Frame
A **frame** in the context of z/OS networking refers to a data unit at the Data Link Layer (Layer 2) of the OSI model. It encapsulates higher-layer network data (such as an `IP packet`) with header and trailer information, enabling reliable transmission across a physical network segment, typically via an `OSA` (Open Systems Adapter) on a mainframe.
Key Characteristics
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- Encapsulation: Each frame encapsulates a higher-layer protocol data unit (e.g., an
IP packet) by adding a header and a trailer, providing the necessary information for link-layer communication. - Addressing: Contains source and destination
MAC addressesin its header, which are used for addressing and forwarding data within a local network segment. - Error Detection: Includes a Frame Check Sequence (FCS), typically a Cyclic Redundancy Check (CRC), in its trailer to detect transmission errors and ensure data integrity at the link layer.
- Boundary Definition: Defines the precise start and end of a data unit for transmission over the physical medium, allowing receiving devices to correctly interpret the incoming bit stream.
- Protocol Specific: The structure and content of a frame are specific to the underlying Data Link Layer protocol being used (e.g., Ethernet frame, Token Ring frame).
- Hardware Interaction: Directly handled by network interface cards, such as the
OSAon z/OS, for physical transmission and reception of data over the network medium.
- Encapsulation: Each frame encapsulates a higher-layer protocol data unit (e.g., an
Use Cases
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TCP/IPCommunication: z/OS systems use frames to send and receiveTCP/IPdata overOSAadapters, facilitating communication with other LPARs, external servers, and network devices.VTAMoverIP: WhenVTAM(Virtual Telecommunications Access Method) traffic is encapsulated withinIP(e.g., usingEnterprise Extender), the underlyingIPpackets are then carried within frames across the physical network.- Network Diagnostics: Analyzing frame headers and content (e.g., using network sniffers on connected devices) is a crucial technique for diagnosing connectivity issues, performance problems, or security incidents involving z/OS network traffic.
HiperSockets: WhileHiperSocketsprovides a virtual internal network for high-speed LPAR-to-LPAR communication without physical cables, it conceptually handles data units analogous to frames for efficient data transfer within the mainframe.
Related Concepts
Frames are fundamental to the TCP/IP stack on z/OS, residing at the Data Link Layer, directly above the Physical Layer and below the Network Layer. They are transmitted and received by OSA (Open Systems Adapter) hardware, which acts as the network interface for z/OS. An IP packet is encapsulated within a frame, and multiple frames can carry segments of a TCP connection. MAC addresses are embedded in frame headers for local network addressing, distinguishing them from IP addresses which operate at the Network Layer.