tutorials: FAQ1

Back to Tutorials

Ethernet Frequently Asked Questions:

Ethernet Basics

What is Ethernet?
What is an 802.3 network?
What is CSMA/CD?
What is a baseband network?
What is a broadband network?
What is the OSI Model?
What does an Ethernet packet look like?
What is the difference between an Ethernet frame and an IEEE802.3 frame? Why is there a difference?
What is a SNAP header ?
What is a MAC address?
Why must the MAC address have to be unique?
Is there a special numbering scheme for MAC addresses?
What is a preamble?
What is a Start Frame Delimiter (SFD)?
What does CRC mean?
What is a broadcast address?

Q: What is Ethernet?

A: Ethernet is a type of network cabling and signaling specifications (OSI Model layers 1 [physical] and 2 [data link]) originally developed by Xerox in the late 1970. In 1980, Digital Equipment Corp. (DEC), Intel and Xerox (the origin of the term DIX, as in DEC/Intel/Xerox) began joint promotion of this baseband, CSMA/CD computer communications network over coaxial cabling, and published the "Blue Book Standard" for Ethernet Version 1. This standard was later enhanced, and in 1985 Ethernet II was released. The IEEE’s (Institute of Electrical and Electronics Engineers’) Project 802 then (after considerable debate) used Ethernet Version 2 as the basis for the 802.3 CSMA/CD network standard. The IEEE 802.3 standard is generally interchangeable with Ethernet II, with the greatest difference being the construction of the network packet header. A complete description of all Ethernet specifications is far outside the scope of this document. If this area interests you, you are encouraged to obtain copies of the IEEE 802.3 documents, and perhaps the ISO 8802-3 documents as well.

Q: What is an 802.3 network?

A: That’s IEEE-ism for Ethernet, but with a few small differences. The physical layer specifications are identical (though DIX Ethernet never specified standards for UTP and Fiber-Optic media) and the MAC sublayer are somewhat different.

Q: What is CSMA/CD?

A: CSMA/CD is the media access control mechanism used by Ethernet and 802.3 networks; in other words, it determines how a packet of data is placed on the wire. CSMA/CD stands for "Carrier Sense Multiple Access, with Collision Detection". Before an Ethernet device puts a packet "on the wire", it listens to find if another device is already transmitting. Once the device finds the wire is clear, it starts sending the packet while also listening to hear if another device started sending at the same time (which is called a collision). Refer to the Q&A on collisions for more info about this phenomenon.

Q: What is a baseband network?

A: A baseband network is one that provides a single channel for communications across the physical medium (e.g., cable), so only one device can transmit at a time. Devices on a baseband network, such as Ethernet, are permitted to use all the available bandwidth for transmission, and the signals they transmit do not need to be multiplexed onto a carrier frequency. An analogy is a single phone line such as you usually have to your house: Only one person can talk at a time—if more than one person wants to talk everyone has to take turns.

Q: What is a broadband network?

A: Simplistically, it is the opposite of a baseband network. With broadband, the physical cabling is virtually divided into several different channels; each with its own unique carrier frequency, using a technique called "frequency division modulation". These different frequencies are multiplexed onto the network cabling in such a way to allow multiple simultaneous "conversations" to take place. The effect is similar to having several virtual networks traversing a single piece of wire. Network devices "tuned" to one frequency can’t hear the "signal" on other frequencies, and visa-versa. Cable-TV is an example of a broadband network: multiple conversations (channels) are transmitted simultaneously over a single cable; you pick which one you want to listen to by selecting one of the frequencies being broadcast.

Q: What is the OSI Model?

A: The Open Systems Interconnect (OSI) reference model is the ISO (International Standards Organization) structure for the "ideal" network architecture. This Model outlines seven areas, or layers, for the network. These layers are (from highest to lowest): 7.) Applications: Where the user applications software lies. Such issues as file access and transfer, virtual terminal emulation, interprocess communication and the like are handled here. 6.) Presentation: Differences in data representation are dealt with at this level. For example, UNIX-style line endings (CR only) might be converted to MS-DOS style (CRLF), or EBCIDIC to ASCII character sets. 5.) Session: Communications between applications across a net- work is controlled at the session layer. Testing for out-of-sequence packets and handling two-way communication are handled here. 4.) Transport: Makes sure the lower three layers are doing their job correctly, and provides a transparent, logical data stream between the end user and the network service s/he is using. This is the lower layer that provides local user services. 3.) Network: This layer makes certain that a packet sent from one device to another actually gets there in a reasonable period of time. Routing and flow control are performed here. This is the lowest layer of the OSI model that can remain ignorant of the physical network. 2.) Data Link: This layer deals with getting data packets on and off the wire, error detection and correction and retransmission. This layer is generally broken into two sub-layers: The LLC (Logical Link Control) on the upper half, which does the error checking, and the MAC (Medium Access Control) on the lower half, which deals with getting the data on and off the wire. 1.) Physical: The nuts and bolts layer. Here is where the cable, connector and signaling specifications are defined. There is also the undocumented but widely recognized ninth network layer: 9.) Bozone (a.k.a., loose nut behind the wheel): The user sitting at and using (or abusing, as the case may be) the networked device. All the error detection/correction algorithms in the world cannot protect your network from the problems initiated at the Bozone layer.

