Knowing the OSI reference model and the TCP/IP protocol model will come in handy when you learn about how data is encapsulated as it moves across a network. It is not as simple as a physical letter being sent through the mail system. In this article, I will be talking about some of the facts that you need to know about data encapsulation in Networking.
In theory, a single communication, such as a video or an email message with many large attachments, could be sent across a network from a source to a destination as one massive, uninterrupted stream of bits.
However, this would create problems for other devices needing to use the same communication channels or links.
These large streams of data would result in significant delays. Further, if any link in the interconnected network infrastructure failed during the transmission, the complete message would be lost and would have to be retransmitted in full.
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.
The animation shows a small LAN with two hosts and a server. When the Segmentation button is pressed a large message from the first host is broken up into smaller messages that are sent across the network to the server. Then the Multiplexing button is pressed messages from both hosts are sent onto the network one after the other to the server.
Sequencing
The challenge to using segmentation and multiplexing to transmit messages across a network is the level of complexity that is added to the process. Imagine if you had to send a 100-page letter, but each envelope could only hold one page. Therefore, 100 envelopes would be required, and each envelope would need to be addressed individually. It is possible that the 100-page letter in 100 different envelopes arrives out-of-order. Consequently, the information in the envelope would need to include a sequence number to ensure that the receiver could reassemble the pages in the proper order.
In network communications, each segment of the message must go through a similar process to ensure that it gets to the correct destination and can be reassembled into the content of the original message, as shown in the figure. TCP is responsible for sequencing the individual segments.
The figure shows two computers sending messages on a network to a server. Each message has been divided up into multiple pieces shown as yellow and orange envelopes, some are interleaved and numbered. The text reads: Multiple pieces are labelled for easy direction and re-assembly. Labelling provides for ordering and assembling the pieces when they arrive.
Protocol Data Units
As application data is passed down the protocol stack on its way to being transmitted across the network media, various protocol information is added at each level. This is known as the encapsulation process. Note: Although the UDP PDU is called a datagram, IP packets are sometimes also referred to as IP datagrams.
The form that a piece of data takes at any layer is called a protocol data unit (PDU). During encapsulation, each succeeding layer encapsulates the PDU that it receives from the layer above in accordance with the protocol being used. At each stage of the process, a PDU has a different name to reflect its new functions. Although there is no universal naming convention for PDUs, in this course, the PDUs are named according to the protocols of the TCP/IP suite. The PDUs for each form of data is shown in the figure.
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.
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.
Three Addresses
Network protocols require that addresses be used for network communication. Addressing is used by the client to send requests and other data to a server. The server uses the client’s address to return the requested data to the client that requested it.
The OSI transport, network, and data link layers all use addressing in some form. The transport layer uses protocol addresses in the form of port numbers to identify network applications that should handle client and server data. The network layer specifies addresses that identify the networks that clients and servers are attached to and the clients and servers themselves. Finally, the data link layer specifies the devices on the local LAN that should handle data frames. All three addresses are required for client-server communication, as shown in the figure.
The figure shows two columns. In each column are the 7 layers of the OSI model (application presentation session transport network data link physical). There is an arrow (line with arrows on each end) that goes between the transport box in the left column to the transport layer box in the right column and the words protocol address. There is an arrow between the two boxes labelled network and the words network host address. There is an arrow between the two data link boxes and the words physical address. There is an arrow going between the two physical boxes with the following bits as electrical or radio-frequency signals 0 1 0 0 1 1 1 0 0 1 0 0 0 1 1.
Encapsulation Example
When messages are being sent on a network, the encapsulation process works from top to bottom. At each layer, the upper layer information is considered data within the encapsulated protocol. For example, the TCP segment is considered data within the IP packet. 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.
This process is reversed at the receiving host and is known as de-encapsulation. De-encapsulation is the process used by a receiving device to remove one or more of the protocol headers. The data is de-encapsulated as it moves up the stack toward the end-user application. You saw this animation previously in this module. This time, click Play and focus on the de-encapsulation process.
The animation shows a small network with a Server and a Client. The client receives a string of binary bits from the server. The client takes the binary string of bits and converts it into an Ethernet frame. The Frame contains the Ethernet header, the IP packet, the TCP segment, and the data. Each protocol header is processed and then removed in the opposite order it was added. The Ethernet information is processed and removed, followed by the IP protocol information, the TCP information, and finally the HTTP information. The HTML web page information is then passed on to the web browser software of the client
Understanding Ethernet Encapsulation In Networking
Ethernet Encapsulation
Ethernet and wireless LANs (WLANs) are the two most commonly deployed LAN technologies. Unlike wireless, Ethernet uses wired communications, including twisted pair, fibre-optic links, and coaxial cables. In this article, we are going to talk about 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.
The diagram shows the fields of an Ethernet frame. From left to right the fields and their lengths are Preamble and SFD, 8 bytes; destination MAC address, 6 bytes; source MAC address, 6 bytes; type/length, 2 bytes; data, 46 – 1500 bytes; and F C S, 4 bytes. Excluding the first field, the total number of bytes in the remaining fields is between 64 – 1518 bytes.
Ethernet Frame Fields
Field
Description
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.
The figure is three columns showing the decimal and hexadecimal equivalents of select 4-bit binary numbers. From left to right, the column headings are decimal, binary, and hexadecimal. Each column has 16 rows below the header.
Decimal and Binary Equivalents of 0 to F Hexadecimal
An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (4 bits per hexadecimal digit). Hexadecimal digits use the numbers 0 to 9 and the letters A to F. The figure shows the equivalent decimal and hexadecimal values for binary 0000 to 1111. Hexadecimal is commonly used to represent binary data. IPv6 addresses are another example of hexadecimal addressing.
Figure 1 shows a table with Decimal values between 0 and 15 with the Binary and Hexadecimal equivalents. This table demonstrates why Hexadecimal has letters A through F along with numbers 0 through 9. Figure 2 shows that a MAC address can be represented with dashes, colons or periods.
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