comp.dcom.lans.ethernet FAQ

Archive-name: LANs/ethernet-faq Posting-Frequency: monthly Last-modified: 2005/03/29 Version: 050329 URL:

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(c) 2005 James Messer Maintainer: James Messer

comp.dcom.lans.ethernet Frequently Asked Questions --------------------------------------------------

This document is provided as is without any express or implied warranties. While every effort has been taken to ensure the accuracy of the information contained in this article, the authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The contents of this article reflect my opinions only and not necessarily those of my employer.

FAQ Table of Contents


1.0 FAQ Administration [1.1] What is this FAQ? [1.2] Who maintains this FAQ? [1.3] Where can this FAQ be found? [1.4] Who provides information to the FAQ? [1.5] Can I use this FAQ on my web page? [1.6] License Information

2.0 Introduction to Ethernet

[2.1] What is Ethernet? [2.2] What is the history of Ethernet? [2.3] What is CSMA/CD? [2.4] What is the Open Systems Interconnect (OSI) model? [2.5] Where are the IEEE specifications?

3.0 Ethernet Physical Layer

[3.1] What are the different physical Ethernet network types? [3.2] What does baseband and broadband mean? [3.3] What is the difference between a bus topology and a star topology? [3.4] What physical Ethernet topologies are no longer popular? [3.5] What are the most common physical Ethernet networks used today? [3.6] What digital signal encoding is used in an Ethernet network? [3.7] What types of cabling are used for Ethernet? [3.8] What pin assignments are used in twisted-pair Ethernet cabling? [3.9] Can two Ethernet stations be directly attached with 10BASE-T? [3.10] How many stations are supported by a single Ethernet network? [3.11] What is propagation delay? [3.12] What is an interframe gap?

4.0 Ethernet Data Link Layer

[4.1] What are the different Ethernet frame formats? [4.2] What is transparent bridging? [4.3] What is the spanning tree protocol? [4.4] What is Ethernet switching?

5.0 Ethernet Errors and Troubleshooting

[5.1] What is a collision, and how many collisions are bad? [5.2] What is Signal Quality Error (SQE)? [5.3] What is jam? [5.4] What is a late collision, and why is it bad? [5.5] What is a runt? [5.6] What is jabber? [5.7] What is a CRC/Alignment error? [5.8] What non-commercial software is available to monitor an Ethernet network?

1.0 FAQ Administration

[1.1] What is this FAQ?

This FAQ will attempt to explain and decipher the intricacies of Ethernet networking and answer some of the most common questions relating to Ethernet networks. Although it contains technical information, this FAQ is best used as an introduction to Ethernet networking.

[1.2] Who maintains this FAQ?

This FAQ is maintained by James Messer ( Questions, comments, corrections, and contributions are encouraged!

[1.3] Where can this FAQ be found?

This FAQ will be posted to the comp.dcom.lans.ethernet newsgroup on the first of each month. An official archive of the FAQ can be found at:

The HTTP version of this FAQ can be found at:

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[1.4] Who provides information to the FAQ?

The FAQ questions and answers are summarized from the comp.dcom.lans.ethernet newsgroup. Much of this information is also obtained from the IEEE standards

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and related technical documents.

Send any corrections or FAQ additions to Our thanks to all who have provided information to this FAQ! Your contributions are crucial for keeping this FAQ updated and technically correct.

This FAQ is always in a state of change. There are other questions that should be added to this FAQ, questions that need to be answered, and there are probably inaccuracies with the answers that already exist in the FAQ. If you have any suggestions or corrections for the FAQ, please e-mail with additions or corrections.

[1.5] Can I use this FAQ on my web page?

Since this FAQ changes constantly, a copy of the FAQ on your web page would be out of date in a very short time. Please don't do this! A more appropriate method would be to set a hyperlink to the URL found in the secondary header of this FAQ. Please send an e-mail to if you plan on adding a link to this FAQ to your web page.

I strongly encourage the use of this FAQ for educational purposes. If you are an instructor or student using this information, I look forward to your comments!

