포스팅 해주신 S-GW 기능이 좀 이상하다 싶어서 출처를 따라가 보니, 답변해주신 분께서 S-GW, P-GW 둘의 기능을 모두 합쳐서 나열하셨군요. 위의 기능은 S-GW, P-GW의 기능을 모두 나열한 것입니다. 그 중 S-GW의 기능만 추린다면 UE(가입자), eNB 기반의 기능이 위주가 됩니다. 패킷 흐름에 대한 상세 제어나 , UE의 IP 할당, 과금등은 P-GW에서 수행합니다. S-GW는 eNB에게 user data를 전달하기 위한 Core쪽의 최종단 NE 입니다. 따라서 UE의 mobility와 관련된 기능들을 많이 수행합니다.

변동우님 답변 대답히 감사드립니다.
며칠동안 기다렸는데 알찬 정보를 주셔서 감사드려요.
답변 정리하면

SGW 가 하는일은
1.eNodeB와 eNodeB간, 3GPP네트웍과 E-UTRAN간의 단말이동에 대한 anchoring 기능을 담당
2. SGW는 eNB에게 user data 전달 하기 위한 NE

으로 정리가 되는군요.

그동안 찾아 본게 있는데

SGW (Serving Gateway): The SGW 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.

[출처]http://en.wikipedia.org/wiki/System_Architecture_Evolution

여기에서
1. For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE.

>> UE의 Idle상태가 되기위해, DL data가 도착했을때, SGW는 downlink data path와 trigger paging 을 종료한다.
이렇게 이해하면 될까요?

2.It manages and stores UE contexts

>>UE Context 를 관리하고 저장한다는게 무슨의미인지 알수 있을까요. ?

한창섭님//
1. UE가 Idle 상태이면 SGW는 downlink data path를 해제하고, downlink data가 UE에게 전달될 필요가 있는 시점에 paging을 trigger하는걸로 이해하시면 됩니다.
2. SGW도 MME와 마찬가지로 UE 단위로 서비스를 관리합니다. 가입자 기반의 user data 서비스에 필요한 정보관리를 UE Contexts로 지칭하는 듯 합니다.

아 그렇군요 ~ 변동우님 정말 감사드립니다.

덕분에 며칠동안 궁금했던 것이 풀렸습니다.

다시한번 머리숙여 감사드립니다!!

개인적인 의견하나 올립니다.
변동우님이 지적하신데로, S-GW 기능과 P-GW 기능이 섞여 있습니다.
그러나, 위의 기능 대부분 기능들은 S-GW에서 수행합니다. 7)번, 8)번 만 제외하고는....
해당 기능들에 대해 약간 부가 설명을 아래에 하였습니다.

1) Mobility anchoring between LTE and non-3GPP technologies(e.g., WiMax, 3GPP2)
-> 위 기능은 P-GW 기능에 가깝고요, S-GW는 LTE-3G(WCDMA, UTRAN) 간 Mobility를 지원합니다.
즉, 3G(WCDMA)의 SGSN과 S-GW 간 S4 인터페이스를 통하여 Mobility를 지원합니다.
위의 LTE - Non 3GPP 간 Mobility는 P-GW 기능으로 보는 것이 맞을 것 같습니다.

2) UE connection management
-> 당근 기능이고요.

3) Policy enforcements via interworking with PCRF. : PCRF와 연동을 통한 정책 강화
-> 3GPP TS23.203 등 다수의 표준에서 보면, S-GW가 BBERF(Bearer Binding and Event Reporting Function)
기능을 수행하게 되어 있는데, 이 때, S-GW가 PCRF와 Gxx 인터페이스로 연동하게 되어 있습니다. 아마도,
위의 설명은 그 기능을 설명하고 있는 것 같습니다.

4) Charging support
-> 이 기능도 보통은 P-GW만 수행하는 기능으로 잘 못들 알고 계신데, S-GW도 해당 기능을 수행합니다.
단, Inbound/Outbound 로밍시에 주로 수행합니다. 우리나라 사람이 해외에 나가면, outbound 로머로
표현을 하고, 외국사람이 우리나라로 들어오면 Inbound 로머로 표현을 하는데, 예를들면, 외국사람이
우리나라로 들어와서 KT LTE망을 이용할 경우, KT망의 S-GW와 외국망의 P-GW가 연동을 하는데,
이 때, S-GW에서 수집한 과금 정보와 외국의 P-GW에서 수집한 과금 정보를 가지고 사업자 간 요금 정산을
수행합니다.

5) Per-user based packet filtering
-> 당근이고요, 여기에서 packet filtering 이라함은 QoS 수행을 의미합니다. QoS 정책은 PCRF에서 관리하지만,
그 QoS 정책이 "P-GW -> S-GW -> eNB -> UE"로 각각 전달되어 저장되고, 실제 트래픽을 제어(즉, packet filtering 혹은 Traffic Management)하는 기능은 각 노드에서 모두 수행하지요.

6) Lawful interception
-> 사실, 이 부분은 "합법적 감청"이라고 할 수 있죠.

좀 더 정확히 학습을 하실려면, 3GPP TS23.401을 보시면 됩니다.
그 표준에서 S-Gw, P-GW 및 MME 기능을 퍼와서 아래에 첨부하였습니다. 참고하십시오.

