Specifications

4-26 Cisco MGX 8220 Reference, Release 4.1
Frame Relay Access Service Module
An application of a STUN connection is shown in Figure4-12. An SNA/SDLC device is connected
to a FRASM port using SDLC protocol. The traffic is first converted to frame relay and then to ATM
cells for transmission over the network. At the other end, the traffic is first converted back to frame
relay and the SDLC traffic is then extracted for transmission to a front end communication processor
and then to the IBM mainframe.
Figure4-12 Using FRASM for a STUN connection
Using STUN, the FRASM supports:
• Point-to-point SDLC lines.
• Multidropped SDLC devices over a single logical port. These can be assigned to different
connections.
• Multiple logical connections over a single logical port.
• Virtual multidrop in which SDLC devices attached to FRASMs on different MGX 8220 shelves
can be configured to appear as multidropped devices on a single multidrop line.
BSTUN Connections
BSTUN, short for Block Serial TUNnel, is an IBM technique for transmitting bisynch traffic over
frame relay networks by encapsulating the bisynch frames within frame relay frames using the
protocol of RFC 1490.
There are two methods of achieving this:
1 passthrough (or transparent)
The passthrough method encapsulates the entire bisynch data stream including data and control
fields for transmission over the frame relay network. In this method, the frame relay network is
entirely transparent to the bisynch network. Passthrough mode is supported for 2780, 3780 and
3270 IBM devices.
2 local acknowledgment
The local acknowledgment method terminates the bisynch traffic at the frame relay network
interface and encapsulates data only, the bysynch frames are then reconstructed at the other.
Local acknowledgment mode is supported for 3270 devices.
For both methods, bisynch traffic received by the FRASM is converted first to a frame relay format
and is then further converted into cells for transmission over an ATM network, the process is then
performed in reverse order at the other end.
BTUN can also be used for a transparent text mode which permits the unrestricted coding of data
(for example, binary, floating point, and so forth).
BSTUN is used where the requirements call for bisynch in and bisynch out with the intervening
frame relay and ATM segments being used merely to transport the bisynch traffic.