DOC: adding feature list

Signed-off-by: Raphael Defosseux <raphael.defosseux@openairinterface.org>

docs/FEATURE_SET.md

+<table style="border-collapse: collapse; border: none;">
+  <tr style="border-collapse: collapse; border: none;">
+    <td style="border-collapse: collapse; border: none;">
+      <a href="http://www.openairinterface.org/">
+         <img src="./images/oai_final_logo.png" alt="" border=3 height=50 width=150>
+         </img>
+      </a>
+    </td>
+    <td style="border-collapse: collapse; border: none; vertical-align: center;">
+      <b><font size = "5">OpenAirInterface Core Network Feature Set</font></b>
+    </td>
+  </tr>
+</table>
+
+**Table of Contents**
+
+1. [OpenAirInterface Core Network Block Diagram](#1-openairinterface-core-network-block-diagram)
+2. [OpenAirInterface Core Network Fundamentals](#2-oai-core-network-fundamentals)
+3. [OpenAirInterface Core Network Deployment](#3-oai-core-network-deployment)
+4. [OpenAirInterface SPGW-CUPS Fundamentals](#4-oai-spgw-cups-fundamentals)
+5. [OpenAirInterface SGW Feature List](./FEATURE_SET_SGW.md)
+
+# 1. OpenAirInterface Core Network Block Diagram #
+
+![Block Diagram](./images/oai_cn_block_diagram.png)
+
+# 2. OAI Core Network Fundamentals #
+
+*  Network Access Control Functions
+   -  Authentication and authorization
+   -  Admission control
+   -  Policy and charging enforcement
+*  Packet Routing and Transfer Functions
+   -  IP header compression function
+   -  Packet screening.
+*  Mobility Management Functions
+   -  Reachability management for UE in ECM IDLE state
+*  Security Functions
+*  Radio Resource Management Functions
+*  Network Management Functions (O&M)
+   -  GTP C signaling based Load and Overload Control
+   -  Load balancing between MME instances
+   -  MME control of overload
+   -  PDN GW control of overload
+
+# 3. OAI Core Network Deployment #
+
+*  Target OS
+   -  Ubuntu 18.04 (bionic) server edition
+   -  Red Hat Entreprise Linux 8
+*  Hardware Requirements
+   -  x86-64 Intel/AMD CPU
+   -  At least one 1 network device
+*  Linux Kernel
+   -  Generic kernel is enough
+*  Deployment feasible on:
+   -  PC
+   -  Server
+   -  Container
+   -  Virtual Machine
+
+# 4. OAI SPGW-CUPS Fundamentals #
+
+CUPS = Control-User Planes Separation
+
+So SPGW is composed of almost 2 network functions:
+
+*  1 SPGW-C
+*  1 SPGW-U
+
+Fully written in C++ (-std=c++17)
+
+*  Internal design still asynchronous (ITTI based API)
+*  Usage of submodules: spdlog, libfolly (nolock collections)
+
+Main differences with previous SPGW (tag `v0.7.0`)
+
+*  Handles GTP fragmentation
+*  Enables Network Address Translation (NAT), based on iptables
+*  Easier to install (no kernel dependencies)
+*  Switch-talking natively PFCP
+*  Data is copied/handled in user space

