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When data is sent from one computer to another computer or from one node to another node, it is divided into small chunks of data.
The data that we send in a network is generally called messages.
As we move from one layer to another layer in the OSI model or TCP/IP model, the message’s name will change.
The different message names depend on the type of protocol the layer uses and the technologies on which the layer is based.
On the contrary, the name of the message clarifies the type of protocols used in a different layer of the OSI model or TCP/IP model.
What is Encapsulation?
Encapsulation is the process of adding headers and trailers around data.
You will find the use of encapsulation in the four-layer TCP/IP model and the OSI reference model.
How encapsulation work in networking (Basic Concept)
To understand the process of encapsulation, let us consider you are sending an email using an email client like Outlook or Thunderbird.
The data sent through an application like Outlook will first move from the application layer to the transport layer.
The transport layer will first encapsulate the data with its own information and pass it to the internet layer.
The received data is again encapsulated with this layer of information in the internet layer and passed to Network Access Layer.
In the Network Access Layer, the received data is again encapsulated with the layer information and lastly pass it to the physical network link.
Moreover, concerning the OSI model, the data in each layer is termed as Protocol Data Unit (PDU), and the PDU of one layer become Service Data Unit (SDU) of another layer.
Additionally, the PDU in each layer of the OSI model or TCP/IP model is named separately corresponding to the protocol in that layer.
First, we will understand what Protocol Data Unit (PDU) and Service Data Unit (SDU) are.
Next, we will go through the messages or data in each layer termed Segment, Datagram, Packets, Frame, Bit, and Cell.
What is Protocol Data Unit (PDU) in networking?
A protocol data unit (PDU) is a single unit of information or a specific block of information transferred over a network.
It is composed of protocol-specific control information and user data.
It is used in the reference of the OSI model, which defines the data state when it is transferred from one layer to another layer.
In other words, we can say that protocol data unit (PDU) is used as a generic term for the block of information in the OSI model.
What is Service Data Unit (SDU) in networking?
Service Data Unit (SDU) is the block of information or data received from the upper layer of the OSI model.
Generally, the PDU of one layer becomes the SDU of another layer when it is transferred from one layer to another.
The protocol data unit (PDU) of the OSI model are:
- The transport layer (Layer 4) PDU is the segment or the datagram
- The network layer (Layer 3) PDU is the packet
- The data link layer (Layer 2) PDU is the Frame
- The physical layer (Layer 1) PDU is the bit.
The protocol data unit (PDU) of the Internet Protocol Suite (TCP/IP model) are:
- The transport layer PDU is the segment for TCP or the datagram for UDP
- The Internet layer PDU is the packet
- The link-layer or network access layer PDU is the frame.
The type of message or PDU in a different layer of the OSI or TCP/IP model.
What are segments?
A segment is the unit of data used in the TCP (Transmission Control Protocol) of the transport layer.
Moreover, the segment is the PDU for TCP at the transport layer.
When the application provides the data stream, it is first sliced into multiple chunks of data called segments.
Moreover, one segments carry data along with the acknowledgment received from another segment.
A segment contains multiple information which conveys different functions and controls (called headers).
We won’t go into much detail about the segment, but we will look after some of the most common components of the segment.
- Source port (which can be 59365 and above)
- Destination port (mostly 80 and above)
- Flags (it has a long list of flags that convey different meanings)
- Checksum (it verify the segment integrity)
- Sequence number (helpful in keeping track and identifying different segments)
- Options (any other important settings)
What are datagrams?
Datagrams are very similar to packets, and it is also used by the network layer protocol of the OSI reference model.
The only difference between datagram and packet is that datagrams do not require any confirmation at the receiving end.
The datagrams are ideal for streaming sites or voice calls where the constant flow of data is more important than 100% accuracy.
The datagram is also referred to as “IP datagrams” when Internet Protocol (IP) is used in the network layer.
A datagram is the PDU that is used for UDP in the transport layer.
What are Packets?
A packet is a bit of data transmitted over a packet-switched network through multiple paths from source to destination.
A packet is a PDU (Protocol Data Unit) used in the network layer of the OSI model and the Internet layer of the TCP/IP model.
The device in a TCP/IP network divides data into a small unit, and when it reaches the network layer, it is called packets.
Moreover, the size of the packet may vary depending on the bandwidth of the network.
A segment received from the transport layer is added with an additional IP address and a packet.
It is sometimes also referred to as “IP Packets” or “network packet.”
Similar to a segment, the packet has its own field along with a segment inside it. The important fields added on the top of the segment when it reaches the network layer are mentioned below.
- Source IP address
- Destination IP address
Each packet can take its own path, and if the packets are interrupted or lost, they will be retransmitted to a network.
When the packets reach the destination, they are reassembled to form a single file or other contiguous blocks of data.
Moreover, you will also find that packets are generically referred to as any message.
What are Frames?
Frames can be a fixed or variable block of data or information formatted or encoded for digital transmission over a network.
Moreover, the Frames are the PDU at the data link layer of the OSI reference level and a network interface layer of the TCP/IP model.
When any communication occurs between two computers or two network devices, they are done using a MAC address.
MAC address is simply the physical address of the device associated with the network interface card.
When any message is sent from one node to another node, it will search for the device’s physical address. This process takes place by mapping the IP address and corresponding MAC address.
The mapping or translation of IP address to MAC address is done through ARP (Address Resolution Protocol).
Like segments and packets, Frame has its own header and the IP packet from the network layer.
The Frame contains the following information as mentioned below.
- Source Mac Address
- Destination Mac Address
- Payload (the data packet that is received from the network layer)
- Length (total length of the data)
What is a Cell in networking?
A cell is a 53-byte fixed-length message used by Asynchronous Transfer Mode (ATM) network.
It serves the same purpose as the Frame. Moreover, it is used by technologies operating at the lower layers of the OSI model like in the Ethernet and Frame Relay networks.
What are Bits or Data Bits in networking?
Bits is the PDU (Protocol Data Unit) at the physical layer of the OSI Model and Network Interface Layer of the TCP/IP Model.
Bits are generally encoded using electrical signals and pulses of light, then transferred through a physical medium in a computer network.