EPS (Evolved Packet System) - 1

Reference:

http://www.cvt-dallas.org/Aug12-Sridhar.pdf

http://ece.engineering.ualberta.ca/en/Graduate/~/media/ece/Graduate/Documents/GradDocs/Intro_to__Evolved_packet_core_network.pdf

Compared to the Legacy RAT, the biggest difference is that LTE has only PS domain, (all IP based)

Why EPC?

GSM, CDMA etc. were based on circuit switching and the services developed were specifically concentrated on the typical applications of telecommunications. But the introduction of mobile internet in early 1990’s brought a huge change or we can say the revolution in telecommunication world. But at that time the mobile equipment were not designed enough to support the services. Another reason was the bandwidth; the BW of radio was not enough to support the services.

Now the trend has been changed with the evolution of new mobile broadband access technologies and developments in semiconductor chips made it possible to support he mobile internet services.

In November 2004, 3GPP(Third generation partnership project) started its work on 4G technologies that was like a successor of Universal mobile telecommunication system(UMTS), particularly a work item named system architecture evolution(SAE) along with LTE which is responsible for evolution of packet core network(EPC), which will support the high bandwidth services at high data rate.

3GPP wanted to create a global standard for 4G technologies. Because, firstly, to give an operator a full freedom to choose a vendor. It means whatever vendor the operator will use, its end users would not have any disruption in services in moving from one vendor equipment to another. It will also increase the competition between vendors. Secondly, the creation of global standard will be helping in removing the separation between various players like operators and vendors involved in providing services to the end users. As an example, in no separation case, the semiconductor chip maker company will have one larger market. So the larger the market is then larger its users. It would help in reducing overall cost of the production and the company can achieve high profits at lowest price levels. So the main target behind the evolution of core networks is to provide affordable and reliable communications networks to the users.

In 2006, I started to work, at that time, CS service was important, just some of people used mobile internet by WAP, sending MMS by using GPRS/EDGE.

Nowadays, being with Smartphone, everybody uses mobile internet, CS service is just basic function.

What is EPS???

The answer is EPC + EUTRAN

What is EPC?

End-to-end IP (All-IP)

Clear delineation of control plane and data plane

Simplified architecture: flat-IP architecture with a single core

See below picture for difference between legacy and EPS

There are SGW, PGW, MME and PCRF in EPS.

Let's see 3GPP 23.002

Entities of the EPC PS Domain, 

1. MME (Mobile management equipment)

Mobility Management,

- NAS signalling and security;

- Inter CN node signalling for mobility between 3GPP access networks;

- Tracking Area list management;

- PDN GW and Serving GW selection;

- SGSN selection for handovers to 2G or 3G 3GPP access networks;

- Roaming ;

- Authentication;

- Bearer management functions including dedicated bearer establishment.

- Lawful Interception of signalling traffic.

The CS fallback enabled MME supports the following additional functions according to TS 23.272 [81]:

- Deriving a VLR number and LAI out of the TAI

- Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE

- Initiating IMSI detach at EPS detach

- Initiating paging procedure towards eNodeB when MSC pages the UE for CS services

- Supporting SMS procedures for CS Fallback.

- Support CS Fallback interface and related functions for 1xRTT CDMA access.

When the MME supports the interworking to 3GPP CS, the MME supports the following functions as specified in

TS 23.216 [83]:

- Performing the PS bearer splitting function by separating the voice PS bearer from the non-voice PS bearers.

- Handling the non-voice PS bearers handover with the target cell as according to Inter RAT handover procedure

as defined in TS 23.401 [76].

- Initiating the SRVCC handover procedure for handover of the voice component to the target cell.

- Coordinating PS handover and SRVCC handover procedures when both procedures are performed,

- support interworking and SRVCC related functions for 1xRTT CDMA access.

Looks MME is very very very very very important in LTE, looks it handles all of NAS procedures. 

