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1000Mbs on a 66MHz bus - How do they do that?
It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs
yet the host bust is running at 66MHz. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? Don |
#2
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1000Mbs on a 66MHz bus - How do they do that?
Don wrote:
It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs yet the host bust is running at 66MHz. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? Don The host bus uses parallel data transfer - 16 or 32 bits at a time. -- LSR |
#3
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1000Mbs on a 66MHz bus - How do they do that?
Don wrote:
It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs yet the host bust is running at 66MHz. PCI sockets typically run at 33MHz (the standard allows up to 64-bit x 66MHz, but this is rarely seen except in high-end gear). That provides a 32-bit data path, with a theoritical transfer rate of 33M x 4bytes / second = 133MBypes/sec. This is more than adequate to saturate the gigabit ethernet capacity. PCI-X (64-bit, 133MHz) and PCI-e (1 bit, 2.5GHz)(per lane) can do it even easier. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? You'll rarely see throughput of 1000Mbit because overhead comes in from a number of places. Ethernet frame headers, IP headers, and propogation delays because the other end needs to ACK each packet (delays can be minimised by properly configured TCP stacks). Crappy protocols that limit packet size to something way too small, etc. Plus, of course, you have to get the data in the first place, which probably means disk I/O with its inherent delays (mechanical and processing overheads), and of course sharing the same PCI bus on which the CPU-NIC transfer is taking place. The speed quoted for ethernet refers to the available signalling bandwidth on the network, aka 'wire rate'. Everything is signalled at that rate, but there are gaps between packets. |
#4
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1000Mbs on a 66MHz bus - How do they do that?
Don wrote:
It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs yet the host bust is running at 66MHz. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? Don If that 66 MHz host bus is the PCI bus, it transfers 32 (or, maybe, 64) bits in parallel at 66 MHz, for a peak transfer rate of 2112 (or 4224) Mb/s; hence, not a bottleneck for a 1000 Mb/s NIC. And other host buses are commonly 64 (or more) bits wide as well -- no problem. Also, 1000 Mb/s is indeed the burst speed of a 1Gb/s NIC: the rate at which two successive bits are transmitted within a packet. There are gaps between packets, and there is protocol overhead, so that the STR (Sustained Transfer Rate) on a 1Gb/s ethernet is well below 1000 Mb/s, even if the end-nodes to the transfer are blazing fast. -- Cheers, Bob |
#5
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1000Mbs on a 66MHz bus - How do they do that?
In article ,
"Don" writes: It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs yet the host bust is running at 66MHz. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? It only requires parallelisation by a factor of 16 to achieve that. -- SAm. |
#6
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1000Mbs on a 66MHz bus - How do they do that?
"Don" wrote in message ... It seems odd to me that a Gigabit NIC deal with data at a rate of 1000Mbs yet the host bust is running at 66MHz. How is it possible? Could it be that the NIC is only capable of burst speeds of 1000Mbs? Don Thank you to all respondents. All very welcome and clear explanations. I am very grateful to you all for taking the time to educate me. Don |
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