Information Technology Reference
In-Depth Information
media-access system. It is based on contention access, bus topology, and baseband signaling, and runs
on shielded twisted-pair media at 230.4 kbps. The physical interface is EIA/TIA-422 (formerly RS-422),
a balanced electrical interface supported by EIA/TIA-449 (formerly RS-449). LocalTalk segments can span
up to 300 meters and support a maximum of 32 nodes.
The Network Layer
This section describes AppleTalk network-layer concepts and protocols. It includes discussion of
protocol address assignment, network entities, and AppleTalk protocols that provide OSI reference
model Layer 3 functionality.
Protocol Address Assignment
To ensure minimal network administrator overhead, AppleTalk node addresses are assigned
dynamically. When a Macintosh running AppleTalk starts up, it chooses a protocol (network-layer)
address and checks whether that address is currently in use. If it is not, the new node has successfully
assigned itself an address. If the address is currently in use, the node with the conflicting address sends
a message indicating a problem, and the new node chooses another address and repeats the process.
Figure 9-2 shows the AppleTalk address selection process.
The mechanics of AppleTalk address selection are media dependent. The AppleTalk Address Resolution
Protocol (AARP) is used to associate AppleTalk addresses with particular media addresses. AARP also
associates other protocol addresses with hardware addresses. When either AppleTalk or any other
protocol stack must send a packet to another network node, the protocol address is passed to AARP.
AARP first checks an address cache to see whether the relationship between the protocol and the
hardware address is already known. If it is, that relationship is passed up to the inquiring protocol stack.
If it is not, AARP initiates a broadcast or multicast message inquiring about the hardware address for
the protocol address in question. If the broadcast reaches a node with the specified protocol address, that
node replies with its hardware address. This information is passed up to the inquiring protocol stack,
which uses the hardware address in communications with that node.
