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The IEEE 802.3 SNAP Frame Format

Posted in Ethernet Frame Formats

Introduction

While the original 802.3 specification worked well, the IEEE realized that some upper layer protocols required an Ethertype to work properly. For example, TCP/IP uses the Ethertype to differentiate between ARP packets and normal IP data frames. In order to provide this backwards compatibility with the Version II frame type, the 802.3 SNAP (SubNetwork Access Protocol) format was created.

The SNAP Frame Format consists of a normal 802.3 Data Link Header followed by a normal 802.2 LLC Header and then a 5-byte SNAP field, followed by the normal user data and FCS.

You can see the above mentioned headers in the 3D diagram of the frame below:

THE DATA LINK HEADER

Ethernet 802.3 SNAP Frame Format - Analysis

The Ethernet II Frame Format

Posted in Ethernet Frame Formats

Introduction

The following is a description of the frame format described by the original Ethernet Version II specification as released by DEC, Intel, and Xerox. Like the 802.3 spec, the Version II spec defines a Datalink Header consisting of 14 bytes (6+6+2) of information, but the Version II spec does not specify an LLC Header.

Let's now have a closer look at the Ethernet II frame format:

THE DATA LINK HEADER

Ethernet II Frame Format, Datalink, header, DATA & CRC (FCS) analysis

Offset 0-5: The Destination Address

The IEEE 802.3 Frame Format

Posted in Ethernet Frame Formats

The following is a description of the Ethernet Frame Format described in the IEEE 802.3 Specification. The 802.3 Specification defines a 14 byte Data Link Header followed by a Logical Link Control Header that is defined by the 802.2 Specification.

The Diagram below analyses the Ethernet 802.3 Frame:

THE DATA LINK HEADER

ethernet-frames-802.3-1

Manchester Signal Encoding

Posted in Ethernet Protocol, CSMA/CD, Collisions

Introduction

The device driver software receives a frame of IP, IPX, NetBIOS, or other higher-layer protocol data. From this data, the device driver constructs a frame, with appropriate Ethernet header information and a frame check sequence at the end.

The circuitry on the adapter card then takes the frame and converts it into an electrical signal. The voltage transitions in the transmitted bit stream are in accordance to the format called Manchester Signal Encoding. Manchester encoding describes how a binary ONE and ZERO are to be represented electrically. Manchester encoding is used in all 10 Megabit per second Ethernets; for example, 10BASE2 Thin Ethernet, 10BASE5 Thick Ethernet and 10BASE-T Twisted-Pair Ethernet.

IEEE 802.3 Interframe Spacing

Posted in Ethernet Protocol, CSMA/CD, Collisions

Introduction

The IEEE 802.3 specification states that before a station can attempt to transmit on the wire, it must first wait until it has heard 9.6 microseconds of silence. Many popular myths have arisen surrounding the reasons for the 9.6 microsecond interframe gap. The purpose of this section is to clarify the true reason for the 9.6 microsecond interframe gap.

CCENT/CCNA

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