Migrating X.25 over PSI to XOL

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X.25 over LLC2
X.25 over LLC2 (XOL) enables X.25 communication over Ethernet LAN cards. The ISO 8881 standard
specifies the implementation of X.25 communication over the LAN interface. The X.25 over LLC2
architecture is similar to the X.25 over PSI architecture. The protocol implementation in XOL is as
follows:
 The serial WAN interface (Layer 1) in X.25 over PSI is replaced by the LAN interface in XOL.
 The LAP-B layer (Layer 2) in X.25 over PSI is replaced by LLC2 in XOL.
 The PLP layer (Layer 3) is the same in X.25 over PSI and XOL. The PLP layer uses DLPI primitives to
communicate over LLC2. At the physical interface level, the X.25 packets are transmitted over the
LAN card to a router that supports XOL routing functionality.
Figure 3 shows the three layers of protocols in XOL.
Figure 3. Three layers of protocol in XOL
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Features of XOL
XOL supports the following features:
 Number of XOL logical ports supported
XOL allows a maximum of eight XOL logical ports per LAN interface if configurable SAP support is
available on the XOL router. If configurable SAP support is not available on the XOL router, XOL
allows one XOL logical port per LAN interface.
 Number of XOL logical ports per server slot supported
The maximum number of XOL logical ports supported on each server slot is 32 (4 * 8). This limit
requires a 4-port Ethernet card in the slot and configurable SAP support on the router. With PSI cards,
four X.25 ports are available with the 4-port J3526A card.
 Level 3 packet size supported
The maximum Level 3 packet size is 1024 bytes when an Ethernet card is used. The maximum Level 3
packet size is 4096 bytes when native LAPB is used on a PSI card. The MTU size for Ethernet (1500)
limits the maximum Level 3 packet size in Ethernet cards.