Ethernet Throughput

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March 31, 2010

ETHERNET DATA THROUGHPUT

EXECUTIVE SUMMARY This paper provides an insight on the Ethernet frame, the data suppression and compression techniques available to improve Ethernet frame throughput, and the differences between L1 and L2 throughput figures.

Increasingly, data throughput on Ethernet wireless links is being quoted at Layer 1 (L1) rather than Layer 2 (L2).

L2 throughput has traditionally been used to indicate Ethernet throughput performance. Throughput, along with latency, frame loss and back-to-back frames, is provided using an automated RFC 2544 test on dedicated Ethernet/IP test equipment. RFC 2544 testing provides an independent industry-standard means to fully validate and benchmark an Ethernet network connection.

Other means to measure throughput include PC-based TCP or UDP test suites. These operate at Layer 4 (L4) and are represented by products such as Iperf and Netperf. Their results can be highly variable and are not recommended for validation testing. But in the hands of operators who are aware of their operation and limitations, they can provide a quick and useful indication of L2 performance.

More recently, L1 throughput figures have appeared as a measure of raw bit-rate throughput. Unlike L2 throughput, which provides figures based on the Ethernet frame size, L1 measures on the frame space; the Ethernet frame size plus the inter-frame IFG and Preamble bytes. L1 figures are usually indicated for smallest, 64-byte frames to provide a maximized L1 throughput figure. Some suppliers are using these maximized L1 figures for their products without reference to the methodology, leading to exaggerated and misunderstood claims within an industry more used to throughput figures provided under RFC 2544.

Throughputs can be improved using bit-saving data suppression and compression techniques. Non-essential framing data on a link between L2 switches is removed at the input to the link, and then restored on exit, to provide an effective means to increase throughput and hence data efficiency.

2 AVIAT NETWORKS March 31, 2010

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TABLE OF CONTENTS

EXECUTIVE SUMMARY ........................................................................................................................... 1

Ethernet Framing ....................................................................................................................................... 3

L1 vs. L2 Throughput ................................................................................................................................. 6

L1 or L2 Throughput .................................................................................................................................. 7

Measurement Considerations .................................................................................................................... 8

RFC 2544 ..................................................................................................................................................................... 8

Other Test Methods ..................................................................................................................................................... 8

CONCLUSION ........................................................................................................................................... 8

Glossary ..................................................................................................................................................... 8


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ETHERNET FRAMING IP data packets are encapsulated within Ethernet frames for transport over Ethernet networks. Figure 1 illustrates Ethernet framing of an IP packet for standard and VLAN tagged (802.1Q and 802.1Q-inQ) frames.

Figure 1. Ethernet frame

Description Bytes

IFG Inter-Frame Gap 12

PRE Preamble: Synchronization + SFDConsists of seven preamble synchronization bytes (alternating ones and zeros), plus one start of frame delimiter (SFD) byte.

8

MAC DA and MAC SA (Media Access Control)

Destination Address 6 bytesSource Address 6 bytes

12

L/T Length/Type 2

Q Tag VLAN 802.1Q Tag 4

Q-in-Q Tag VLAN 802.1Q-in-Q Tag 4

Packet Includes: IP Header 20 bytes typical TCP header 20 bytes Application data max 1460 bytes (std packets)

46 to 1500

FCS Frame Check Sequence 4

Total Frame Space: Std Frame 84 to 1538

Total Frame Space: Q Frame 88 to 1542

Total Frame Space: Q-in-Q Frame 92 to 1546

Table 1: Ethernet Frame Content (802.3)

Standard IP data packets range in size from 46 to 1500 bytes. Each packet is encapsulated within Ethernet framing, which adds frame address and check bytes, plus an inter-frame gap (IFG) and a Preamble (PRE).

For each packet

sent, the IFG and

Preamble consume

20 bytes.


