Examining how much throughput of actual throughput can be achieved on a Gigabit Ethernet based network and how much this increases by using Jumbo Frames. Also covered is how that relates to throughput of a Wireless Link with Gigabit Ethernet interfaces.
Gigabit Ethernet Physical Layer
On a Fibre Optic Gigabit Ethernet Network (1000BaseSX, 1000BaseLX), the raw line rate is 1.25Gbps. This raw data rate is chosen to include 8b10b Line Coding. Line Coding is used to ensure “DC balance” of the data stream, remove long runs of consecutive 0’s and 1’s, which makes the physical transceivers easier to design, implement, and maximises performance of the fibre optic transceiver range capability. When the 8b10b line coding is removed from the raw data stream by the Gigabit Ethernet chipset, this allows an uncoded payload of exactly 1.0Gbps.
On a copper based Gigabit Ethernet Network (1000BaseT), transmission uses four lanes over all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation with adaptive equalization and five-level pulse amplitude modulation (PAM-5). The symbol rate is identical to that of 100BASE-TX (125 megabaud).
Gigabit Ethernet Net Data rate
The Basic parameters of the Ethernet standard allow us to calculate the theoretical maximum throughput.
All frames must have a 8byte preamble, a 12byte inter-frame gap, and a minimum length of 64 bytes which includes Destination MAC (6 Bytes), Source MAC(6 Bytes), Protocol Type (2Bytes), Payload (46 Bytes) and CRC(4 Bytes).
Frames, Preamble, Interframe Gap
Given this, the frame size including the preamble and inter-frame gap is 84bytes (8+12+64).
The number of frames per second can be calculated as:
Rate / Frame size = frames per second
1000Mbps / (84bytes x 8) = frames/s
1,000,000,000 bits / 672 bits = 1,488,000 frames/s.
As a result, the maximum theoretical throughput is calculated as:
frames per second x frame size
1,488,000 x 512 bits** = 761Mbps
Note: 64byte frame x 8 bits = 512bits
However, we also lose some bandwidth from the preamble and the inter-frame gap. They can be calculated as follows:
Preamble (recall that it is 8bytes):
frames per second x 8 bytes x 8 bits (to convert it to Mbps)
1,488,000 x 8 x 8 = 95Mbps
Inter-frame gap (recall that it is 12bytes):
frames per second x 12 bytes x 8 bits (to convert to Mbps)
1,488,000 x 12 x 8 = 143Mbps
So the actual maximum, given 64 byte frames, is 523Mbps (761 – 95 – 143) or 65MB/s.
Now, let’s do the calculation quickly using a 1518byte frame.
Add in the preamble and the inter-frame gap:
1000Mbps / (1538bytes x 8) = 81,274 frames/s
81,274frames/s x 12144bits*** = 986Mbps
Note: 1518bytes x 8 bits = 12,144bits
Preamble overhead: 81,274 x 8 x 8 = 5Mbps
Inter-frame gap overhead: 81,274 x 12 x 8 = 7Mbps
So the max throughput, given a 1518 byte frames, is 974Mbps (986 – 5 – 7) or 121MB/s.
Note that these numbers do not include Ethernet frame, IP, TCP or UDP overhead, so we will take an additional hit there.
Let’s take a look at a 9k MTU (jumbo frames) with all accompanying overhead for TCP.
Frame size = 9000bytes
IP Header = 20bytes
TCP Header = 20bytes
TCP Options = 12 bytes
Frames per second:
9000+12+8 = 9020 x 8 = 72,160 bits
1,000,000,000 bits / 72,160 bits = 13,858 frames/s
Max throughput with no overhead:
13,858 x 72,000 = 997Mbps
13,858 x 8 x 8 = .886
13,858 x 12 x 8 = 1.33Mbps
Ethernet Header overhead:
13,858 x 14 x 8 = 1.55Mbps
Ethernet first customer ship (FCS) overhead:
13,858 x 4 x 8 = .443Mbps
IP Header overhead:
13,858 x 20 x 8 = 2.21Mbps
TCP Header overhead:
13,858 x 20 x 8 = 2.21Mbps
TCP Options overhead:
13,858 x 12 x 8 = 1.33Mbps
Theoretical throughput of Gigabit Ethernet with jumbo frames, and using TCP:
997Mbps – .886 – 1.33 – 1.55 – .443 – 2.21 – 2.21 – 1.33 = 987Mbps or 123MB/s.
The approximate throughput for Gigabit Ethernet without jumbo frames and using TCP is around 928Mbps or 116MB/s.
Limitations in Real-world applications
This raw capacity is that of the actual link itself. However, note that this is still not an accurate representation of what your customer can expect in the real world. Other factors will influence the real-world throughput. These include, but are not limited to, file sizes, types of transactions, cache hits/misses, cpu power, cpu utilization, network utilization, disk utilization, protocol (Network File System (NFS), Common Internet File System protocol (CIFS), etc), client type, kernel version, etc.
Wireless Links with Gigabit Ethernet Interfaces
Note that for Wireless links such as Microwave, Radio, Millimeter Wave or Free Space Optics, the Airside Interface often uses different coding and modulation than the network side interface. This difference is often due to limitations in the amount of RF spectrum available (for example, a 40MHz, 56MHz, 60MHz, 80MHz or even 112MHz channel) from the regulatory body and channel planning, the modulation used (for example, up to 256QAM or 1024QAM) which affects both transmit power and receiver sensitivity, aggregation features such as MIMO or XPIC, and especially for longer links, the corresponding Link Budget between the two ends which includes the Antenna Gain at both sides, plus any losses caused by transmission waveguides, connectors, plus atmospheric fade effects. This airside interface may therefore impose a lower capacity for the “end to end” wireless link even if the network interfaces at each end are connected at 1Gbps Gigabit Ethernet rate.
Transparent Wireless Links
Note that only some wireless technologies such as Free Space Optics (FSO) are capable of fully transparent transmission using the exact same modulation used on Fibre Optic networks, so the full 1.25Gbps line rate, along with all packet structure is maintained exactly. The advantages of transparent transmission is that throughput is easily predicted, and latency is the lowest possible as transmission is generally one bit at a time.
The default Gigabit Ethernet without Jumbo Frames of around 928Mbps (116MB/s). For networking equipment where Jumbo Frames are supported, by increasing the MTU can deliver even more data on the same bandwidth link, thanks to the decreased amount of overhead by utilising a lower number of frames. Jumbo Frames can therefore potentially far more of the theoretical Gigabit Ethernet bandwidth to carry data, which means 987Mbps (123MB/s) capacity.
For Further Information
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