Question

Given the following frame, answer the questions that follow. No Ethernet header!

Answer 1

Ethernet frame:

In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload.^[1]

An Ethernet frame is preceded by a preamble and start frame delimiter (SFD), which are both part of the Ethernet packet at the physical layer. Each Ethernet frame starts with an Ethernet header, which contains destination and source MAC addresses as its first two fields. The middle section of the frame is payload data including any headers for other protocols (for example, Internet Protocol) carried in the frame. The frame ends with a frame check sequence (FCS), which is a 32-bit cyclic redundancy check used to detect any in-transit corruption of data.

The header features destination and source MAC addresses (each six octets in length), the EtherType field and, optionally, an IEEE 802.1Q tag or IEEE 802.1ad tag.

The EtherType field is two octets long and it can be used for two different purposes. Values of 1500 and below mean that it is used to indicate the size of the payload in octets, while values of 1536 and above indicate that it is used as an EtherType, to indicate which protocol is encapsulated in the payload of the frame. When used as EtherType, the length of the frame is determined by the location of the interpacket gap and valid frame check sequence (FCS).

The IEEE 802.1Q tag or IEEE 802.1ad tag, if present, is a four-octet field that indicates virtual LAN (VLAN) membership and IEEE 802.1p priority. The first two octets of the tag are called the Tag Protocol IDentifier (TPID) and double as the EtherType field indicating that the frame is either 802.1Q or 802.1ad tagged. 802.1Q uses a TPID of 0x8100. 802.1ad uses a TPID of 0x88a8.

Ethernet packet and frame structure

The optional 802.1Q tag consumes additional space in the frame. Field sizes for this option are shown in brackets in the table above. IEEE 802.1ad (Q-in-Q) allows for multiple tags in each frame. This option is not illustrated here.

Ethernet packet – physical layer

Preamble and start frame delimiter

An Ethernet packet starts with a seven-octet preamble and one-octet start frame delimiter

The preamble consists of a 56-bit (seven-byte) pattern of alternating 1 and 0 bits, allowing devices on the network to easily synchronize their receiver clocks, providing bit-level synchronization. It is followed by the SFD to provide byte-level synchronization and to mark a new incoming frame. For Ethernet variants transmitting serial bits instead of larger symbols, the (uncoded) on-the-wire bit pattern for the preamble together with the SFD portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011;^{[3]:sections 4.2.5 and 3.2.2} The bits are transmitted in order, from left to right.^{[3]:sections 4.2.5}

The SFD is the eight-bit (one-byte) value that marks the end of the preamble, which is the first field of an Ethernet packet, and indicates the beginning of the Ethernet frame. The SFD is designed to break the bit pattern of the preamble and signal the start of the actual frame.^{[3]:section 4.2.5} The SFD is immediately followed by the destination MAC address, which is the first field in an Ethernet frame. SFD is the binary sequence 10101011 (0xAB, decimal 171 in the Ethernet LSB first bit ordering).^{[3]:sections 3.2.2, 3.3 and 4.2.6}

Physical layer transceiver circuitry (PHY for short) is required to connect the Ethernet MAC to the physical medium. The connection between a PHY and MAC is independent of the physical medium and uses a bus from the media independent interface family (MII, GMII, RGMII, SGMII, XGMII). Fast Ethernet transceiver chips utilize the MII bus, which is a four-bit (one nibble) wide bus, therefore the preamble is represented as 14 instances of 0xA, and the SFD is 0xA 0xB (as nibbles). Gigabit Ethernet transceiver chips use the GMII bus, which is an eight-bit wide interface, so the preamble sequence followed by the SFD would be 0xAA 0xAA 0xAA 0xAA 0xAA 0xAA 0xAA 0xAB (as bytes).

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