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The Ethernet Frame comprises the IP Packet plus MAC header (MAC DA and MAC SA), Length/Type and FCS bytes. The number of error-free frames that can be sent over an Ethernet link represents the L2 throughput for that link

The Ethernet Frame Space comprises the Ethernet Frame plus IFG and Preamble bytes. The number of error-free frame-spaces that can be sent over an Ethernet link represents the L1 throughput for that link

The first items of interest are the IFG and Preamble. Together they add 20 bytes to the Ethernet frame size and as overheads deliver no useful content data.

The Ethernet specification (IEEE 802.3) requires the IFG to be not less than 96 bits = 12 bytes

The 8 byte Preamble (Preamble + SFD) at the start of each frame heralds the arrival of the frame

This means that when an Ethernet frame is sent, 20 bytes are added for the transmission of the frame

The next items of interest are the MAC destination address (MAC DA) and MAC sender address (MAC SA). These sit within the Ethernet frame, and at 6 bytes each represent a 12-byte total. Again, they are an overhead – they deliver no useful content data.

This is where modern Ethernet link equipment, such as the Aviat Eclipse Packet Node, act to provide throughput efficiencies. They do this through suppression of the IFG and Preamble, and compression of the MAC DA and MAC SA. This is illustrated in Figure 2 for a standard Ethernet frame.

IFG + Preamble suppression and MAC Header compression particularly apply to rate or bandwidth limited links within an Ethernet network, such as microwave links or leased-line connections, where making most efficient use of available capacity is paramount. They are typically applied over links between L2 switches where the dedicated nature of the connection means there’s no need to retain IFG and Preamble bytes and the MAC address data. Instead, these overhead bytes are suppressed or compressed at the sending end and then reinserted at the receiving end.

Figure 2. IFG and Preamble suppression and MAC DA/SA compression

The standard

frames accepted at

the input are

reduced in size for

transmission over

the link and then

reinstated at the far

end.


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Item Description

1 Standard Ethernet frame. Frame size 64 to 1518 bytes. Frame space 84 to 1538 bytes.

2 Application of IFG and Preamble suppression. The 20 bytes are replaced by 4 bytes; 2 bytes each for transport channel start-of-frame and end-of-frame delineation, and frame scrambling.

This 16-byte reduction in frame space represents a 23.5 percent throughput improvement for 64-byte frames. For average-size 260-byte frames, it represents a 6 percent traffic throughput improvement. For 1518-byte frames, it represents a 1 percent improvement.

3 Addition of MAC header compression. The 12 bytes are replaced by 2 bytes to support an address look-up-table across the link.

When this 10-byte reduction is added to the 16 bytes of IFG and Preamble suppression, the total represents a 44.8 percent throughput improvement for 64-byte frames. For average-size 260-byte frames, it represents a 10 percent traffic throughput improvement. For 1518-byte frames, it represents a 1.7 percent improvement.

Table 2. Frame suppression and compression action

Let’s look at the math to support this. We have assumed a 100 Mbit/s link capacity for simplicity, and that the full data bandwidth of the link is available for payload transport. In practice, on Ethernet wireless links where there is a need to match Ethernet framing to the air-link framing, a small percentage of the payload will be consumed by the shaping and handshaking overheads required.

The number of frames sent per second can be calculated by dividing the link capacity in bit/s by the frame space in bit/s. For the smallest 64-byte frame, the number of standard Ethernet frames sent per second on a 100 Mbit/s link is: 100,000,000 / (64 + 20) x 8 = 148,809.5 FPS. Note again that the frame space is the Ethernet frame size plus the 20 bytes of IFG + Preamble.

This is where suppression of the IFG and preamble bytes provides real value. Suppression reduces the frame space by 16 bytes; from 20 to just 4 bytes. So for the same 64-byte frame, we can now send through 100,000,000 / (64 + 4) x 8 = 183,823.5 FPS. The extra FPS sent compared to standard frame transmission is 35,014, which represents a 23.5 percent increase.