4.4.2 MME
MME functions include:
- NAS signalling;
- NAS signalling security;
- Inter CN node signalling for mobility between 3GPP access networks (terminating S3);
- UE Reachability in ECM-IDLE state (including control and execution of paging retransmission);
- Tracking Area list management;
- Mapping from UE location (e.g. TAI) to time zone, and signalling a UE time zone change associated with mobility,
- PDN GW and Serving GW selection;
- MME selection for handovers with MME change;
- SGSN selection for handovers to 2G or 3G 3GPP access networks;
- Roaming (S6a towards home HSS);
- Authentication;
- Authorization;
- Bearer management functions including dedicated bearer establishment;
- Lawful Interception of signalling traffic;
- Warning message transfer function (including selection of appropriate eNodeB);
- UE Reachability procedures;
- Support Relaying function (RN Attach/Detach).
NOTE: The Serving GW and the MME may be implemented in one physical node or separated physical nodes.
The MME shall signal a change in UE Time Zone only in case of mobility and in case of UE triggered Service Request, PDN Disconnection and UE Detach. If the MME cannot determine whether the UE Time Zone has changed (e.g. the UE Time Zone is not sent by the old MME during MME relocation), the MME should not signal a change in UE Time Zone. A change in UE Time Zone caused by a regulatory mandated time change (e.g. daylight saving time or summer time change) shall not trigger the MME to initiate signalling procedures due to the actual change. Instead the MME shall wait for the UE's next mobility event or Service Request procedure and then use these procedures to update the UE Time Zone information in the PDN GW.

4.4.3 Gateway

4.4.3.1 General

Two logical Gateways exist:
- Serving GW (S GW);
- PDN GW (P GW).
NOTE: The PDN GW and the Serving GW may be implemented in one physical node or separated physical nodes.

4.4.3.2 Serving GW
The Serving GW is the gateway which terminates the interface towards E-UTRAN.
For each UE associated with the EPS, at a given point of time, there is a single Serving GW.
The functions of the Serving GW, for both the GTP-based and the PMIP-based S5/S8, include:
- the local Mobility Anchor point for inter-eNodeB handover;
- sending of one or more "end marker" to the source eNodeB, source SGSN or source RNC immediately after switching the path during inter-eNodeB and inter-RAT handover, especially to assist the reordering function in eNodeB.
- Mobility anchoring for inter-3GPP mobility (terminating S4 and relaying the traffic between 2G/3G system and PDN GW);
- ECM-IDLE mode downlink packet buffering and initiation of network triggered service request procedure;
- Lawful Interception;
- Packet routing and forwarding;
- Transport level packet marking in the uplink and the downlink, e.g. setting the DiffServ Code Point, based on the QCI of the associated EPS bearer;
- Accounting for inter-operator charging. For GTP-based S5/S8, the Serving GW generates accounting data per UE and bearer;
- Interfacing OFCS according to charging principles and through reference points specified in TS 32.240 [51].
Additional Serving GW functions for the PMIP-based S5/S8 are captured in TS 23.402 [2].
Connectivity to a GGSN is not supported.

4.4.3.3 PDN GW
The PDN GW is the gateway which terminates the SGi interface towards the PDN.
If a UE is accessing multiple PDNs, there may be more than one PDN GW for that UE, however a mix of S5/S8 connectivity and Gn/Gp connectivity is not supported for that UE simultaneously.
PDN GW functions include for both the GTP-based and the PMIP-based S5/S8:
- Per-user based packet filtering (by e.g. deep packet inspection);
- Lawful Interception;
- UE IP address allocation;
- Transport level packet marking in the uplink and downlink, e.g. setting the DiffServ Code Point, based on the QCI of the associated EPS bearer;
- Accounting for inter-operator charging;
- UL and DL service level charging as defined in TS 23.203 [6]
(e.g. based on SDFs defined by the PCRF, or based on deep packet inspection defined by local policy);
- Interfacing OFCS through according to charging principles and through reference points specified in TS 32.240 [51].
- UL and DL service level gating control as defined in TS 23.203 [6];
- UL and DL service level rate enforcement as defined in TS 23.203 [6]
(e.g. by rate policing/shaping per SDF);
- UL and DL rate enforcement based on APN-AMBR
(e.g. by rate policing/shaping per aggregate of traffic of all SDFs of the same APN that are associated with Non-GBR QCIs);
- DL rate enforcement based on the accumulated MBRs of the aggregate of SDFs with the same GBR QCI
(e.g. by rate policing/shaping);
- DHCPv4 (server and client) and DHCPv6 (client and server) functions;
- The network does not support PPP bearer type in this version of the specification. Pre-Release 8 PPP functionality of a GGSN may be implemented in the PDN GW;
- packet screening.
Additionally the PDN GW includes the following functions for the GTP-based S5/S8:
- UL and DL bearer binding as defined in TS 23.203 [6];
- UL bearer binding verification as defined in TS 23.203 [6];
- Functionality as defined in RFC 4861 [32];
- Accounting per UE and bearer.
The P GW provides PDN connectivity to both GERAN/UTRAN only UEs and E UTRAN capable UEs using any of E UTRAN, GERAN or UTRAN. The P GW provides PDN connectivity to E UTRAN capable UEs using E UTRAN only over the S5/S8 interface.