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Cisco MGX 8220 Installation and Configuration
Release 5.0, Part Number 78-6430-03 Rev. D0, November 2003
Chapter
Frame Relay Service Modules (FRSMs)
An application of a STUN connection is shown in Figure 4-5. 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.
Figure 4-5
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 devices can be assigned to different
connections.
•
Multiple logical connections over a single logical port.
•
Virtual multidrop in which SDLC devices attached to FRASM modules 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 bisync traffic over Frame
Relay networks by encapsulating the bisync frames within Frame Relay frames using the protocol of
RFC 1490.
There are two methods.
1.
passthrough (or transparent)
The passthrough method encapsulates the entire bisync 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 Bisync network. Passthrough mode is supported for 2780, 3780, and
3270 IBM devices.
2.
local acknowledgment
The local acknowledgment method terminates the Bisync traffic at the Frame Relay network
interface and encapsulates data only. The Bisync frames are then reconstructed at the other. Local
acknowledgment mode is supported for 3270 devices.
For both methods, Bisync 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 Bisync in and Bisync out with the intervening Frame
Relay and ATM segments being used merely to transport the Bisync traffic.
Remote SDLC
End User Device
SDLC
FRASM
ATM network
Frame Relay
over ATM
MGX
8220
MGX
8220
FRASM
SDLC
Front-end
Processor
NCP1
Mainframe
VTAM1
11760