2. Serving Gateway (SGW)

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. For detailed S-GW

functions, see TS 23.401 [76] and TS 23.402 [77].

Connectivity to a GGSN is not supported.

The functions of the Serving GW include:

- the local Mobility Anchor point for inter-eNodeB handover;

- Mobility anchoring for inter-3GPP mobility;

- ECM-IDLE mode downlink packet buffering and initiation of network triggered service request procedure;

- Lawful Interception;

- Packet routeing and forwarding;

- Transport level packet marking in the uplink and the downlink;

Accounting on user and QCI granularity for inter-operator charging;

- A local non-3GPP anchor for the case of roaming when the non-3GPP IP accesses connected to the VPLMN;

- Event reporting (change of RAT, etc.) to the PCRF;

- Uplink and downlink bearer binding towards 3GPP accesses as defined in TS 23.203 [73];

- Uplink bearer binding verification with packet dropping of "misbehaving UL traffic";

- Mobile Access Gateway (MAG) functions if PMIP-based S5 or S8 is used;

- Support necessary functions in order for enabling GTP/PMIP chaining functions.

3. PDN Gateway

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.

The PDN GW provides PDN connectivity to both GERAN/UTRAN only UEs and E-UTRAN capable UEs using any of

E-UTRAN, GERAN or UTRAN. The PDN GW provides PDN connectivity to E-UTRAN capable UEs using

E-UTRAN only over the S5/S8 interface. The PDN GW may also provide PDN connectivity to UEs using non-3GPP

access networks with the procedures defined in TS 23.402 [77].

For detailed PDN GW functions, see TS 23.401 [76] and TS 23.402 [77].

PDN GW 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;

- UL and DL service level charging, gating control, rate enforcement as defined in TS 23.203 [73];

- UL and DL rate enforcement based on APN-AMBR;

ETSI

3GPP TS 23.002 version 11.6.0 Release 11 32 ETSI TS 123 002 V11.6.0 (2013-06)

- 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;

Additionally the PDN GW includes the following functions for the GTP-based S5/S8/S2a/S2b:

- UL and DL bearer binding as defined in TS 23.203 [73];

- UL bearer binding verification;

4. Policy and Charging Rules Function (PCRF)

The Policy and Charging Rules Function (PCRF) acts as a policy decision point for policy and charging control of

service data flows/applications and IP bearer resources. The PCRF selects and provides the applicable policy and

charging control decision to the PCEF and, if applicable, application detection and control decision to the TDF or PCEF

with application detection and control feature. When the Gxx interface applies, the PCRF maintains the correlation

between the GW control session over Gxx interface and the IP-CAN session over Gx. The PCRF also acts as an

information exchange relay between BBERF and PCEF to forward event triggers, which can't be transferred directly.

When the Sd interface applies, the PCRF maintains the correlation between the IP-CAN session and the TDF session.

Events subscribed by the TDF are reported by the PCRF.

When S9a interface applies, the PCRF provides to the BPCF the UE/H(e)NB local IP address and UDP port number,

the QoS rules and PCC rules over S9a interface. PCRF maintains the correlation between the GW control session over

S9a interface and the IP-CAN session over Gx interface (if IP-CAN session over Gx interface is available).

When S15 interface applies, the PCRF provides dynamic QoS control policies to the BPCF for the purpose of allocation

of QoS resources in the Fixed Broadband Access Network for HNB CS traffic.

PCRF is the policy and charging control element. PCRF functions are described in more detail in TS 23.203 [73].

In non-roaming scenario, there is only a single PCRF in the HPLMN associated with one UE's IP-CAN session.

In a roaming scenario with local breakout of traffic and/or when a Gxx interface applies there are two PCRFs associated

with one UE's IP-CAN session:

- H-PCRF that resides within the H-PLMN;

- V-PCRF that resides within the V-PLMN.

A single logical PCRF entity may be deployed by means of multiple and separately addressable PCRFs in the PLMN

to be continued.. 

 

 

 

 

 

 

 

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