- 1 Sequencing
- 2 Protocol Data Units
- 3 Three Addresses
- 4 Encapsulation Example
- 5 De-encapsulation Example
- 6 Ethernet Encapsulation
- 7 Ethernet Frame Fields
A better approach is to divide the data into smaller, more manageable pieces to send over the network. Segmentation is the process of dividing a stream of data into smaller units for transmissions over the network. Segmentation is necessary because data networks use the TCP/IP protocol suite to send data in individual IP packets. Each packet is sent separately, similar to sending a long letter as a series of individual postcards. Packets containing segments for the same destination can be sent over different paths.
This leads to segmenting messages having two primary benefits:
- Increases speed – Because a large data stream is segmented into packets, large amounts of data can be sent over the network without tying up a communications link. This allows many different conversations to be interleaved on the network called multiplexing.
- Increases efficiency – If a single segment fails to reach its destination due to a failure in the network or network congestion, only that segment needs to be retransmitted instead of resending the entire data stream.
Protocol Data Units
Note: Although the UDP PDU is called a datagram, IP packets are sometimes also referred to as IP datagrams.
That data is passed down the stack and encapsulated into a new PDU at each layer. At the top, the email data is divided into smaller chunks of data. Below that, a transport header is added in front of the chunk of data and it becomes a segment. Below that, a network header is added in front of the transport header and it becomes a packet. Below that, a frame header is added in front of the network header and a frame trailer is added behind the data and it becomes a frame (medium dependent).
The frame is shown as a stream of bits prior to being received by a router that is connected to the cloud. Text at the bottom reads: Data – The general term for the PDU used at the application layer; Segment – Transport layer PDU; Packet – Network layer PDU; Frame – Data Link layer PDU; Bits – Physical layer PDU used when physically transmitting data over the medium. Note: If the Transport header is TCP, then it is a segment. If the Transport header is UDP then it is a datagram.
You saw this animation previously in this module. This time, click Play and focus on the encapsulation process as a web server sends a web page to a web client.
The animation shows a small network with a Web Server and a Web Client. There’s is a graphic that shows the components that make up a message. An Ethernet Frame, an IP Packet, a TCP segment, and the user data. The animation begins with the webserver preparing the Hypertext Markup Language (HTML) page as data to be sent. The application protocol HTTP header is added (prepended) to the front of the HTML data.
The header contains various information, including the HTTP version the server is using and a status code indicating it has information for the web client. The HTTP application layer protocol delivers the HTML-formatted web page data to the TCP transport layer. The transport layer protocol prepends additional information to the HTTP data to manage the exchange of information between the web server and web client. The IP information is prepended to the TCP information. IP assigns the appropriate source and destination IP addresses. This information is known as an IP packet. The Ethernet protocol prepends and adds to the end (appends) information to the IP packet to create a data link frame. The frame is then converted into a string of binary bits that are sent along the network path to the web client.
You saw this animation previously in this module. This time, click Play and focus on the de-encapsulation process.
Understanding Ethernet Encapsulation In Networking
Ethernet operates in the data link layer and the physical layer. It is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards. Ethernet supports the following data bandwidths:
- 10 Mbps
- 100 Mbps
- 1000 Mbps (1 Gbps)
- 10,000 Mbps (10 Gbps)
- 40,000 Mbps (40 Gbps)
- 100,000 Mbps (100 Gbps)
As shown in the figure, Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.
Ethernet and the OSI Modes
Ethernet Frame Fields
The minimum Ethernet frame size is 64 bytes and the maximum is 1518 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.
Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame” and is automatically discarded by receiving stations. Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.
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. They are considered invalid. However, the Fast Ethernet and Gigabit Ethernet interfaces of some Cisco Catalyst switches can be configured to support larger jumbo frames.
Ethernet Frame Fields
|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.|
|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.|
|Source MAC Address Field||This 6-byte field identifies the originating NIC or interface of the frame. A source MAC address can only be a unicast address.|
|Type / Length||This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.
Note: You may also see this field referred to as EtherType, Type, or Length.
|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.|
|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.|
MAC Address Format
Decimal and Binary Equivalents of 0 to F Hexadecimal
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