Carrier Ethernet 1 Native Ethernet Explained

1/28/2008 Native Ethernet Transmission Page 1 of 5 Copyright © 2007 Harris Stratex Networks, all rights reserved.

www.harrisstratex.com

Whitepaper

Native Ethernet Transport Explained Introduction Several vendors of point-to-point wireless systems claim to support ‘native’ Ethernet data transport. But what do they really mean? What is meant by ‘Native’ Ethernet transport?

This paper provides an introduction to wireless Ethernet transport and demystifies the concept of Native Ethernet transmission and how it applies to point-to-point (PTP) radio systems designed to operate in licensed or Common Carrier frequency bands. The Ethernet data transport mechanism supported by most solutions available on the market today is also presented and compared to the Eclipse platform approach.

What Does ‘Native’ Ethernet Really Mean? The term Native Ethernet is not defined by any recognized standards or organizational body, and consequently does not have a well understood meaning. However, this does not prevent many vendors from claiming the high ground with systems that they say support native Ethernet, as opposed to those that supposedly don’t.

Probably the best way to define native Ethernet is to state what it is not, and here there is some agreement as to the meaning – native Ethernet transmission is when the Ethernet frames do not undergo encapsulation into other formats such as E1/DS1, E3/DS3 or SDH/SONET frames, so that the Ethernet format and its structure are retained ‘untouched’ during transmission. Encapsulation is performed when a system adapts data from an upper layer protocol into a lower layer protocol, which in this case is Ethernet at the Data Link Layer 2 into a TDM frame at the Physical Layer 1.

This encapsulation or adaptation between layers has enabled carriers to make use of the extensive network of legacy PDH and SDH/SONET equipment to provide new Ethernet-centric services, but the trade-off is that encapsulation introduces delay and reduces the available throughput available to the Ethernet packets, due to the additional overhead data added to facilitate the encapsulation process.

Are there really ‘non-Native’ radios on the market? The short answer is no! When Ethernet capabilities were initially added to traditional TDM microwave transmission systems a few years ago, it was generally acheived using external adapters or multiplexers, which mapped the Ethernet data into TDM frames using standardized protocols, such as GFP (General Framing Protocol).

Today, most contemporary radio systems do not use encapsulation, but map the Ethernet data more or less directly into the ‘radio frame’, ready for transmission over the radio frequency link. However, all radio systems incorporate specific additional measures that are not found on wired systems to ensure high quality and uniform transmission performance, including data scrambling and coding (see ‘Following the Ethernet Data Transmission Path’ below).

Wireless Ethernet Performance – what’s really important? There’s a lot to consider when examining the different solutions available for transporting Ethernet data over wireless. For a list, see the Harris Stratex eNews article ‘The Top 10 Things to Consider About Wireless Ethernet’ at: http://hstx.metricnews.com/enews_0407/ethernet2.html.

No. 1 This is White

Paper No. 1 in a

series produced

by Harris Stratex

Networks on

Carrier Ethernet.

‘There is no

Standards-based

definition of

‘Native’

Ethernet.’.

‘There is no such

thing today as a

non-native

Ethernet radio.’

http://hstx.metricnews.com/enews_0407/ethernet2.html�

July 2007 Native Ethernet Transmission Page 2 of 5 Copyright © 2007 Harris Stratex Networks, all rights reserved.

White Paper White Paper

But there are several parameters that are key to evaluating overall performance.

• Throughput. This is the maximum amount of data that can be transported from source to destination. This is not the maximum data bytes from within an Ethernet packet, but rather the total bytes (including headers and tags) that are transported.

• Latency. The total time taken for a frame to travel from source to destination, and a critical parameter for delay-sensitive applications such as live video and VoIP.

• Quality of Service Controls. QoS controls and features are becoming more important for providing packet management for specific applications such as VoIP, video streaming or conferencing.

When comparing solutions that operate in licensed exempt spectrum, some additional considerations should be evaluated:

• Interference and system reliability. License exempt wireless systems utilize unregulated frequency bands that are available for anyone to use, anytime, for any purpose. An interference-free link today is no guarantee for the future.

• Best Efforts. Unlicensed systems, especially those based upon OFDM/WiFi techniques, transmit as much data as is permitted by existing conditions, which may only be a fraction of the full rated throughput advertised. Be wary of extra-ordinary claims from some vendors where mutually-exclusive performance statements are often made, for example: up to 200km link lengths, 300Mbit/s throughput, 99.999% availability, and even non-line-of sight (NLOS) operation.

Following the Ethernet Data Transmission Path Let’s take a look at a transmission path for Ethernet data over a typical wireless transmission system.

Almost all PTP microwave radios on the market are Frequency Division Duplex (FDD) systems, meaning the transmitter and the receiver operate on different RF frequencies creating two unidirectional paths.

The following steps are found in all wireless transmission systems:

Digital Interface and QoS Processing When Ethernet frames enter the digital block it determines where the frames have come from and where they are going to. The digital block stores this information within the frames as ‘Source’ and ‘Destination’ MAC addresses.