Q: What does an Ethernet packet look like?

A. The Ethernet packet preamble is normally generated by the chipset. Software is responsible for the destination address, source address, type, and data. The chips normally will append the frame check sequence. | Preamble - | 62 bits | A series of alternating 1’s and 0’s used by the Ethernet receiver to acquire bit synchronization. The chip generates this. | Start Of Frame Delimiter - | 2 bits | Two consecutive 1 bits used to acquire byte alignment. The chip generates this. Destination Ethernet Address - | 6 bytes | Address of the intended receiver. The broadcast address is all 1’s. + Source Ethernet Address - | 6 bytes | The unique Ethernet address of the sending | | station. Length or Type field - | 2 bytes | For IEEE 802.3 this is the number of bytes of data. For Ethernet I&II this is the type of packet. Types codes are > 1500 to allow both to coexist. The type code for IP packets is 0x800 46 bytes | Data - | to | Short packets must be padded to 46 bytes. | 1500 bytes | Frame Check Sequence - | 4 bytes | The FCS is a 32 bit CRC calculated using the AUTODIN II polynomial. The chip normally generates this field.

Q: What is the difference between an Ethernet frame and an IEEE802.3 frame? Why is there a difference?

A: Ethernet was invented at Xerox Palo Alto Research Center and later became an international standard. IEEE handled making it a standard; and their specifications are slightly different from the original Xerox ones. Hence, two different types. 802.3 uses the 802.2 LLC to distinguish among multiple clients, and has a "LENGTH" field where Ethernet has a 2-byte "TYPE" field to distinguish among multiple client protocols. TCP/IP and DECnet (and others) use Ethernet_II framing, which is that which Xerox/PARC originated.

Q: What is a SNAP header ??

A: Sub-Network Access Protocol, an extension to the original 802.2 data link level format. (SNAP is described in IEEE 802-1990) The 802.2 data link format replaced the Ethernet Protocol Type concept with two 8-bit fields, Source SAP, and Destination SAP. Unfortunately that causes problems with migration of protocols, and the lack of SAP space that is available. So one SAP as allocated for this scheme which greatly expands the available protocol space. When using the SNAP SAP the first 5 bytes of data are used as a protocol ID. The first 3 bytes should be a value allocated to you as a vendor id, the same as you get for Source address values. The is called the OUI (Organizationally Unique ID) The second 2 bytes is a protocol type. Note that this is 802.2 and applies across all 802 LAN media types. For translation bridging, there is a convention, if you set the OUI to zero, you are representing a mapped Ethernet frame. So that a bridge will translate such a frame back into Ethernet format, and not into an 802.3 frame format. 802.2 SNAP frame: MAC | DSAP | SSAP | UI | OUI | Type | data | | Header| 0xAA | 0xAA | 0x03 | 3bytes|2bytes| | This will appear the same on all 802 compliant LAN media. On 802.3, there will be a Length field between the SA and the DSAP but not on 802.5 or FDDI.

Q: What is a MAC address?

A: It is the unique hexadecimal serial number assigned to each Ethernet network device to identify it on the network. With Ethernet devices (as with most other network types), this address is permanently set at the time of manufacturer, though it can usually be changed through software (though this is generally a Very Bad Thing to do).

Q: Why must the MAC address to be unique?

A: Each card has a unique MAC address, so that it will be able to exclusively grab packets off the wire meant for it. If MAC addresses are not unique, there is no way to distinguish between two stations. Devices on the network watch network traffic and look for their own MAC address in each packet to determine whether they should decode it or not. Special circumstances exist for broadcasting to every device.

Q: Is there a special numbering scheme for MAC addresses?

A: The MAC addresses are exactly 6 bytes in length, and are usually written in hexadecimal as 12:34:56:78:90:AB (the colons may be omitted, but generally make the address more readable). Each manufacturer of Ethernet devices applies for a certain range of MAC addresses they can use. The first three bytes of the address determine the manufacturer. RFC-1700 (available via FTP) lists some of the manufacturer-assigned MAC addresses. A more up-to-date listing of vendor MAC address assignments is available on ftp.lcs.mit.edu in pub/map/Ethernet-codes.

Q: What is a preamble?

A: A seven octet field of alternating one and zero binary bits sent prior to each frame to allow the PLS circuitry to reach its steady state synchronization with received frame timing. (802.3 standard, page 24,42).

Q: What is a Start Frame Delimiter (SFD)?

A: A binary sequence of ‘10101011’ immediately following the preamble and indicating the beginning of a frame. (802.3 standard, page 24).

Q: What does CRC mean?

A: Cyclical Redundancy Check - A method of detecting errors in a message by performing a mathematical calculation on the bits in the message and then sending the results of the calculation along with the message. The receiving workstation performs the same calculation on the message data as it receives it and then checks the results against those transmitted at the end of the message. If the results don’t match, the receiving end asks the sending end to send again.

Q: What is a broadcast address?

A: The unique address that identifies a packet as appropriate to all receiving stations. In 802.3 any address in which the second byte is an odd number. (1,3,...F).