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                       CFE S12 SAS5 FRAME HANDLER Help
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    The Frame Handler function is activated on a ISTB Directory Number 
by the registering of that DN with the LAS Frame Handler which is an SLTC device
conncected with a IPTMN module.  Each SLTC can have up to 128 customers who
have dynamic connections with the assigned SLTC. (these connections drop after 25
seconds of no traffic)

The SLTC multiplexs the channels on a BD-SPC to the TNS and hence tke PKT HANDLER.

CFE checks for FHL registration.  If it is then observe the allocated
	TEI, LI1, LIC2 and LIC3 values.

CFE checks for states of the customer datalink (remember these are dynamic)

CFE Checks for states of the FHREF (frame handler) and the Bd channels.

CFE checks for SPC between LAS and the TNS

                Generic Alcatel Desription of X25 CONNECTIVITY
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A packet switched call between two subscribers connected to ISDN exchanges will always be 
routed via a Packet Handler (PH) which is physically located outside the exchanges. This 
will be true even when the two subscribers are connected to the same exchange. Before 
making use of the packet switching services of the ISDN, subscribers must first establish 
a connection with the Packet Handler. The PH is connected to a CRF-P by means of the PHI 
(Packet Handler Interface).

The hardware modules involved in packet switching are: 

User-side modules:
ISM : ISDN subscriber module; used for local Basic Access (BA) interfaces

IRSU : ISDN remote subscriber unit ; used for remote Basic Access interfaces

IRIM : ISDN remote interface module

IPTMu (IPTM-PRA) : Integrated packet trunk module at user side (with PRA functionality); 
used for Primary rate interface. An IPTM combines a 2Mbit/s interface with a pool of 
4 HDLC (high level data link control (device)) terminations; the IPTMu in particular sees 
its interface as a primary rate access and uses one HDLC for D-channel termination.

IPTMn (IPTM-Bd) : IPTM at network side (with trunk functionality + Bd-channel handling) ; 
the IPTMn in particular sees its interface as a digital trunk and uses all 4 HDLCs for 
Bd-channel termination. 

DTM : Digital trunk module ; used for termination of trunks, controlled by ISUP signalling. 
IPTMp (IPTM-PHI) : IPTM at PH side (with PHI functionality) ; used for primary rate 
interface of the PHI. The IPTMp is essentially identical to the IPTMu, with a slightly 
adapted signalling protocol according to the PHI standard. 
CRF-S

The Connection Related Function at Subscriber side (CRF-S) is really a local ISDN exchange.
Each ISDN subscriber belongs to only one CRF-S. A subscriber connected to a CRF-S can 
establish an access connection to the PH: 

Packet switching over B-channel
Only one data link is allowed on a given B-channel. From the CRF-S the call is sent over a 
trunk to the CRF-P where it is switched to a Bb -channel of the PHI. The Bb-channel is a 64 
kbit/s channel on PHI for B-channel services. The Bb-channel is established in the same way
as for subscribers directly connected to the host exchange, i.e. the B-channels will be 
circuit switched to the Packet Handler in the PSPDN (Packet Switched Public Data Network). 
There are two packet switching procedures:

A static semi-permanent Bb-channel out of the PHI, (a semi-permanent packet switched ISDN 
line). Only semi-permanent static connections are provided (via MMC commands), the complete
path is predefined by population, the need for a semi-permanent path is stored in each 
exchange, and the path is established independently in each exchange. 

In ISDN_V2, subscribers are connected to System 12 exchange access Packet Switching services
via either one or more B-channels and/or the D-channel. Over this access X.25 (data 
communication user-network protocol) is run. The S12 exchange has only to provide a 
transparent path (channel or logical link) through the exchange, with the subscriber at 
one end and Packet Handler at the other end. 

Packet switching over D-channel
In the case of circuit switching the D-channel is used only for conveying signalling 
messages. However for Packet Switching (PS) the D-channel can also be used for transporting 
X.25 packets from one user to another. To support this function data links must be 
stablished between users and the PH. Multiple (LAPD: link access protocol on D-channel) 
data links are allowed on the D-channel. In the CRF-S, D-channel packet data frames from 
different users are multiplexed onto a Bd-channel to the PH (Bd-channel is a 64 kbit/s 
timeslot on PHI for D-channel services). The actual transportation consists of multiplexing 
packets (frames) arriving at the subscribers' D-channels onto a number of B-channels (design
ated Bd channels) between the CRF-S and the PH. This is accomplished by means of a Frame 
Handler (FH) function in the CRF-S. The FH consists of a Frame Switching (FS) function and 
a Frame Relaying (FR) function. The multiplexing (i.e. switching) is done in the FS, while 
the transport from the CRF-S through the CRF-P is by means of SPCs. To support D-channel PS 
it is necessary to create these data links and Bd channels.


 