A look-up table is then used to determine which backplane port needs to be used, to send the frames to the correct destination.

Some wireless systems provide more than a simple Layer 2 bridging function, and include a built-in Layer 2 Ethernet switch that supports QoS features to enable prioritization and control of the Ethernet packets. These features often include:

• VLAN tagging per 802.1q;

• Priority queuing (802.1p): port-based, VLAN or DiffServ priority bytes;

• 802.3x Flow Control.

Incoming packets are first analyzed at the Layer 2 (L2) switch for various QoS features that might be supported.

‘Throughput

efficiency and low

latency are key.’’



Scrambling After the L2 switch, the payload, including the Ethernet data and any TDM channels, is scrambled or randomized into a data frame so that any long strings of all one’s or all zero’s are forwarded as a pseudo-random stream. This is standard practice for all radio systems so that the far end receiver can be synchronized for clock and carrier recovery. It also ensures a uniform transmission spectrum.

Framing Ethernet packets are inherently asynchronous and modems are synchronous, so regardless of the radio design, buffers must be used to synchronize the framed data for the digital modem. The data is framed into a format that enables the modem to work efficiently, for example an 8 or 10 bit modem.

For radios that support both TDM (E1/DS1) and Ethernet traffic, the TDM line pulses are terminated, translated and framed. Then, they are multiplexed with Ethernet data so that the buffer now holds two kinds of data in proportion to the capacity allocated to each.

Coding Coding is also a standard feature of wireless transmission systems. Various signal processing techniques are performed on the digital signal aimed at optimizing performance over a radio path. These techniques include error control, where bits are added to support forward error correction (FEC). The number of control bits added varies by vendor and equipment type. For example, unlicensed OFDM systems typically have higher ‘overheads’ to help counter the effects of likely interference, thereby lowering the effective user data throughput. Additional coding bits also introduce delays in the transmission path, to result in higher latencies.

Modulation A digital modem converts the signal to either an intermediate or radio frequency for transmission over the radio path. The type of modem used determines the frequency efficiency (in bits/hertz) of the resulting signal and the amount of bandwidth required.

The modulation process does not differ greatly between most wireless products. The frames (which are simply a string of 1’s and 0’s) are arranged into blocks of symbols, modulated onto a carrier frequency and passed onto the RF section for up-conversion and amplification on the required frequency and power output.

Eclipse backplane operation Eclipse provides highly efficient transport of Ethernet and TDM data over the same radio path, without any compromise or degradation as a result of encapsulation, adaptation or circuit emulation.

The Eclipse nodal solution utilizes a high speed backplane architecture to route data signals between customer interface units and multiple radio paths. The backplane clock speed is user configurable between two modes. A lower clock rate supports speed increments of 2Mb/s, while a higher clock rate option supports capacity increments of 155Mb/s for Gigabit Ethernet applications.

The Ethernet interface cards (DAC ES and DAC GE for Fast and Gigabit Ethernet applications respectively) accurately predict the start and finish of each Ethernet frame (or group of frames) using High-Level Data Link Control (HDLC) to achieve maximum throughput with minimum overhead, with no encapsulation of frames. A known series of overhead bits

‘Eclipse supports

high performance

native Ethernet

transport.’



are added before and after the frame, to ensure good signal detection and resynchronization at the receiver.

HDLC frames are then placed onto the backplane in timeslots which are determined by the clock speed.

Eclipse throughput compared to CAT 5 cable Throughput is the total capacity of the wireless link that is available to transport user packets. In back-to-back throughput testing (per RFC 2544), the packet throughput of Eclipse is shown to be identical to that of a standard CAT 5 cable.

CAT 5 Cable Eclipse (DAC GE) Fast Ethernet

Frames/sec Throughput (Mbps) Frames/sec Throughput (Mbps)

64 148,809 76.19 148,809 76.19

128 84,459 86.49 84,459 86.49

256 45,289 92.75 45,289 92.75

512 23,496 96.24 23,496 96.24

1024 11,973 98.08 11,973 98.08

1518 8,127 98.69 8,127 98.69

Eclipse Latency compared to a Cisco 2950 Switch Latency is total time taken for a frame to travel from source to destination, and is a critical parameter for delay-sensitive applications such as real-time video and VoIP.

The following chart compares the one-way latency of Eclipse with a Cisco 2950 switch. This testing was performed using RFC 2544, and shows that Eclipse performance in most cases exceeds that of an industry standard switch.



Conclusions • Native Ethernet has no meaning in terms of standards, but has been adopted as an

industry term to mean that Ethernet frames do not undergo encapsulation into another format.

• Transporting Ethernet data over microwave radio links requires specific processing that is common to all radio transmission systems.

• There is very little difference between the operation of modern digital microwave systems designed to operate in licensed frequency bands.

• Harris Stratex Networks’ Eclipse is a particularly efficient platform for Ethernet, or for Ethernet with companion TDM traffic.

Documents

Carrier Ethernet 1 Native Ethernet Explained