Functions of main LTE packet core elements - MME, SGW, PGW

Now when I have to start writing, I'm looking deep into back of my skull, to figure out with what I have my first LTE-like problem. And you know what? I don't remember.
So maybe the best way of starting anything is to start (writing) from the beginning.
Let's have a closer look at three main packet core elements of LTE network.

MME - Mobility Management Entity
MME is the key control node for LTE access network. It is responsible for tracking and paging procedure including retransmissions, and also for idle mode of User Equipment (UE). MME is also involved in bearer activation and its deactivation procedures, to its task also belongs choosing the SGW for a UE in process of initial attach and when the intra-handover take place which involves Core Network (CN) node relocation.
MME is responsible for authenticating user towards the HSS, if user is roaming MME terminates S6a interface towards user's home HSS. All Non Access Stratum (NAS) signaling terminates at the MME point, which is also responsible for generation and allocation of temporary UE identities (GUTI). Among its duties is also authorization UE to Public Land Mobile Network (PLMN) and enforcing UE roaming restrictions if there are any. MME is also termination point of ciphering and integrity protection for NAS signaling. Lawful Interception (LI) of signaling could be also supported by MME entity. It also provides the control plane function for mobility between LTE and 2G/3G networks by the S3 interface( from SGSN to MME).


Functions mentioned above as a list, according to 23.401 3GPP documentation.
MME functions include:

• NAS signalling;
• NAS signalling security;
• Inter CN node signalling for mobility between 3GPP access networks (terminating S3);
• UE Reach ability 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 Reach ability procedures.

The MME shall signal a change is 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 theUE's next mobility event or Service Request procedure and then use these procedures to update the UE Time Zone information in PDN GW.

SGW - Serving Gateway

Serving GW is the gateway which terminates the interface towards E-UTARN. For each UE associated with the EPS, at given point of time, there is a single Serving GW. 

SGW is responsible for handovers with neighboring eNodeB's, also for data transfer in terms of all packets across user plane. To its duties belongs taking care about mobility interface to other networks such as 2G/3G. SGW is monitoring and maintaining context information related to UE during its idle state and generates paging requests when arrives data for the UE in downlink direction. (e.g. somebody's calling). SGW is also responsible for replication of user traffic in case of LI.

SGW functions as a list, according to 23.401 3GPP documentation.

SGW functions 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

Please bury in mind, that the connectivity to a GGSN is not supported.

PDN GW - Packet Data Network Gateway

The PGW is the gateway which terminates the SGi interface towards PDN. If UE is accessing multiple PDNs, there may be more than one PGW for that UE, however a mix of S5/S8 connectivity and Gn/Gp connectivity is not supported for that UE simultaneously.

PGW is responsible to act as an "anchor" of mobility between 3GPP and non-3GPP technologies. PGW provides connectivity from the UE to external PDN by being the point of entry or exit of traffic for the UE. The PGW manages policy enforcement, packet filtration for users, charging support and LI.

Possible to use non-3GPP technologies are: WiMAX, CDMA 1X and EvDO.

PGW functions as a list, according to 23.401 3GPP documentation.

PGW functions include:

• 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 (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.

Aditionally the PDN GW includes the following functions fot 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 PGW provides PDN connectivity to both GERAN/UTRAN only UEs and E-UTRAN capable UEs using any of E-UTRAN, GERAN or UTRAN. The PGW provides PDN connectivity to E-UTRAN capable UEs using E-UTRAN only over the S5/S8 interface.

Hope you like it. Do not hesitate, please comment.

You found grammar mistake, word spelled wrong, or maybe something is wrong with the content please let me know.

As I said once - I'm not a thief, that's why I'm putting here links to sources from above was taken.

Sources:

23.401 3GPP documentation,

knol - a unit of knowledge,

MME - www.fullchipdesign.com,

SGW - www.fullchipdesign.com,

PGW - www.fullchipdesign.com,