4-12
Cisco MGX 8220 Installation and Configuration
Release 5.0, Part Number 78-6430-03 Rev. D0, November 2003
Chapter
Frame Relay Service Modules (FR SMs)
An application of a STUN connection is shown in Fi g ure 4 - 5. An SNA/SD LC devic e is co nn ect ed to a
FRASM port using SDLC protocol. T he 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 t ra nsm ission to a front-end communi cat io n pr o ce ss or an d
then to the IBM mainframe.
Figure4-5 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 Bloc k Serial TUNnel, is a n IBM technique for transmitting bisync tr affic over Frame
Relay networks by encapsulating the bisync frames within Frame Relay frames using the protocol of
RFC1490.
There are two methods.
1. passthrough (or transparent)
The passthrough method encapsu l ate s 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 t er min at es t he B is y nc traffic at th e Fr am e Relay network
interface and encapsulates data only. The Bisync f r ame s 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 in to cell s for tra nsmissi on ov er an ATM network, the process is then performe d
in reverse order at the ot he r end.
BTUN can also be used for a transparent text mode which permits the unrestricted coding of data (for
example, binary, floati ng p oint, and so forth) .
BSTUN is used where the req ui rem en ts call for Bisync in and Bisy nc ou t w it h t he in t er ven in g Fra m e
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