EPS UE BEARERS AND CONNECTIVITY
When you are done with this LTE section, you will be capable of learning EPS UE bearers and connectivity. EPS bearer terms of its endpoints as well as the QoS features. Also, you can Lean radio bearer S1 bearer and S5/S8 bearer are related to the EPS bearer, etc more.
- define EPS bearer terms of its endpoints as well as the QoS features
- describe why the radio bearer S1 bearer and S5/S8 bearer are related to the EPS bearer.
- clarify the role for the QCI value when defining the QoS that is applied to bearers of EPS
- explain how various types of traffic are carried by different bearers of EPS for the same UE.
- define how uplink and downlink data packets are organized and transmitted through appropriate EPS bearers that connect to one UE
- clarify the importance and the operation of the default bearer
- explain the process for establishing an EPS bearer
- identify and define the primary identities of the EPS networks’ hierarchical regions
- identify the identities employed in relation to the users and UEs.
- explain the process used by the UE to sign up as a user on EPS along with the IMS
- provide the most important methods for managing mobility
- explain the operation principle of CS fallback.
- Explain how home eNB (femtocell) is linked to the LTE EPS
LTE EPS Table Of Content
- The EPS as an IP-CAN.
- EPS Quality of Service.
- EPS Bearer QoS Class Identifiers.
- ARP (Allocation and Retention Priority)
- QoS Levels.
- EPS Bearer Types.
- EPS Bearer Composition.
- EPS Bearer Establishment.
- EPS Area Identities.
- Subscriber Identities.
- LTE State Management.
- EPS Service Concepts.
- Device Selection.
- EPS Initial Attach.
- Default Bearer Establishment.
- IMS Registration.
- CS Fallback Attach.
- EPC Support for Idle Mode.
- TAU (Tracking Area Update).
1. The EPS as an IP-CAN
The EPS offers no services to the user apart from connectivity to an external packet data network. In this respect, the EPS can be considered to function only as an IP-CAN. The specific services that user receives are provided through service platforms that are also implemented in the external PDN.
2. EPS Quality of Service.
QoS in the EPS is defined by a combination of four parameters:
⦁ QCI (QoS Class Identifier)
⦁ ARP (Allocation and Retention Priority)
⦁ GBR (Guaranteed Bit Rate)
⦁ MBR (Maximum Bit Rate)
3. EPS Bearer QoS Class Identifiers.
In order that common levels of QoS can be provided in different operators’ networks, the 3GPP has defined a limited set of QoS levels that are standardized for all operators. Each QoS level is identified with a QCI value. The QoS targets for each QCI value are shown in the table. It can be seen that there is a broad division between GBR and non-GBR services. There are also targets for the delay budget and packet loss rate. Additionally, each QCI is allocated a priority level. This is used for prioritizing resource allocations at the eNB.
4. ARP (Allocation and Retention Priority).
The EPS ARP facility allows the system to deal with overloaded and busy periods. Every QoS class is linked with an ARP level, that determines the relative importance for the EPS bearers based on the classes. There are fifteen levels of ARP that range between Priority 1 (highest) up to Priority 15 (lowest).
The other elements of ARP are mere “yes” or “no” parameters: the connection’s Pre-emption Capability, which determines the possibility of being capable of preempting other low-priority connections as well as Pre-emption Vulnerability that determines if it is able to be protected from higher-priority connections.
5. QoS Levels.
The QoS Levels of the EPC is currently defined using the three levels GBR MBR, and AMBR (Aggregate Maximum Bit Rate).
GBR connections have an unrestricted data rate which makes them suitable for carrying specific types of delay-sensitive and real-time traffic. MBR connections are not guaranteed, variable-bit rate services that have a predetermined max data rate. If a connection’s rate exceeds the maximum set, the network could decide to stop removing the traffic that is not being used.
