HP OpenVMS I/O User's Reference Manual

Local Area Network (LAN) Device Drivers

Configuring the Ethernet Media Type from the Console

Format Parameter

Frame Formats  

The frame formats available in the LAN media are shown in LAN Frame Formats.  

Figure 3  LAN Frame Formats

Note that CSMA/CD provides two frame formats and the FDDIprovides one frame format. The 802.1 header is an optional extensionto the 802.2 header.

CSMA/CD Frames  

There are two headers for CSMA/CD frames:

• Ethernet header



• IEEE 802.3 header

CSMA/CD Frame with Ethernet Header illustratesa CSMA/CD frame with an Ethernet header.  

Figure 4  CSMA/CD Frame with Ethernet Header

The Ethernet header consists of the DA, SA, and PTY fields.Ethernet frames must be at least 64 bytes in length, which meansthat the minimum data length is 46 bytes. Applications select Ethernetformat by specifying NMA$C_LINFM_ETH (the default) as the valuefor NMA$C_PCLI_FMT in their P2 characteristics buffer. If the amountof actual data to be transmitted is less than 46 bytes, the CSMA/CD driverstransmit extra bytes of zero after the application data.

CSMA/CD Frame with IEEE 802.3 Header illustratesa CSMA/CD frame with an IEEE 802.3 header.  

Figure 5  CSMA/CD Frame with IEEE 802.3 Header

The IEEE 802.3 format is similar to the Ethernet format, exceptthe PTY field is replaced by the LEN field.

FDDI Frames  

FDDI Frame Format illustratesthe format of FDDI frames.  

Figure 6  FDDI Frame Format

The FDDI header consists of the FC, DA, and SA fields.

Token Ring Frames (Alpha Only)  

Token Ring Frame Format (Alpha Only) illustratesthe format of Token Ring frames. 

Figure 7  Token Ring Frame Format (Alpha Only)

ATM ELAN Frames (Alpha Only)  

LAN Emulation Data Frame Format with IEEE 802.3/Ethernet Header illustratesthe format of LAN emulation data frame format for the IEEE 802.3and Ethernet Header.  

Figure 8  LAN Emulation Data Frame Format with IEEE802.3/Ethernet Header

802.2/802.1Headers 

The 802.2 header follows the 802.3 header in a CSMA/CD frameand follows the FDDI header in an FDDI frame.

The 802.2 header is illustrated in 802.2 Header.  

Figure 9  802.2 Header

This 802.2 header is followed by the 802.1 header illustratedin 802.1 Header if the DSAP fieldis the SNAP SAP (AA hex), the SSAP field is the SNAP SAP, and theCTL field is UI (03 hex).  

Figure 10  802.1 Header

The PID field consists of two subfields: the company portionof the PID and the implementation-specific portion of the PID (see 802.1 Header Subfields ).  

Figure 11  802.1 Header Subfields

Token Ring Source Routing Header (Alpha Only)  

Source Routing Field (Alpha Only) detailsthe field of the source routing header.  

Figure 12  Source Routing Field (Alpha Only)

The routing control (RC) contains the information about howthe packet is to be routed.

The Sn contains the segment identifier for the hop. Each segmentidentifier contains a ring number and bridge number used in thehop.

Configuring the Ethernet Media Type from the Console

Format Parameter