Service Access Points

A service access point (SAP) identifies a particular user service that sends and receives a specific class of data.

This user service allows different classes of data to be routed separately to their corresponding service handlers. Those DLCs that support multiple concurrent SAPs have addresses known as destination SAP and source SAP embedded in their packet headers. DLCs that can only support a single SAP do not need or use SAP addressing, but still have the concept of enabling the one SAP. In general, SAP is enabled for each DLC user on each port.

Most SAP address values are defined by IEEE standardized network-management entities or user-defined values as specified in the Token-Ring Network Architecture Reference. Some of the common SAP addresses are:

Item Description
null SAP (0x00) Provides some ability to respond to remote nodes even when no SAP has been enabled. This SAP supports only connectionless service and responds only to exchange identification (XID) and TEST Link Protocol Data Units (LPDU).
SNA path control (0x04) Denotes the default individual SAP address used by Systems Network Architecture (SNA) nodes.
PC network NETBIOS (0xF0) Used for all DLC communication that is driven by Network Basic I/O System (NetBIOS) emulation.
discovery SAP (0xFC) Used by the local area network (LAN) name-discovery services.
global SAP (0xFF) Identifies all active SAPs.
Note: See Request for Comment (RFC) 1060 for examples of IEEE 802 Local SAP values. RFCs are available from the Network Information Center at SRI International, Menlo Park, California.