With MAC header compression, the frame space reduces by a further 10 bytes. For the same 64-byte frame, we can now send through 100,000,000 / (64 + 4 - 10) x 8 = 215,517.2 FPS. The extra FPS sent compared to standard frame transmission is 66,707.7, which represents a 44.8 percent increase.

Figure 3 summarizes the percent throughput improvements for several frames sizes, including an average-size 260-byte frame.

The smaller the frame size, the more frames it takes to fill the “pipe.” And with more frames, there are more overheads, so overheads consume a proportionally larger slice of the available Ethernet bandwidth. Hence, the smaller the frame size the greater the gains to be made by employing frame suppression and compression techniques. Put another way, by reducing overheads more useful (packet) data is sent inside each frame.

Frame Size

Standard Frame Frame with IFG & Preamble Suppression

Frame with IFG & Pre Supp. + MAC Header Compression

Frame Space

FPS Frame Space

FPS FPS %Increase

Frame Space

FPS FPS %Increase

64 84 148809.5 68 183823.5 23.5 58 215517.2 44.8 128 148 84459.5 132 94697.0 12.1 122 102459.0 21.3 260 280 44642.9 264 47348.5 6.1 254 49212.6 10.2 512 522 23496.2 516 24224.8 3.1 506 24703.6 5.1 1518 1538 8127.4 1522 8212.9 1.1 1512 8267.2 1.7

Figure 3. Summary of throughput improvements by frame size for a 100 Mbit/s link

Ethernet overhead

varies and is a

function of frame

size/distribution.

The average frame

size is about 260

bytes.


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L1 VS. L2 THROUGHPUT L1 throughput represents the total bit rate through an Ethernet user port based on frame space; the Ethernet frame plus the full 20-byte IFG + Preamble.

Where a link does not support IFG + Preamble suppression and MAC Header compression, the L1 throughput represents the link throughput

Where a link incorporates these suppression and compression capabilities it can accept more incoming frame spaces – more bit/s – than would otherwise be the case. This is frame-size dependent – smaller frames have a greater proportion of their frame space taken up by the IFG + Preamble overhead – so they have more to gain with the application of IFG + Preamble suppression

L1 throughput is sometimes referred to as the port utilization rate

L2 throughput represents the bit rate count on Ethernet frames through a user port. The count does not include the IFG + Preamble bytes.

Throughput is highly frame-size-dependent on links that do not support IFG and Preamble suppression

On links that do, the reduction in overhead from 20 to just 4 bytes improves L2 throughput on all frame sizes. But smaller frames receive most benefit, resulting in L2 throughput being much less affected by frame size

MAC header compression reduces the frame size; standard frames accepted at the input are reduced in size for transmission over the link, and then reinstated at the far end. The 10-byte reduction benefits both L1 and L2 throughput figures. Smaller frame sizes benefit most.

Figure 4 shows a comparison of L1 and L2 throughputs on a 100 Mbit/s link. From this, it can be seen that:

Without IFG + preamble suppression or MAC header compression:

L1 port throughput matches the link capacity of 100 Mbit/s and is independent of frame size

L2 throughput is significantly affected by frame size. Larger frames are transported more efficiently – the smaller the frame the higher the proportion of IFG + Preamble overhead, and the lower the throughput. Throughput on smallest 64-byte frames is nominally 76 Mbit/s (76 percent of link capacity). On average size 260-byte frames, throughput is 93 Mbit/s (93 percent). On 1518-byte frames, throughput is 98.7 Mbit/s (98.7 percent)

With IFG and preamble suppression:

The reduced overhead on frames sent over the link allows more frames to enter the link.

