The Frame - Encapsulating the Packet

The Frame - Encapsulating the Packet
The Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent.

Both the Ethernet header and trailer have several sections of information that are used by the Ethernet protocol. Each section of the frame is called a field. There are two styles of Ethernet framing: IEEE 802.3 (original) and the revised IEEE 802.3 (Ethernet).

The differences between framing styles are minimal. The most significant difference between the IEEE 802.3 (original) and the revised IEEE 802.3 is the addition of a Start Frame Delimiter (SFD) and a small change to the Type field to include the Length, as shown in the figure.

Ethernet Frame Size

The original Ethernet standard defined the minimum frame size as 64 bytes and the maximum as 1518 bytes. This includes all bytes from the Destination MAC Address field through the Frame Check Sequence (FCS) field. The Preamble and Start Frame Delimiter fields are not included when describing the size of a frame. The IEEE 802.3ac standard, released in 1998, extended the maximum allowable frame size to 1522 bytes. The frame size was increased to accommodate a technology called Virtual Local Area Network (VLAN). VLANs are created within a switched network and will be presented in a later course.

If the size of a transmitted frame is less than the minimum or greater than the maximum, the receiving device drops the frame. Dropped frames are likely to be the result of collisions or other unwanted signals and are therefore considered invalid.

The Preamble (7 bytes) and Start Frame Delimiter (SFD) (1 byte) fields are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to get the attention of the receiving nodes. Essentially, the first few bytes tell the receivers to get ready to receive a new frame.

Destination MAC Address Field

The Destination MAC Address field (6 bytes) is the identifier for the intended recipient. As you will recall, this address is used by Layer 2 to assist devices in determining if a frame is addressed to them. The address in the frame is compared to the MAC address in the device. If there is a match, the device accepts the frame.

Source MAC Address Field

The Source MAC Address field (6 bytes) identifies the frame's originating NIC or interface. Switches also use this address to add to their lookup tables. The role of switches will be discussed later in the chapter.

Length/Type Field

The Length/Type field (2 bytes) defines the exact length of the frame's data field. This is used later as part of the FCS to ensure that the message was received properly. Either a length or a type may be entered here. However, only one or the other may be used in a given implementation. If the purpose of the field is to designate a type, the Type field describes which protocol is implemented.

The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version because both uses were common. The Ethernet II Type field is incorporated into the current 802.3 frame definition. Ethernet II is the Ethernet frame format that is used in TCP/IP networks. When a node receives a frame, it must examine the Length/Type field to determine which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal or 1536 decimal, then the contents of the Data Field are decoded according to the protocol indicated.

Data and Pad Fields

The Data and Pad fields (46 - 1500 bytes) contains the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, an IPv4 packet. All frames must be at least 64 bytes long. If a small packet is encapsulated, the Pad is used to increase the size of the frame to this minimum size.

Frame Check Sequence Field

The Frame Check Sequence (FCS) field (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame.

The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, no error occurred. Calculations that do not match are an indication that the data has changed; therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.

The Frame - Encapsulating the Packet
The Frame - Encapsulating the Packet
The Frame - Encapsulating the Packet
The Frame - Encapsulating the Packet
The Frame - Encapsulating the Packet
2 comments for "The Frame - Encapsulating the Packet"

how recent is this curriculum, can i study this material and sit for my CCNA now..? Im worried about IPV6 has it been added as a full topic or is it still just brief, are we still subnetting using the IPV4 way...?

great explanation.