[1.6] License Information

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs License. To view a copy of this license, visit

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or send a letter to

Creative Commons 559 Nathan Abbott Way Stanford, California 94305, USA.

2.0 Introduction to Ethernet [2.1] What is Ethernet?

Ethernet is the IEEE 802.3 series standard, based on the CSMA/CD access method that provides two or more stations to share a common cabling system. This access method, Carrier Sense Multiple Access with Collision Detection, is the basis for Ethernet systems which range from speeds of 1 Mb/s through 1000 Mb/s.

The design goals for Ethernet were to create a simply defined topology that made efficient use of shared resources, was easy to reconfigure and maintain, provided compatibility across many manufacturers and systems, and had a low cost.

[2.2] What is the history of Ethernet?

The original Ethernet specification began in the early 1970's by Xerox PARC, and was eventually improved upon by Digital Equipment Corporation, Intel, and Xerox (DIX) in 1980 with the release of Ethernet Version 1. By 1982, the specification was updated and Ethernet Version 2 was released.

In 1983, Novell created their own proprietary Ethernet frame type prior to the release of the IEEE 802.3 specification (See Section [4.1]). By 1985, the IEEE 802.3 specification was completed and provided a specification for Ethernet connectivity over thick coax and thin coax. In 1990, the specification was updated to include Ethernet over twisted pair copper wiring with 10Base-T. The current IEEE 802.3 specification includes thick coax, thin coax, twisted pair cabling and fiber optic cabling, with speeds of 10 Mb/s, 100 Mb/s (Fast Ethernet), 1,000 Mb/s (Gigabit Ethernet), and 10,000 Mb/s (10 Gigabit Ethernet).

[2.3] What is CSMA/CD?

Carrier Sense Multiple Access with Collision Detection is the basis for the Ethernet standard. This provides specific rules for allowing stations to communicate over the same transmission medium. There are a number of steps involved in communicating with CSMA/CD.

Stations must listen for a carrier on the wire. If no carrier is detected, stations can begin transmitting. While transmitting, the station continues to listen on the wire to ensure successful communications. If two stations attempt to transmit information at the same time, the transmissions overlap and cause a collision.

If a collision occurs, the transmitting station recognizes the interference on the network and transmits a bit sequence called jam. The jam helps to ensure that the other transmitting station recognizes that a collision has occurred. After a random delay, the stations attempt to retransmit the information and the process begins again.

[2.4] What is the Open Systems Interconnect (OSI) Model? ** Please contribute! See Section [1.4] ** [2.5] Where are the IEEE specifications? The IEEE specifications can be purchased from the IEEE at:

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Information on all IEEE standards can be found at:

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3.0 Ethernet Physical Layer [3.1] What are the different physical Ethernet network types?

Some of the physical Ethernet types as defined in the 802.3 specification are:

10BASE5 - 10BASE5 is the original design of the traditional Ethernet backbone, designed to be left in place permanently or for extended periods. 10BASE2 - 10BASE2 is the original design for a departmental or workgroup sized Ethernet environment. It is designed to be simple, inexpensive, and flexible as people and stations move. 10BROAD36 - 10BROAD36 is a seldom used Ethernet specification which uses a physical medium similar to cable television, with CATV-type cables, taps, connectors, and amplifiers. 1BASE5 - 1BASE5 is a specification of Ethernet that runs at 1 Mb/s over twisted pair wiring. This physical topology uses centralized hubs to connect the network devices. 10BASE-T - 10BASET provides Ethernet services over twisted pair copper wire.

FOIRL - Fiber Optic Inter-Repeater Link - This specification of the 802.3 standard defines a standard means of connecting Ethernet repeaters via optical fiber.

10BASE-F - 10BASE-F is a set of optical fiber medium specifications which define connectivity between devices. 100BASE-T - 100BASE-T is a series of specifications that provides 100 megabit speeds over copper or fiber. These topologies are often referred to as Fast Ethernet.