L1 port throughput increases to 123 Mbit/s for 64-byte frames, to 106 Mbit/s for average-size 260-byte frames and to 101 Mbit/s for 1518-byte frames

L2 throughput increases to 94 Mbit/s for 64-byte frames, to 98.5 Mbit/s for 260-byte frames and to 99.7 Mbit/s for 1518-byte frames. Throughput is much less affected by frame size

The percent improvement in throughput for L1 and L2 compared with a standard frame (no suppression) is 23.5 percent for 64-byte frames, 6.1 percent for 260-byte frames and 1.1 percent for 1518-byte frames


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With IFG and preamble suppression, plus MAC header compression:

L1 port throughput increases to 144.8 Mbit/s for 64-byte frames, to 110 Mbit/s for 260-byte frames and to 101.7 Mbit/s for 1518-byte frames

L2 throughput increases to 110 Mbit/s for 64-byte frames, to 102 Mbit/s for 260-byte frames and to 100.4 Mbit/s for 1518-byte frames

The percent improvement in throughput for L1 and L2 compared with a standard frame (no suppression or compression) is 44.8 percent for 64-byte frames, 10.2 percent for 260-bytes frames and 1.7 percent for 1518-byte frames

60,000,000

70,000,000

80,000,000

90,000,000

100,000,000

110,000,000

120,000,000

130,000,000

140,000,000

150,000,000

160,000,000

0.0 500.0 1000.0 1500.0

Frame Byte Size

Bit

s/s

Standard Frame L1 Port Speed

IFG + Pre Suppression L1 PortSpeed

IFG + Pre Suppression plusMAC Compression L1 PortSpeed

Standard Frame L2 Throughput

IFG + Pre Supression L2Throughput

IFG + Pre Suppression plusMAC Compression L2Throughput

Figure 4. L1 and L2 Mbit/s as a function of frame size

L1 OR L2 THROUGHPUT Figures quoted at L1 are universally based on 64-byte frames to provide a maximized indication of bits-per-second throughput. L1 figures are increasingly being used within the industry.

Layer 2 throughput figures have traditionally and more realistically represented Ethernet throughput, as they look only at the Ethernet frame; IFG and preamble bytes are ignored. Measurement practice is specified in RFC 2544 to provide a standardized test methodology for the industry.

L1 and L2

throughput figures

are now in use.

Check to see which

is being

represented.


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MEASUREMENT CONSIDERATIONS Data throughput refers to the maximum amount of data that can be transported from source to destination with zero errors or lost frames. As a test, it measures the rate-limiting capability of an end-end Ethernet connection.

RFC 2544 Throughput is normally benchmarked using an automated RFC 2544 test:

RFC 2544 defines a specific set of tests to measure and report the performance characteristics of network devices. It provides a standardized test methodology for the industry

It tests for throughput, latency, frame loss and back-to-back frames

The test suite supports seven predefined frame sizes (64, 128, 256, 512, 1024, 1280 and 1518 bytes) to simulate various traffic conditions

RFC 2544 testing is supported on all professional Ethernet test equipment

L1 throughput can be calculated using the L2 frame rate data provided from an RFC 2544 test. L1 throughput = frame rate FPS x frame space bytes x 8, where the frame space is the frame size plus the full 20 bytes of IFG + Preamble.

OTHER TEST METHODS Application test procedures using software such as Iperf or Netperf are frequently used to measure throughput. These are layer 4 networking evaluation tools and test figures can be misleading unless users are familiar with their operation, limitations and the results that can be expected. Results must be treated with caution – raw results can be pessimistic by 10 percent or more.

For information on Iperf testing, refer to the Aviat application note: Iperf Testing of DAC ES Performance.

CONCLUSION Both L1 and L2 throughput figures are being used within the industry. If the measurement methodology is not stated, ask.

RFC 2544 is the preferred validation tool for Ethernet network testing.

IFG + Preamble suppression and MAC Header compression significantly improve data transport efficiencies.

GLOSSARY IETF Internet Engineering Task Force. IETF recommendations, Internet standards or network protocols

are published as RFCs. MAC Media access protocol. RFC Request for comments. RFC 2544 An IETF recommendation that specifies test criteria to characterize Ethernet link performance. VLAN Virtual LAN. IEEE 802.1Q tagging mechanism.

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© Aviat Networks, Inc. (2009 – 2010). All Rights Reserved.

Data subject to change without notice.

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Ethernet Throughput

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