This MIB module manages the features and configuration
for PDN Gateway(PGW) and Serving Gateway(SGW) in Evolved
Packet Core(EPC) architecture.
        
The PGW is the permanent IP point-of-attachment for 
access via the E-UTRAN. The PGW performs IP policy and
charging enforcement on packet flows to and from mobile 
devices.
        
The SGW is a local mobility anchor for E-UTRAN mobility,
switching packets between the S5 interface and the GTP
based S1-U interface for mobiles in connected mode.
        
+------------------------------------------------+  
|                                                |
|                                                |
|           +=======+__________                  |
|           | UTRAN |          |S12              | 
|           |      +======+    |                 | 
|           +======|      |    |                 |
|           +======| SGSN |___ |                 |
|           |      +======+   ||                 |
|           | GERAN |  |    S4||                 |
|           +=======+  |(GTP-C||                 |
|                    S3| VO,V1||                 |
|                      |  GTPU||                 |   
|                      |VO,V1)||                 |
|                  +======+   ||                 | 
|              ____| MME  |__ ||                 |
|              |   +======+  |||                 |
|        S1-MME|      S11    |||                 |
|              |     GTP-C   |||                 |
|              |  (V0,V1,V2) |||                 |
|+====+ +========+        +======+     +======+  |  +===+
|| UE |-|E-Utran |        | SGW  |     | PGW  |-SGi-|PDN|
||    | |(EnodeB)|        |      |     |      |  |  |   |
|+====+ +========+        +======+     +======+  |  +===+
|              |            |  |  GTP-C   |      | 
|              |            |  |(vo,v1,v2)|      | 
|              |            |  |GTP-C     |      |  
|              |            |  |  (vo,v1) |      | 
|              |  GTP v2-u  |  |          |      |
|               ------------   -----------       |
|                  (S1-U)          S5            |
|                                                |
+------------------------------------------------+
        
This architecture supports the non-roaming and roaming
architectures using GTP based protocol on the PGW and SGW
with a number of functions. It support high number of
subscribers with their associated control messages and
data traffic. Each of this association forms an entry in
PGW and SGW and is commonly know as bearers.
        
The various features used by this architecture and their
attributes are captured in this MIB:
        
1. Overload protection, handles congestion control, SGW and
PGW represent congestion as a DFP metric considering the
current CPU, memory usage and number of bearers open. On
reaching the low congestion threshold, based on the ARP, high
priority calls are accepted and those with a lower priority
are rejected. Once the high threshold is reached all new
calls are rejected.
        
2. Buffering agent, the SGW buffers the data packets in the
downlink direction till the downlink path/peer information is
ready. It buffers all packets coming from any gateway based
on the default/configured values of the buffer size, discard
period and number of packets. Packets will be buffered on a
per bearer basis and retrieved in the order it was buffered
using the handle given by the buffering agent initially. If
the buffer is full then no more bearers will be accepted. 
        
Acronyms and terms:
        
 3GPP     3rd Generation Partnership Project
 DFP      Dynamic Feedback Protocol
 EDGE     Enhanced Data rates for GSM Evolution
 EPC      Evolved Packet Core
 E-UTRAN  Evolved UTRAN
 GERAN    GSM EDGE Radio Access Network
 GPRS     General Packet Radio Service
 GSM      Global System for Mobile communications
 GTP      GPRS Tunneling Protocol
 PDN      Public Data Network
 PGW      PDN Gateway
 SAE      System Architecture Evolution
 SGW      Serving Gateway
 SP GW    An integrated SGW and PGW Gateway
 UE       User Equipment
 UMTS     Universal Mobile Telecommunications System
 UTRAN    UMTS Radio Access Network
        
        
REFERENCE
        
[1] 3GPP: Evolved GPRS Tunneling Protocol for Control
    Plane (GTPv2-C). 3GPP TS 29.274 V1.3.1.
        
[2] 3GPP: GPRS enhancements for E-UTRAN access.
    3GPP TS 23.401 V8.3.0.
        
[3] 3GPP: Circuit Switched fallback in Evolved
    Packet System(EPS). 3GPP TS 23.272.