An SDU network, more precisely understood as the role of Service Data Units (SDUs) within network communication, does not refer to a specific type of network but rather to a fundamental concept in the Open Systems Interconnection (OSI) model. An SDU is not a network in itself. It's a unit of data that moves between different layers of the OSI model.
Understanding Service Data Units (SDUs)
Definition
According to OSI terminology, a Service Data Unit (SDU) is:
A unit of data passed down from an OSI layer or sublayer to a lower layer.
Crucially, this data is not yet packaged into a Protocol Data Unit (PDU). Think of the SDU as the raw ingredients for a message; the lower layer's job is to prepare and package these ingredients for transmission across the network.
Key Concepts
- Layer Interaction: SDUs are the interface between different layers within the OSI model. An upper layer provides the SDU to the lower layer.
- No Encapsulation Yet: SDUs represent data before the lower layer adds its own headers and control information to make a PDU.
- Focus on Service: The SDU represents the data unit related to the service that the lower layer will provide.
Analogy
Imagine a restaurant:
- Upper Layer (Customer): The customer (upper layer) gives an order (SDU) to the waiter.
- Lower Layer (Waiter/Kitchen): The waiter or kitchen staff (lower layer) receives this order (SDU) and then prepares it, including adding necessary elements (like the plate, utensils, etc.) and delivering it (PDU).
In this analogy, the food order itself, before being put on a plate, is like the SDU.
Practical Example: TCP/IP Model
In the TCP/IP model (a practical simplification of the OSI model), an example includes:
- Application Layer: Generates data (e.g., an email message) as an SDU.
- Transport Layer (TCP): Receives the email message SDU, adds TCP headers (e.g., port numbers, sequence numbers) and transforms it into a TCP segment (a PDU).
- Network Layer (IP): Receives the TCP segment (now an SDU for this layer), adds IP headers (e.g., source and destination IP addresses), and forms an IP packet (a PDU).
Key Differences: SDU vs PDU
| Feature | Service Data Unit (SDU) | Protocol Data Unit (PDU) |
|---|---|---|
| Definition | Data passed down from an upper layer. | Data with layer-specific control information (headers, etc.) added by a specific layer. |
| Encapsulation | Not encapsulated, it's the raw user data or the data for a specific service | Encapsulated by the lower layer with its specific protocol information |
| Purpose | Represents the data for the service request to a lower layer | Data ready to be transmitted using a specific layer's protocol |
Summary
- SDUs are crucial for layering in network protocols.
- SDUs exist before the lower layer encapsulates the data into its own PDU.
- The concept is crucial for understanding how data is processed and transmitted through the various layers of network models such as the OSI model or the TCP/IP model.
Therefore, when we refer to "SDU Network", we're referring to data transmission that involves the exchange of Service Data Units between network layers, emphasizing the flow of data through different levels of abstraction.