Gigabit Ethernet - Gigabit Ethernet provides speeds of 1000 Mb/s over copper and fiber.

[3.2] What does baseband and broadband mean?

A baseband network has a single channel that is used for communication between stations. Ethernet specifications which use BASE in the name refer to baseband networks.

A broadband network is much like cable television, where different services communicate across different frequencies on the same cable. Broadband communications would allow a Ethernet network to share the same physical cable as voice or video services. 10BROAD36 is an example of broadband networking.

[3.3] What is the difference between a bus topology and a star topology?

A bus topology is a networking architecture that is linear, usually by using one or more pieces of cable to form a single line, or bus. The signals sent by one station extend the length of this cable to be heard by other stations.

A star topology is an architecture that includes a central device or hub to connect all stations together. Signals sent by a station must pass through (and are usually regenerated) by these central hubs. Since the hub sits in the center and all other stations are linked through the hub, the architecture resembles a star.

[3.4] What physical Ethernet topologies are no longer popular?

There are a number of physical networking components specified in the IEEE 802.3 specification, but many of those early physical networking components are not used in most modern Ethernet networks. However, there may be instances where an existing legacy network still exists which uses these older components. Since these older pieces of equipment are still part of the 802.3 specification, there are no technical reasons why an Ethernet network would not operate properly with these components. The two most popular older Ethernet technologies are 10BASE5 and 10BASE2.

10BASE5 ------- 10BASE5 is the original Ethernet backbone, and is occasionally referred to as thicknet or thick Ethernet because of the thick 50 ohm coax that was used as the physical medium. 10BASE5 is a bus topology that uses transceiver cables to attach stations to the central 10BASE5 cable.

Maximum segment length: 500 meters Maximum number of segments connected with repeaters: 5 (2500 meters) Maximum attachments per segment: 100 Minimum separation between attachments: 2.5 meters

10BASE2 ------- 10BASE2 is designed as a smaller and less expensive alternative to 10BASE5, and is sometimes referred to as Thinnet or Thin Ethernet because of the much smaller cables. 10BASE2 is also a bus topology, but each of the workstations use a 'T' BNC connector to connect workstations to the central bus.

Maximum segment length: 185 meters Maximum number of segments connected with repeaters: 5 (1000 meters) Maximum attachments per segment: 30 Minimum separation between attachments: .5 meters [3.5] What are the most common physical Ethernet networks used today?

Most modern Ethernet networks use twisted pair copper cabling or fiber to attach devices to the network. The 10BASE-T, 100BASE-T, and Gigabit Ethernet topologies are well suited for the modern cabling and fiber infrastructures. [3.6] What digital signal encoding is used in an Ethernet network?

** Please contribute! See Section [1.4] ** [3.7] What types of cabling are used for Ethernet? ** Please contribute! See Section [1.4] ** [3.8] What pin assignments are used in twisted-pair Ethernet cabling? 10BASE-T and 100BASE-T use a RJ-45 connector, which is an eight-pin modular connector:

Contact 1 Transmit + Contact 2 Transmit - Contact 3 Receive + Contact 4 Not Used Contact 5 Not Used Contact 6 Receive - Contact 7 Not Used Contact 8 Not Used

When looking at an RJ-45 wall jack (female), contact 1 is on the left and contact 8 is to the right. When looking at the RJ-45 connector on the end of a cable (male) with the tab on the bottom and the contacts on the top, contact 8 is on the left and contact 1 is to the right.

Gigabit Ethernet, or 1000BASE-T, uses the same RJ-45 connector, but uses a different wiring scheme that uses all four cable pairs:

[3.9] Can two Ethernet stations be directly attached with 10BASE-T?

Two Ethernet stations can be directly attached to each other, but the cabling will be wired differently than a normal 10BASE-T Ethernet network connection. The 802.3 specification refers to this direct connection between two stations as a crossover function.

