there any higher-level system implications of the above? Once again, the short answer is – yes. Higher-level protocols generally have timers to cover the error-recovery process. Windows of 100ms and upwards are often set to bound the amount of time devoted to a re- transmission at the transport layer. If this happens occasionally, the impact on throughput would be negligible. If, on the other hand, re-transmissions are occurring every few seconds or less, we could experience significant throughput degradation, especially if a high-level of damaged frames are associated with a single device, such as a server. It’s interesting that the main twisted-pair LAN technologies have maximum BER specifications that correspond to an average frame error period of 10 seconds when fully loaded (100% data transfer rate). Question 4: How realistic are the above figures? The short answer is that they are very pessimistic. They represent the limit of reality, or what might conceivably happen in extreme worst case. I would offer the following qualifications by way of a reality check: 1. the correlation between BER and packet loss will not be as pessimistic as stated in the presence of real- life statistical noise, which will contain bursts of multiple bit errors occurring less frequently. Burst errors will typically damage a single frame, allowing many other frames targeted in this analysis to go undamaged. 2. maximum BER performance corresponds to a minimally-compliant system, comprising LAN equipment and cabling, operating in the worst electromagnetic environment for which it was designed. This is very seldom seen in practice. 3. scientific analysis is a more meaningful process than taking measurements on a single experimental configuration. Worst case is almost impossible to simulate in practice. Question 5: So, is there any substance in the claim that 1% packet re-transmission can reduce throughput by as much as 80%? Well, 1% packet loss corresponds to a BER in the range of 10-5 to 10-7, depending on packet size. This would lead to multiple re-transmissions per second, even for modest data transfer rates of 1 to 10 Mbit/s. This could indeed have a serious impact on throughput when using higher-level protocols, such as TCP/IP. As much as 80% degradation? Possibly. Of course, a BER of 10-7 or worse will only be seen in a non-compliant system, hence the claim is unrealistic. There are one to five orders of magnitude difference between BERs associated with 1% packet loss and worst case BERs specified for twisted-pair LAN technologies. Add another two orders of magnitude difference when W using optical fibre. This represents a significant margin of safety. I rest my case. h i t e P a About Brand-Rex p Brand-Rex is a designer and manufacturer of copper and fibre based cabling systems, headquartered in e Glenrothes, Scotland with facilities across Europe. Brand-Rex has two primary businesses: Connectivity and Speciality. Its Connectivity division designs and manufactures cabling systems (both copper and r fibre) for data communications and is the No.2 player in Europe. The Speciality division exclusively produces cables that are used for control, communications, power and instrumentation within hostile environments. viewfield industrial estate • glenrothes • fife • united kingdom • KY6 2RS T: +44 (0) 1592 772124 F: +44 (0) 1592 775314 E: [email protected] W: www.brand-rex.com .
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