XID3c(neg) . snd_XID(s,3) . send_xid(s,info=3,neg)
• •• •
XID3c(neg) · XID_rcvd(s,3) · rcv_xid(s,info=3,neg)
•• ••
~.
.- XID3
-+· snd_XID,rcv_XID. send_xid,rcv_xid
..
~.. •••
~.
Chapter 11. Operation of Link Stations
11-11
c = LPDU command frame r = LPDU response frame k = request correlator
n = number of port or adjacent link station requests s = adjacent link station identifier
sp = link station parameters
cc = routing information control field
routingJnformation = routing information (control field and segment numbers) sap
=
local and remote LSAP addressesIspid = link station process identifier pcp
=
path control connection parameters mrp = mode/role parametersThe following notes are keyed to Figure 11-3 on page 11-11.
1. The connectJn_als from the Physical Unit causes DLC.LAN.MGR to reserve link station resources for n links pending the initiation of link activation by remote nodes.
2. The access channel control receives a TEST command LPDU and routes it to DLC.LAN.MGR because the combination of the destination and source
addresses and DSAP and SSAP does not match the addressing information for an active link station. DLC.LAN.MGR responds with a TEST response LPDU.
DLC.LAN.MGR in the remote node obtains the routing information from the TEST response LPDU.
3. Once again, the access channel control is unsuccessful in searching for a control block based on the destination and source addresses and DSAP and SSAP of the received XID frame. The access channel control passes the null XID command LPDU to DLC.LAN.MGR. DLC.LAN.MGR interprets the null XID as a request for a connection and notifies the Physical Unit with
als_connectedJn. A correlator is included to keep the subsequent address assignments associated with the proper connections.
Note: This operation assumes that the routing information (if any) accompa-nying the XID is passed to the access channel control by the MAC sub-layer.
The access channel control will pass the entire frame, including the routing information, to DLC.LAN.MGR.
4. The Physical Unit passes the adjacent link station address to DLC.LAN.MGR in the assign_als_address record. DLC.LAN.MGR then activates the new link station and updates the access channel control with addressing information about the new link. Once the access channel control bookkeeping is complete, DLC.LAN.MGR passes the previously received null XID to the Physical Unit.
Simultaneously, the Physical Unit, after passing assign_als_address to
DLC.LAN.MGR, sends a null XID. Again, as stated in the previous example, the type of the first XID sent is dependent upon configuration. For T2.1 nodes, it may be either a null or format 3 XID. The XID being sent in this example is a null XID response LPDU.
5. Having received a null XID, the Physical Unit responds with its own XID (for T2.1 nodes, this is XID3). When a non-null XID is received, the Physical Unit validates the adjacent node's identification.
6. Assuming the adjacent node is a valid partner, the Physical Unit completes the definition of the link station by passing any remaining parameters to the DLC in define_als. DLC.LAN.MGR passes the parameters on to the link station.
7. At this point the Physical Unit is ready to negotiate the node role with the adja-cent node. This involves exchanging XlDs until one node. determines it is the
"primary" and the other determines it is the "secondary."
Note: Node role is not relevant to ABME link station operation, but it is required for determining the value of the OAF-OAF Assignment Indicator (aDA/). It is also used to determine which link station sends the SABME command LPDU. The contents of the XID information field are always provided by the Physical Unit. Thus the node role negotiation is performed by the Phys-ical Unit, not by DLC.LAN.MGR.
8. When the Physical Unit determines that the node-role negotiation is over, it issues the contact_als record to DLC.LAN.MGR.
a. The contact_als contains two. parameters. One, path control connection, tells the DLC.LAN.MGR what path control component is to be associated with the link for purposes of routing data from the link station to path control.
b. The other parameter, modelrole, tells DLC.LAN.MGR whether to have the link station send SABME or not. In this case, the modelrole is "sec," and once a SABME has been received, the DLC.LAN.MGR sets the link station to acknowledge an SABME command LPDU with a UA response LPDU.
After having received an SABME command LPDU and prior to sending a UA response LPDU, the link station must reset its status variable that keeps track of the state of the XID exchanges. It is possible for the XID exchange procedure to complete with one or both link stations waiting for a response to a previously sent XID command LPDU. If an XID response LPDU is expected, the response timer should be stopped and the unan-swered XID command LPDU discarded.
9. The UA sender ("secondary") does not send contacted_als to the Physical Unit until it has received some indication that the UA was received. This indication could be an RR or RNR command LPDU or an information LPDU. Here, the SABME sender has sent an RR command LPDU with the P bit set to B '1' , veri-fying the contact success in the remote link station.
Chapter 11. Operation of Link Stations