EPS Procedures Overview

1. EPS Registration and Attach Procedures
When the UE enters the LTE coverage or powers up, it first registers with the EPS network
through the “initial EPS attach” procedure [28]. This attach procedure is used to:
• Register the UE for packet services in EPS,
• Establish (at a minimum) a default EPS bearer that a UE could use to send and receive the
user application data,
• Allocate IPv4 and/or IPv6 addresses.
The overview of the attach procedure is illustrated in Figure 1.29.
The attach procedure usually starts when the UE initiates the request. After establishing an
RRC connection, the UE can send an attach request message to the MME. UE also requests
PDN connectivity along with the attach request.
After all necessary signaling connections are established, EPC may trigger security
functions. HSS downloads user subscriber information to the MME, which processes the UE
request for default EPS bearer set-up. After the default EPS bearer and QoS are negotiated
and agreed to among the MME and S-GW/P-GW, the MME forwards the default bearer
set-up request to the eNB and the UE.
The eNB and theUE then acknowledge the default bearer set-up, and communicate the attach
accept messages to the EPC. The EPS bearer is finally active and data can flow between the
UE and the IP network, in both uplink and downlink directions.
At this point, UE typically registers with a default APN, as per the subscription policies. If
additional APN is available, the process needs to continue setting up another EPS bearer.

Signaling Radio Bearer (SRB)
In order for the control plane information messages in EPS to flow between the UE and the
EPC or E-UTRAN, SRBs (signaling radio bearers) are set up at the initial connection request.
Three SRBs are used to transfer RRC and NAS messages to/from the UE:
• SRB ID 0 – used to establish the RRC connection request when the UE has transitioned
into connected mode. SRB0 carries common control information required to establish the
RRC connection.
• SRB ID 1 – used for RRC messages, as well as RRC messages carrying high priority NAS
signaling.
• SRB ID 2 – used for RRC carrying low priority NAS signaling. Prior to its establishment,
low priority signaling is sent on SRB1.
Once the SRBs are established, control plane messages and parameters are sent to the UE
from the EPC and/or E-UTRAN. The UE will adhere to these parameters to continue the
protocol procedures on the AS. The parameters sent to the UE in the SRB messages will
control all protocol layers for the data transmission.

Default EPS Data Radio Bearer (Default DRB)
One of the significant changes introduced in LTE is that when the mobile device connects to
the network it also implicitly gets an IP address. This is called “default EPS bearer activation”
[28]. This concept is different from the conventional 3G system of packet data protocol (PDP)
context activation.
In 3G systems, the mobile registers to the network first. Then, based on downlink or
uplink activities, the IP address allocation procedure starts as part of the “PDP context
activation.” This procedure is referred to in 3G systems as establishing PS data call. The
procedure of PS data call set-up follows the same as that in CS. When the user initiates or
receives a call, the CS, or PS call is established and all resources are then allocated at the call
set-up stage.
With the default bearer activation in LTE, the packet call is established at the same time as
when the UE attaches to the EPS. This is the concept that makes the LTE’s connectivity be
known as “always-on”.
This procedure, opposed to 3G, can provide a significant signaling reduction on the protocol
layers and also improves the end-user experience in terms of data re-activation delays after a
certain period of inactivity. In 3G, when the user disconnects the data call and then re-initiates
a new one, the PDP context activation may start all over again. However, in LTE, if the same
procedure is done by the user, the call set-up time for a data call is reduced because the default
DRB (Data Radio Bearer) has been already assigned to the user when first attached to the
EPS system.

Dedicated EPS Data Radio Bearer (Dedicated DRB)
Even though the default DRB is enough for the downlink and uplink data transfer in an EPS
network, the default bearer comes without any QoS guarantees. For real-time streaming applications,
QoS may be needed, especially on the air interface. Such IP packets associated with
these types of applications may need to be assigned with a higher priority than other packets,
especially when the bandwidth is limited.
To exploit the services differentiation, LTE has also introduced another EPS bearer known
as a “dedicated EPS data bearer” which is initiated for an additional data radio bearer [28].
The dedicated bearer becomes important in order to support different types of applications
in EPS network. Dedicated DRB can be set up right after default DRB in the procedures shown
in Figure 1.29.

2 EPS Quality of Service (QoS)
In order to support a mixture of non-real-time and real-time applications, such as voice
and multimedia, the delay and jitter may become excessive if the flows of traffic are not
coordinated. Packet Switches should be able to classify, schedule, and forward traffic based
on the destination address, as well as the type of media being transported. This becomes
possible with QoS-aware systems.
The QoS for data radio bearers is provided to the eNB by the MME using the standardized
QoS attributes. Based on these configured attributes by the EPS, the protocol layers between
the UE and eNB can manage the ongoing scheduling of uplink and downlink traffic.
Various parameters are used to control and identify the QoS. The overall QoS parameters
are shown in Figure 1.30.



EPS Bearer QoS
EPS bearer QoS depends on the resource type; either guaranteed bit rate (GBR) or
non-guaranteed bit rate (non-GBR). The default DRB is always set up as a non-GBR. A
dedicated DRB can be either GBR or non-GBR [29].
As illustrated in Figure 1.30, the GBR-based EPS bearer consists of two distinct parameters;
GBR and MBR. The GBR indicates the bit rate that can be expected to be provided by a
GBR-based bearer, while the MBR limits the bit rate that can be expected to be provided by
this EPS bearer.
The GBR-based QoS parameters provide the eNB with information on the uplink and downlink
rates for an E-RAB. E-RAB transports the packets of an EPS bearer between the UE
and the EPC based on these QoS parameters indicating the E-RAB’s maximum downlink bit
rate, maximum uplink bit rate, guaranteed downlink bit rate, and E-RAB’s guaranteed uplink
bit rate.
Non-guaranteed EPS bearers are subject to control through an AMBR (aggregate maximum
bit rate). TheAMBRapplies to both the subscriber and the APN associated with the subscriber,
and is defined as follows:
• UE-AMBR – value applies to the total bit rate that can be allocated to a subscriber for all its
non-GBR services. The UE-AMBR limits the aggregate bit rate across all non-GBR bearers
of a UE (excess traffic may get discarded by a rate-shaping function).
• APN-AMBR – value applies to the total bit rate that can be allocated to the subset of a
subscriber’s services associated with a particular APN. The APN-AMBR limits the aggregate
bit rate across all non-GBR bearers and across all PDN connections of the same APN
(excess traffic may get discarded).
Similar to GBR-based QoS, the non-GBR parameters have uplink and downlink components.

 ARP and QCI
The ARP (allocation and retention priority) controls the priority in bearer establishment, modification,
or bearer release if resources are limited. In addition, it may be used to indicate which
bearers are dropped when there is congestion in the network. This parameter can be used for
GBR or non-GBR QoS.
The priority level of an ARP ranges from 0 to 15. The value 15 means “no priority,” whereas
the value 1 is the highest level of priority, with the value 0 being reserved. In addition, ARP
provides preemption capability on other E-RABs. This indicates whether the E-RAB will not
preempt other E-RABs or the E-RAB may preempt other E-RABs.
QCI (QoS class indicator) is another common QoS parameter in both GBR and non-GBR
EPS bearers. It provides a mapping from an integer value to specific QoS parameters that
controls how bearer level packets are forwarded.
QCI controls the packet forwarding, such as scheduling weights, admission thresholds,
queue management thresholds, and link layer protocol configuration. QCI values for an
E-RAB are typically pre-configured by the operator. QCI are categorized into nine different
indicators, as shown in Table 1.7 [29].