Ethernet: How fast is it really?   

Authored By Jeff Nowling

Industrial Automation Engineering Consultant, Omron Electronics LLC

With the popularity and low cost of Ethernet hardware, industrial control systems are evolving

toward this platform. Users be aware; proper network design and implementation is critical to

fast reliable network throughput.

Introduction

Considerable confusion exists as to the actual capacity, reliability, and usability of Ethernet

networks in the industrial control arena. There are many varieties of Ethernet hardware and

media types, ranging from the traditional half-duplex 10Mbits/Second coax (10Base5) to the

latest 1000Mbits/Second full-duplex Fiber Optic (1000Base-LX). The speed and throughput of

an Ethernet network will vary among the different types. To complicate things further, the use of

switches, routers, and repeaters change the timing characteristics and performance of Ethernet

networks. This article will cover the question of how fast it's possible to communicate via the

various types and configurations of Ethernet networks. It will also describe the effects on

reliability and determinism due to system loading.

Half Duplex Ethernet Overview

Traditional half-duplex Ethernet is based on the IEEE 802.3 standards. Ethernet uses only the

lower 2 layers of the ISO networking model, the Physical layer and the Data Link layer.

Ethernet does not define any specific Network Layer protocol (e.g. TCP/IP). Ethernet

incorporates the use of CSMA/CD (Carrier Sense Multiple Access with Collision Detection) as

the "who gets to talk" means of media access to the network.

Carrier Sense defines that all stations must listen for no traffic on the network for a

predefined period of time (Inter-frame Gap) before transmitting data.

Multiple Access defines that all stations on the network have equal access to transmit

data. Also, any station is allowed to repeat the transmit sequence without waiting for

other stations to transmit their data. This is unlike Token-Ring or Token-Bus networks

that guarantee every other station an opportunity to transmit before a second packet can

be sent.

Collision Detection defines that a transmitting station must detect a collision of data

with any other transmitting stations data. This occurs when 2 stations attempt to

simultaneously transmit data.

Ethernet performance depends on multiple parameters both fixed by the specification and

variable due to the network usage and architecture.

Parameters include:

Bit Rate: Current Ethernet bit rates are 10, 100, and 1000Mbits/Second.

Propagation Delay: Maximum round trip delay between any two stations.

Jam Time: When a transmitter detects a collision it continues to transmit for 32 extra bit

times to ensure all stations reliably detect the collision.

Slot Time: Longest acquisition time. This must be longer than Maximum Propagation

Delay + Maximum Jam Time.

Packet Length: Minimum is 64 Bytes, Maximum is 1518 Bytes.

Number of Stations: Maximum depends on media type.

Cable Length: Maximum depends on media type.

Number of Repeaters: Maximum depends on media type.

The sequence for a station to transmit data is:

1.) The station monitors network for activity. If there is no activity for a duration greater

than or equal to the Interframe Gap time then the station will immediately begin

transmission of the data packet.

2.) During transmission the station monitors the network for a collision (abnormal

voltage on the wire). If a collision is detected, the station transmits at a minimum the

frame preamble plus a 32 bit a Jam Sequence to ensure that other stations can

reliably detect the collision.

3.) The station then waits a random period of time, increments a collision counter then

repeats the sequence starting from step 1. This process known as "backoff" and is

designed to reduce the probability of repeated collisions.

4.) If another collision is detected, the "backoff" process is repeated with the random

time being increased for each collision.

Throughput, Traffic, and Collision Domains'

After examining the sequence for a station to transmit data in a half-duplex Ethernet network, it

becomes evident that "network traffic" is the key to performance within the system. The greater

the number of nodes and/or the greater the number of transmissions, the higher the probability

of collisions and the longer a station will have to wait for "no activity" on the network. These

traffic issues are only relevant within a "Collision Domain" which is defined as any network

segment in which there will be a collision if two devices transmit at the same time. Bus type

networks such as 10-Base5 and 10-Base2 coaxial networks are examples of a single collision

domain type system. Star type networks such as 10-BaseT and 100-BaseT using repeater

hubs are also examples of single collision domains.

The network in figure 1 is an example of a single collision domain. Even though the stations on

the 10-BaseT portion of the network are on individual wires, because they are connected to a

repeating hub they are considered to be on a single network segment. Assuming no network

traffic, it is possible to calculate the time it takes to transmit a packet of given length from any

station to any other station within the network. The IEEE 802.3 specifications provide in

excruciating detail, the complex methods for calculating delay times for each component within

a network. It is beyond the scope of this document to define these. However, the specifications

state "Carrier Sense" requires 96 bit times and the minimum "Slot Time" is 512 bit times. Using

these numbers, it is theoretically possible to send a 64 Byte packet onto a 10 Mbit/second

system network with no other traffic every 60.8 microseconds. In reality, most devices are only

capable of processing several hundred packets per second. This type of Ethernet network,

while perfectly fine in a business environment, is not particularly well suited for use within the

industrial control environment where deterministic and reliable communications are required.

There are several reasons why this is true. There is a phenomenon known as a "Jabbering

Node". This occurs when a node starts transmitting a continuous stream of data onto the

network. This phenomenon normally results from faulty hardware within a network interface

card. Since CSMA/CD requires 96 bit times of quiet before allowing transmission onto the

network, the Jabbering Node disables all other nodes on the network from sending their data

and essentially stops the network.