6. EPS Bearer Types.
As each EPS bearer is able to provide only one definition of QoS, and users may use multiple services and the UE might have multiple EPS bearers in use and each offering various levels of QoS to various services. For instance, a person could be using a browser and also engaged in VoIP calls. This could require two distinct levels of QoS, the need for two EPS bearers with one bearer for each of the different traffic flows. In addition, signaling is necessary to provide the services, and this could require the use of a third EPS bearer.
7. EPS Bearer Composition.
An EPS is the union that a radio bearer via the air interface as well as an S1 bearer, and the S5/S8 bearer. Each bearer is described in a slightly different manner and the binding between these bearers is available at every node. Particularly, the connection between the radio bearer that is on the S1 interface as well as an air bearer in the S1 interface that is present in the eNB, is the E-RAB (E-UTRAN Radio Access Bearer).
8. EPS Bearer Establishment.
The creation of a new EPS bearer establishment occurs through the PCRF. This could be as a result of a request from a service called AF (Application Function) within an external PDN to establish the UE initiated or terminated service or the setting up of the default EPS bearer once the UE is registered with the system.
The PCRF is the one responsible to manage PCC (Policy Control and Charging) and as such is in charge of deciding the time when an EPS bearer is needed and what QCI amount should be to determine the value of the bearer. This information is communicated to the PDN-GW by using the PCC decision Provision message.
9. EPS Area Identities.
The EPS still uses the PLMN identifier used by the older 3GPP systems. It is comprised of two codes: the MCC (Mobile Country Code) as well as MNC (Mobile Network Code). MNC (Mobile Network Code).
It is the MMEGI (MME Group Identifier) is a 16-bit identifier given for a specific MME Pool. The MMEGI does not have to be unique within the PLMN.
The acronym TAI (Tracking Area Identifier) is identical with the LA (Location Area) or RA (Routing Area) identifiers employed by the GERAN/UTRAN as it can be used to identify groups of cells that are part of an access network. For E-UTRAN the TAI (Tracking Area) is the degree to the UE’s location monitored. It also is the area in which the UE is called upon to be paged. The TAI comprises networks’ MCC and MNC which is then followed by a TAC (Tracking Area Code).
10. Subscriber Identities.
The primary method of identifying EPS subscribers is IMSI (International Mobile Subscriber Identity) which is permanently assigned to a subscriber account. In addition, the IMEISV (International Mobile Equipment Identity and Software Version) and MSISDN (Mobile Station ISDN Number) are also used as LTE identifiers.
11. LTE State Management.
The LTE State Management to ensure that it can provide efficient service to UEs for effective service, EPS must be competent in defining and keeping in mind the availability and accessibility of every terminal. It does this by maintaining two contexts for each UE: the EMM (EPS Mobility Management) context, and the ECM (EPS Connection Management) context
12. EPS Service Concepts.
In the Attach process as part of the Attach process, the EPS will determine at a minimum a default bearer to the UE. The details of a default APN’ that will be used for the bearer can be found in the HSS profile of the subscriber or could be chosen automatically via the EPC.
HSS data could also reveal whether dedicated bearers are required to be created in addition to the default bearer at the time of the attachment.
Every UE regardless of how many EPS bearers it’s created as well as the PDN Connectivity Services it is employing, can only be serviced through only one MME or S-GW a moment (except for the short but inevitable overlap when a move is made).
13. Device Selection.
Each bearer in the EPS system will be moved through a certain number of gadgets. In the case of an EPS Bearer that is established between the UE’s E-UTRAN home and the home EPC the set of devices would be an eNB, MME, an S-GW, and a PDN-GW. Since each network is anticipated to have a variety of different types of devices connected to it, the method of the devices that are involved serves a bearer be specified clearly. The selection of devices for EPS connections works in the following manner. The UE selects the eNB it will make use of based on the air interface selection and reselection.
14 EPS Initial Attach.
The goal of the UE when attaching a device is to verify the subscriber’s name and address to the network in order for services to be accessed. When attaching, this UE is assigned an EPS default bearer that allows for continuous connectivity to a PDN. The UE could be provided with details of a local PCSCF that will allow it to sign up to the IMS.