docs/FEATURE_SET_SGW.md

+<table style="border-collapse: collapse; border: none;">
+  <tr style="border-collapse: collapse; border: none;">
+    <td style="border-collapse: collapse; border: none;">
+      <a href="http://www.openairinterface.org/">
+         <img src="./images/oai_final_logo.png" alt="" border=3 height=50 width=150>
+         </img>
+      </a>
+    </td>
+    <td style="border-collapse: collapse; border: none; vertical-align: center;">
+      <b><font size = "5">OpenAirInterface Core Serving Gateway Feature Set</font></b>
+    </td>
+  </tr>
+</table>
+
+**Table of Contents**
+
+1. [SGW Fundamentals](#1-sgw-fundamentals)
+2. [OAI SGW Available Interfaces](#2-oai-sgw-available-interfaces)
+3. [OAI SGW Conformance Functions](#3-oai-sgw-conformance-functions)
+
+# 1. SGW Fundamentals #
+
+(from [Wikipedia.org](https://en.wikipedia.org/wiki/System_Architecture_Evolution))
+
+The Serving Gateway routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PGW).
+
+For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE.
+
+*  It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information.
+*  It also performs replication of the user traffic in case of lawful interception. 
+
+# 2. OAI SGW Available Interfaces #
+
+| **ID** | **Interface** | **Status**         | **Comments**                                   | **Protocols** |
+| ------ | ------------- | ------------------ | ---------------------------------------------- | ------------- |
+| 1      | S5 / S8       | :heavy_check_mark: | Split in control plane only                    | GTP-C/U       |
+|        |               | :x:                | Missing split in User plane                    |               |
+| 2      | S1-U          | :heavy_check_mark: | Split in control plane only                    | GTP-U/UDP     |
+| 3      | S11           | :heavy_check_mark: | S11-C only actually                            | GTP-C/UDP     |
+| 4      | S4            | :x:                | No interconnection with SGSN                   | GTP-C/UDP     |
+| 5      | S12           | :x:                | No interconnection with UTRAN                  | GTP-U/UDP     |
+
+# 3. OAI SGW Conformance Functions #
+
+Based on document **3GPP TS 23.401 V15.5.0 §4.4.3.2**.
+
+| **ID** | **Classification**                                           | **Status**         | **Comments**                                   |
+| ------ | ------------------------------------------------------------ | ------------------ | ---------------------------------------------- |
+| 1      | Local Mobility Anchor point for inter eNodeB handover        | :question:         | Should be, X2HO have to be tested.             |
+|        | (except when user data is transported using the Control      |                    |                                                |
+|        |   Plane CIoT EPS optimization)                               |                    |                                                |
+| 2      | Sending sending of one or more "end marker" to               | :x:                | Could be requested                             |
+|        | -- the source eNodeB,                                        |                    |                                                |
+|        | -- the source SGSN or                                        |                    |                                                |
+|        | -- the source RNC                                            |                    |                                                |
+|        | immediately after the Serving GW switches the path during    |                    |                                                |
+|        | inter-eNodeB and inter-RAT handover(s),                      |                    |                                                |
+|        | especially to assist the reordering function in eNodeB       |                    |                                                |
+| 3      | Mobility anchoring for inter-3GPP mobility                   | :x:                | No Support of 2G and 3G systems                |
+|        | (terminating S4 and relaying the traffic between 2G/3G system and PDN GW) |       |                                                |
+| 4      | ECM-IDLE mode downlink packet buffering and                  | :x:                | Buffering is not supported in idle-mode        |
+|        |  initiation of network triggered service request procedure   |                    |                                                |
+|        |  and optionally Paging Policy Differentiation                |                    |                                                |
+| 5      | Lawful Interception                                          | :x:                |                                                |
+| 6      | Packet routing and forwarding                                | :heavy_check_mark: |                                                |
+| 7      | Transport level packet marking in the uplink & the downlink  | :x:                |                                                |
+|        |  e.g. setting the DiffServ Code Point, based on the QCI, and |                    | Could be supported                             |
+|        |  optionally the ARP priority level, of the associated EPS bearer |                |                                                |
+| 8      | Accounting for inter-operator charging.                      | :x:                |                                                |
+|        | -- For GTP based S5/S8, the SGW generates accounting data per UE and bearer |     |                                                |
+| 9      | Interfacing OFCS according to charging principles and        | :x:                |                                                |
+|        |   through reference points specified in TS 32.240            |                    |                                                |
+| 10     | Forwarding of "end marker" to the                            | :x:                |                                                |
+|        | -- the source eNodeB,                                        |                    |                                                |
+|        | -- the source SGSN or                                        |                    |                                                |
+|        | -- the source RNC                                            |                    |                                                |
+|        | when the "end marker" is received from PGW and SGW has downlink user plane established. | |                                        |
+|        | Upon reception of "end marker", the Serving GW shall not send Downlink Data Notification. | |                                      |