The crossover function is accomplished by simply wiring the receive pins to the transmit pins:

Contact 1 - Contact 3 Contact 2 - Contact 6 Contact 3 - Contact 1 Contact 6 - Contact 2

[3.10] How many stations are supported by a single Ethernet network? ** Please contribute! See Section [1.4] ** [3.11] What is propagation delay?

The propagation speed of a medium refers to the speed that the data travels through that medium. Propagation delays differ between mediums, which affect the maximum possible length of the Ethernet topology running on that medium.

In the following table, c refers to the speed of light in a vacuum, or 300,000 kilometers per second.

Medium Propagation Speed ------ ----------------- Thick Coax .77c (231,000 km/sec) Thin Coax .65c (195,000 km/sec) Twisted Pair .59c (177,000 km/sec) Fiber .66c (198,000 km/sec) AUI Cable .65c (195,000 km/sec)

From these values, the size of a bit on 10BaseT can be calculated. 10BaseT is twisted pair, which has a propagation delay of 177,000 km/sec. 177,000 km/sec divided by 10 million bits per second is 17.7 meters, or the size of a single bit on a 10BaseT network.

The maximum propagation delay through the network can be calculated by dividing the maximum length by the speed. For 10Base2 thin coax network, this is 185 meters divided by 195,000 km/sec, or 950 nanoseconds. If the actual propagation delay from one end of the network to the other is greater than 950 nanoseconds, late collisions may occur. See section [5.4] for more information on late collisions.

[3.12] What is an interframe gap?

The inteframe gap is the amount of time that is specified between frames transmitted from a workstation. The designers of the Ethernet specification arbitrarily chose 96 bit times to occur between frames from a transmitting station.

This delay is designed to provide the workstations on the Ethernet network with some 'breathing time' between frames to perform normal Ethernet housekeeping functions on the network interface card.

4.0 Ethernet Data Link Layer [4.1] What are the different Ethernet frame formats?

Ethernet Version 2 and IEEE 802.3 Frame Formats -----------------------------------------------

The Ethernet Version 2 frame format was designed before the IEEE specifications, but is almost identical to the 802.3 frame type. With the Ethernet Version 2 frame type, a two-byte Type field follows the source station's six-byte MAC address. In the 802.3 frame type, this two-byte field after the source address is a length field specifying the number of bytes in the LLC and data fields. If these two bytes are greater than 05DC hex (1500 decimal), the frame is a Version 2 frame. Since all type fields are greater than 1500 decimal (the maximum Ethernet frame size), both frame types can easily coexist on the same network. Some network protocol analyzers call a Version 2 frame an Ethertype frame because of this two-byte Type field.

This is an Ethernet Version 2 frame:

+--------------+ | | The preamble consists of 62 bits of alternating | Preamble | ones and zeros that allows the Ethernet card to | 7 bytes | synchronize with the beginning of a frame. | | +--------------+ The Start Frame Delimiter is the sequence | SFD - 1 byte | 10101011, and indicates the start of a frame. +--------------+ | | The destination address is a six byte Media Access | Destination | Control (MAC) address, usually burned into the | 6 bytes | ROM of the Ethernet card. +--------------+ | | The source address is a six byte MAC address, and | Source | can signify a physical station or a broadcast. | 6 bytes | +--------------+ | Type | The Type field (see explanation above). | 2 bytes | +--------------+ | | Any higher layer information is placed in the | Data | data field, which could contain protocol | | information or user data. ~ ~ ~ ~ | 46 to 1500 | | bytes | | | +--------------+ | FCS | The Frame Check Sequence is a cyclic redundancy | 4 bytes | check used by the sending and receiving stations +--------------+ to verify a successful transmission. The FCS is based on the contents of the destination address, source address, type, and data.

The 802.2 Logical Link Control (LLC) Information ------------------------------------------------

The IEEE 802.3 Ethernet specification was intended to be used with the 802.2 Logical Link Control (LLC) specification. The LLC information envelops the data of the frame, and the 802.3 headers envelop this 802.2 LLC protocol data unit (PDU).

