7.1 Ethernet Frames

Ethernet frame

This is the internal structure of the Ethernet frame.

MAC Sublayer

This sublayer (IEEE 802.3, 802.11, or 802.15 for example) is implemented in hardware and is responsible for data encapsulation and media access control. It provides data link layer addressing and is integrated with various physical layer technologies.

Ethernet operates in the _______________ layer and the _______________ layer. It is a family of networking technologies defined in the IEEE _____ and _____ standards.

data link physical 802.2 802.3

Ethernet Frame Fields

Ethernet Frame Total = 70-1526 bytes (8 of Preamble and SFD) - Preamble and Start Frame Delimiter (SFD) Fields (8 bytes) - Destination MAC Address Field (6 bytes) - Source MAC Address Field (6 bytes) - Type / Length Field (2 bytes) - Data Field (46-1500 bytes) - Frame Check Sequence (FCS) Field (4 bytes)

Jumbo frames are usually supported by most _____ Ethernet and _____ Ethernet switches and NICs.

Fast Gigabit

As shown in the figure, the IEEE 802.3 MAC sublayer includes

the specifications for different Ethernet communications standards over various types of media including copper and fiber.

IEEE 802.3 data encapsulation includes the following:

- Ethernet frame - Ethernet Addressing - Ethernet Error detection

Ethernet supports data bandwidths of the following:

- 10 Mbps - 100 Mbps - 1000 Mbps (1 Gbps) - 10,000 Mbps (10 Gbps) - 40,000 Mbps (40 Gbps) - 100,000 Mbps (100 Gbps)

Which part of an Ethernet Frame uses a pad to increase the frame field to at least 64 bytes? - EtherType - Preamble - Start of Frame Delimiter - Data field

- Data field

Which part of an Ethernet Frame describes the higher-layer protocol that is encapsulated? - EtherType - Preamble - Start of Frame Delimiter - Frame Check Sequence

- EtherType

Which part of an Ethernet frame detects errors in the frame? - Preamble - Start of Frame Delimiter - Frame Check Sequence

- Frame Check Sequence

Which data link sublayer controls the network interface through software drivers? - MAC - LLC

- LLC

Which data link sublayer works with the upper layers to add application information for delivery of data to higher level protocols? - MAC - LLC

- LLC

Which part of an Ethernet Frame notifies the receiver to get ready for a new frame? - Start of Frame Delimiter - Frame Check Sequence - Preamble - Data field

- Preamble

What is a function of the MAC sublayer? (Choose three.) - controls access to the media - checks for errors in received bits - uses CSMA/CD or CSMA/CA to support Ethernet technology - communicates between software at the upper layers and the device hardware at the lower layers - allows multiple Layer 3 protocols to use the same network interface and media

- controls access to the media - checks for errors in received bits - uses CSMA/CD or CSMA/CA to support Ethernet technology

2 LAN technologies used today

1) Ethernet- Ethernet uses wired communications, including twisted pair, fiber-optic links, and coaxial cables. 2) Wireless LANs (WLANs)

The minimum Ethernet frame size is _____ bytes and the expected maximum is _____ bytes. This includes all bytes from the destination MAC address field through the frame check sequence (FCS) field. The preamble field is not included when describing the size of the frame.

64 1518

Collision fragment/runt frame

Any frame less than 64 bytes in length is considered a "collision fragment" or "runt frame" and is automatically discarded by receiving stations.

Jumbo/baby giant frames

Frames with more than 1500 bytes of data are considered "jumbo" or "baby giant frames".

Ethernet Error detection

The Ethernet frame includes a frame check sequence (FCS) trailer used for error detection.

Ethernet Addressing

The Ethernet frame includes both a source and destination MAC address to deliver the Ethernet frame from Ethernet NIC to Ethernet NIC on the same LAN.

Frame Check Sequence (FCS) 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.

Preamble and Start Frame Delimiter Fields

The Preamble (7 bytes) and Start Frame Delimiter (SFD), also called the Start of Frame (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.

IEEE 802 LAN/MAN protocols, including Ethernet, use the following two separate sublayers of the data link layer to operate.

They are the Logical Link Control (LLC) and the Media Access Control (MAC), as shown in the figure.

Ethernet Frame Type/Length/EtherType Field

This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame. You may also see this field referred to as EtherType, Type, or Length. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.

Source MAC Address Field

This 6-byte field identifies the originating NIC or interface of the frame.

Destination MAC Address Field

This 6-byte field 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. Can be a unicast, multicast or broadcast address.

LLC Sublayer

This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.

Ethernet Frame Data Field

This field (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, additional bits called a pad are used to increase the size of the frame to this minimum size.

Dropped frames are likely to be the result of

collisions or other unwanted signals. They are considered invalid.

Ethernet LANs of today use switches that operate in _____-duplex. _____-duplex communications with Ethernet switches [DO / DO NOT] require access control through CSMA/CD.

full Full DO NOT