A simplified overview of the attached process in the event that it’s an initial attachment with information stored from a prior context for a UE that uses their H-PLMN (Home Public Land Mobile Network) and accessed through the Home E-UTRAN is illustrated and the steps to be completed are explained below.
15. Default Bearer Establishment.
A default bearer needs to be created and the MME chooses the SGW which will manage it, as well as a PDN- GW that can support that APN. The MME issue a CreateDefault bearer request to the chosen S-GW, which assigns a GTP TEID the bearer of the EPS and forwards the request to the designated PDN-GW. If the network is using dynamic PCC, the PDN GW will search the PCRF that is assigned to for the UE in search of bearer-specific parameters. If not, the bearer is created with local QoS settings that are stored within the PDN-GW.
16. IMS Registration.
With an EPS bearer default installed, the UE is able to register services using the IMS as long as the PLMN uses an IMS and can be reached through its default APN. But before the UE is able to register to the IMS it has to first complete P-CSCF discovery. This could be done in the attachment process in the event that an IP address for the local P-CSCF is provided by the PDN-GW after a default bearer has been created. Or it is possible that the UE could need to launch an independent P-CSCF discovery query. After a local PCSCF is discovered, the UE may send a SIP Register message.
In non-roaming circumstances, the P-CSCF is able to query HSS directly. HSS directly to identify the subscriber’s S-CSCF. If roaming is required, the P-CSCF being visited has to forward the query to one of the I-CSCF as well as an S-CSCF within the home network of the subscriber to take the action.
17. CS Fallback Attach.
If there is a UE and the network support CS Fallback, the UE will ask for CS Fallback registration during the attach process. The service will be accessible in areas that have overlap between E-UTRAN or GERAN/UTRAN coverage.
The Attach Request message that has the attach Type set to EPS/IMSI attach as well as a CS Fallback enabled flag is transmitted via the MME. This triggers the combined attach process within the MME. The MME will calculate its number from VLR VLR accountable for the location Area which is linked to the actual Tracking Area. The information is part of the data build for the MME and is not dynamically accessible. The MME makes use of the SGs interface. It is an improved form of Gs interface that allows the ability to connect with older SGSNs as well as MSCs.
18. EPC Support for Idle Mode.
The MME that is currently serving every UE is accountable to ensure its accessibility. It does this by monitoring the terminal’s TA, which is where the terminal is situated.
The EPS permits a cell to be a part of multiple TA. This permits the UE to move within the contiguous TAs without the need to carry out TAU (Tracking Area Update), which can reduce the amount of signaling related to location that is required, however, it could also enhance the amount of panning that is required for each UE connections.
The MME shows this increased mobility by keeping a TA list for every registered UE in which is the list of TAs for which the UE is currently registered for.
19. TAU (Tracking Area Update).
A TAU is triggered between the UE as well as the MME to which it is registered. It’s initiated by the UE notifying the MME of changes in the TAI following the reselection of a cell. TAUs are also utilized in the initial attachment process. It can also be activated by events like the expiration of the regular TAU timer, or as a part of MME load balancers or Rebalancing.
In the sample message flow of this example, it’s assumed there is a connection between the UE connects to HPLMN and that an SGW modification and MME relocation are not needed. When it is detected that there is a change in TAI the UE sends a TAU Request email to the ENB. The TAU Request contains the old GUTI, the old TAI, and the status of the bearer EPS.
The principal function of the TAU process is to make sure that the MME is aware of the location of each UE is when there is traffic inbound to provide. Paging is typically activated by the receipt of an S-GW Downlink Notification to the MME which indicates that data has reached the S-GW for the S5/S8 part of a parked EPS Bearer.
In the event that it is required to call the idle user (that is an unconnected UE that has entered the state of ECM-IDLE) then the MME uses the paging procedure. Without a comparable router to RNC, the EPS paging process is handled through the MME in conjunction with eNBs. If a paging message needs to be sent it is sent to the MME examines the current list of TAs for the target UE, and adds the paging information into the paging messages S1 delivered to all eNBs that have the TAs indicated.