This is the frame structure for an 802.3 Ethernet frame that contains the 802.2 LLC information:

+----------------+ | | | Preamble | | 7 bytes | | | +----------------+ | SFD - 1 byte | +----------------+ | | | Destination | | 6 bytes | +----------------+ | | | Source | | 6 bytes | +----------------+ | Frame Length | | 2 bytes | +----------------+ | DSAP - 1 byte | The Destination and Source Service Access Point +----------------+ fields determine the protocol used for the upper | SSAP - 1 byte | protocol type of the frame. +----------------+ |Control - 1 byte| The Control field is used for administration by +----------------+ certain protocols. | Data | | | ~ ~ ~ ~ | 46 to 1500 | | bytes | | | +----------------+ | FCS | | 4 bytes | +----------------+

The 802.2 Sub-Network Access Protocol (SNAP) -------------------------------------------- After the 802.2 frame type was defined, many people felt that a single byte for DSAP and SSAP would not be sufficient to handle the growth of protocols into the future. A single byte DSAP or SSAP can only specify 256 separate protocols, and many of those were predefined from the beginning of the 802.2 specification.

To provide future growth, the Sub-Network Access Protocol (SNAP) was created as an extension to the 802.2 specification. To differentiate this protocol from the original 802.2 specification, 802.2 SNAP uses the DSAP and SSAP of 0xAA.

This is an 802.2 SNAP frame encapsulated in an 802.3 frame:

+----------------+ | | | Preamble | | 7 bytes | | | +----------------+ | SFD - 1 byte | +----------------+ | | | Destination | | 6 bytes | +----------------+ | | | Source | | 6 bytes | +----------------+ | Frame Length | | 2 bytes | +----------------+ | DSAP - 1 byte | +----------------+ | SSAP - 1 byte | +----------------+ |Control - 1 byte| +----------------+ The Organizationally Unique ID (OUI) is assigned | OUI - 3 bytes | to unique vendors to help differentiate protocols | | from different manufacturers. +----------------+ | Type - 2 bytes | The two-byte protocol type defines a specific +----------------+ protocol in the SNAP. This also maintains a | | compatibility with Ethernet v2. | Data | | | ~ ~ ~ ~ | 46 to 1500 | | bytes | | | +----------------+ | FCS | | 4 bytes | +----------------+

Novell 802.3 'Raw' Frame Format -------------------------------

Before the final 802.2 LLC specifications were finalized, Novell implemented IPX/SPX over Ethernet. For this reason, Novell originally utilized 802.3 Ethernet without using 802.2 LLC. Because of this lack of LLC header, this frame type was nicknamed 802.3 'raw'. Since Novell created this proprietary frame type for their own use, no other manufacturer uses this frame type.

To implement their 'raw' frame type, Novell used the first two bytes of the 802.3 data field as 0xFFFF. Since the DSAP and SSAP values of 0xFF do not exist, it becomes easy to differentiate between the 802.3 and 802.3 'raw' frame types.

[4.2] What is transparent bridging?

Transparent bridging is a method to connect two similar network segments to each other at the datalink layer. It is done in a way that is transparent to end stations, hence end-stations do not participate in the bridging algorithm.

Transparent bridges are sometimes called learning bridges. When they are turned on and receive data packets from a network segment they:

1) learn the relation between MAC address and segment/port, and 2) forward the packet to all (!) other segments/ports.

The first step in this process is essential to the "learning" aspect of the bridge. After some time the bridge has learned that a particular MAC address, say MACa, is on a particular segment/port, say PORT1. When it receives a packet destined for the MAC address MACa (from any port not being PORT1) it will no longer forward the packet to all ports (step 2). It knows that MACa is associated with PORT1 and will only forward the packet to PORT1.

[4.3] What is the spanning tree protocol?

Spanning tree is a protocol defined in IEEE 802.1D to prevent bridges from creating network loops. Using the spanning tree protocol, bridges communicate to each other and disable certain ports/segments to prevent looping of packets.

