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LLC2 453
Figure 12-19
LLC Frame Format
SNA, being a connection-oriented protocol, prefers type 2. LLC type 2 (LLC2) service
establishes virtual-type circuits between the sender and receiver and is therefore connection-
oriented. LLC2 acknowledges data upon receipt.
The three key fields in Figure 12-19 are the destination service access point (DSAP), the source
service access point (SSAP), and the control field (CTRL).
LLC2-speaking stations can communicate by sending exchange of identification (XID) frames
to each other. These frames identify the stations at a higher level than the MAC address and also
can contain information about the station's configuration. These frames are typically sent only
during setup and configuration periods when it is deemed that sending them is useful.
Two stations communicating with LLC2 do the following:
·
Open and close sessions with each other before and after sending data.
·
Acknowledge the receipt of any frames the other station sends.
·
Control the flow of data between them by limiting the number of frames that may be sent
by one before the other acknowledges any of them.
·
Recover from errors and inform the other station of this.
LLC2 stations communicate in an asynchronous balanced mode (ABM). LLC2-speaking
stations are considered peers and may send data to other LLC2 stations at any time. This ability
to send data at any time is called being asynchronous. Balanced mode means that each station
can send to others without permission.
All frames used in LLC2 communication have both a source and a destination address. LLC2-
speaking stations usually communicate over a local-area network (LAN) on which many
stations communicate. As such, each station needs its own address. Because the LLC2 protocol
runs at the data link layer, the MAC address is typically used.
<--- MAC --->
<----LLC---->
CTRL
DSAP
SSAP
1
1
1-2
Bytes
<----Data---->
FCS
87200333.book Page 453 Wednesday, August 22, 2001 2:53 PM