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IBM Token Ring Concepts D-5
D
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Source Service Access Point (SSAP).
• One or two bytes of control information.
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Zero or more bytes of data information.
The SAPs are used as logical ports, or addresses, from the higher-level
communication and network models, such as OSI, SNA, and TCP/IP, into
the LLC sublayer. Communications between two entities on an IBM Token
Ring network is between addressable MAC/LLC entities. The addressing
for these entities is discussed in the next section.
D.2.1 Addressing
The following sections describe addressing of MAC and LLC entities.
D.2.1.1 MAC Entities
The IEEE Standard 802.5 provides two address formats for Token Ring
networks: 16-bit (2-byte) and 48-bit (6-byte). All nodes on a network must use
the same addressing scheme. Most networks use the 6-byte format, as shown
in the following figure.
Figure D-3 MAC Address Format
The following types of addresses are used with the IBM Token Ring network:
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Ring station address - Used to uniquely identify stations on the Token Ring.
A value of 0 in bit 0 of byte 0 indicates that a ring station address is present
in the frame. A value of 1 in bit 0 of byte 0 indicates either that the address is
a group address or that source routing information is present in the frame.
Note that when the ring station address is a source address, the group
address indicator bit is no longer relevant for group address identification