Many implementations of the spanning tree protocol are configured so an alternate path is available to network traffic, should the original path become disabled.

[4.4] What is Ethernet switching?

From a functional point of view, switching is exactly the same as bridging. However switches use specially designed hardware called Application Specific Integrated Circuits (ASICs) to perform the bridging and packet-forwarding functionality (as supposed to implementations using a central CPU and special software). Consequently, switches are much faster than bridges.

Ethernet switches also offer additional capabilities such as virtual LANs (VLANs) and full duplex connectivity.

5.0 Ethernet Errors and Troubleshooting

[5.1] What is a collision, and how many collisions are bad?

Ethernet networking uses collisions as one of the contention access methods. When the network carrier is not active, any station can send information. If two stations attempt to send information at the same time, the signals overlap with each other, creating a collision.

Collisions are not errors! Many people misinterpret a flashing collision light or a collision counter as a network problem! Although the term 'collision' may bring to mind a terrible crash, be assured that a collision is a normal part of Ethernet networking.

The total number of collisions that occur on a network may be related to traffic patterns or utilization. Because of this variability of collisions, it is not applicable to define a 'good' or 'bad' level of collisions. In most cases, detailed analysis of collisions alone yields very little qualitative network health information.

[5.2] What is the Signal Quality Error (SQE) Test?

The SQE Test is used to test for the collision present circuit between a transceiver and a network interface card (NIC). After data is successfully transmitted, the Ethernet transceiver asserts the SQE signal on the collision presence circuit of the NIC. The NIC sees this test signal as a verification that the transceiver will inform the NIC when a collision occurs.

In most modern Ethernet networks, the SQE test is not used or applicable. Most NICs now have an integrated transceiver and therefore have a hard-wired AUI, so a test for the collision presence circuit is unnecessary.

[5.3] What is jam?

When a collision is recognized by a transmitting station, a bit sequence called jam is transmitted. This jam is 32 bits long.

Interestingly enough, the actual format of jam is unspecified in the 802.3 specifications. Most manufacturers have used alternating 1s and 0s as jam, which is displayed as 0x5 (0101) or 0xA (1010) depending on when the jam is captured in the data stream.

In many Fast Ethernet implementations, the jam has been seen as other arbitrary values, such as 1101000 (0xD0) or 10000110 (0x43). The reasoning for this particular jam pattern isn't very obvious. If anyone has more information on this jam sequence, please email

[5.4] What is a late collision, and why is it bad?

A collision is considered late if the jam occurs after 512 bit-times, or 64 bytes. Collisions that occur after the first 64 bytes of a frame may be indicative of a network design problem (the network is so large the jam cannot traverse the entire length in 32 bit-times), or a hardware or Ethernet firmware issue.

When collisions do not propagate the network quickly enough, a collision could occur between two stations without the stations aware that the packets collided. In this situation, the frames are simply lost, and the upper-layer protocols must begin a retransmission process to retransmit the information. These retransmissions can cause large delays, especially at the application layer.

[5.5] What is a runt?

In Ethernet networks, any frame shorter than the minimum 64 bytes but with a valid CRC is considered a runt. Other frame-length errors in Ethernet are long frames, which are longer than 1518 bytes yet have a valid CRC.

[5.6] What is jabber?

Jabber is described most often as a frame greater than the maximum of 1518 bytes with a bad CRC. A jabbering NIC is often indicative of a hardware problem with a NIC or transceiver.

[5.7] What is a CRC/Alignment error?

When a station sends a frame, it appends a Cyclical Redundancy Check to the end of the frame. This CRC has been generated from an algorithm and is based on the data in the frame. If the frame is altered between the source and destination, the receiving station will recognize that the CRC does not match the actual contents of the packet.

All frames should end on an 8-bit boundary, but problems on the network could cause the number of bits to deviate from the multiple of 8.

Both CRC errors and alignment errors are grouped together as the single CRC/Alignment error counter.

[5.8] What non-commercial software is available to monitor an Ethernet network?

A popular list free software is available at:

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