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Scalability Problems of the mobile wireless networks:

Introduction:

The above evolutionary steps in the architecture of LTE-A resulted in that the radio access networks

became flatten to one single node (i.e., the eNodB).

The flat nature of this architecture focusing on implementing the certain control functions (e.g., the radio

resource control function) at the edge of the networks (e.g., the base stations) concerns the control plane,

but not the user plane. On the other hand, the exponential increase in mobile IP data usage, caused by the

higher data rates that LTE-Advanced offers, results in potentially increasing the data capacity demands

on the core network (i.e. EPC) [Ref: Data offloading techniques in 3GPP Rel-10 Networks: A tutorial,

C.B.Sankaran, IEEE communication magazine, 2012] [Ref: Flat Architectures: Towards Scalable Future

Internet Mobility, Lazlo BOjer, et al., SpringerLink, Vol. 16656, 2011, PP: 35-50 ].

To solve this scalability problem, the 3GPP introduced three categories of architecture based on the

breakout point in release 10 in order to offload and route the certain IP traffic from the packet data

gateway (P-GW) in the operator’s core network (CN) to a local area IP network without traversing the

CN: LIPA (Local Access IP), STPO(Selected Traffic offload), and IP flow mobility functionalities.

The ability to offer these features in the architecture means that without having to significant

investment on upgrading the network structure, the operators can increase the revenue by way of

added-value services to cope with higher throughput and speed that applications such as data transfer

and video streaming would demand.

LIPA:

Local access IP (LIPA) primarily designed for IP-enabled UEs to directly access their local network or

intranet, as well as the visited network, through a local 3GPP access point (HeNB) without involving the

core network. In other words, when a user has the femtocell at home or in the office, mobile devices over

the femtocell (HeNBs) can use LIPA to access other devices or other IP capable entities connecting to

the local network such as TVs, videos, local web servers. The use of LIPA interconnect mobile stations

and local residential/enterprise IP networks called local networks, thus it can improve the user experience

by offering faster and more secure data transfer for high speed applications such as file transfer, video

streaming, and device sharing, since the data would not transvers the mobile core network, as well as the

simultaneous IP access to the Internet from both the local network and the cellular network would be

possible for the user. [Ref: Book: LTE-Advanced: A Practical systems Approach to Understanding the

3GPP LTE Release 10 and 11 Radio Access Technologies, Sassan Ahmadi, Academic Press, 2014].

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LTE-A release 10 supports the UE access to the local network only through the HeNB as the access

medium. The HeNB decides whether to route the data through the core network or to use LIPA to the

local network.

The functionality of LIPA has being extended in release 12 to allow access to the local network when a

UE is in the coverage area of macro-cell (i.e. macro-eNB) to support mobility. The local network does

not require to be connected to the Internet but it can discover and achieve IP connectivity with the UE

via one or more HeNBs of the mobile operator network.

SIPTO:

Selected IP traffic offload is a method in which a certain type of IP traffic on the operator mobile network

is offloaded to a local area network in order to reduce the load on the system. The initial objective is to

add capacity to the primary or the master radio access and the core network path to allow coat-optimized

traffic transport.

SIPTO can be activated in a HeNB. In this case, specific traffic, determined by the operator’s policy or

user subscription, is transferred to/from eNB directly to Internet/intranet with bypassing the core network

(i.e. EPC). For instance, an operator may decide to offload best-effort services to a local network while

still supporting VOIP calls over its core network. Alternatively, SIPTO can be implemented in the

operator’s core network (EPC) to offload traffic to another gateway or node in the network which is

geographically/topologically closer to the UE. Thus, the target MME, which has the UE context, trace,

and detect the UE mobility during the tacking area update procedure, may redirect a packet data

connection (PDN) towards a different gateway that is more appropriate for the UE’s current location. In

other words, SIPTO can be triggered (i.e. activated or deactivated) by events like UE mobility or other

occasions which are more related to traffic changes. As a result, mobility and service continuity is

possible when the UEs move between different eNBs.

Network architecture variants and network elements for LIPA and SIPTO:

3GPP has proposed two types of breakout architectures for traffic offloading [REF: 3GPP R 23.829:

“Local IP Access and Selected IP Traffic Offload (LIPA-SIPTO)]. A breakpoint in the settings of

LIPA/STOP is the location in the architecture where the offload of data traffic flow (i.e. EPS bearers)

occurs. The breakpoint can be at the local private network (LIPA and femto STPO cases) or at/above

radio access network ( macro SIPTO and some femto SIPTO cases).

Alternative one: “Architectures with breakpoint at the private/ enterprise/residential IP network”.

In this type of breakpoint, the operator’s core network is bypassed and the user plane (i.e. data traffic) is

routed via a local packet data network gateway (L-GW). The L-GW logically is co-located with the

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HeNB and it connects to the local IP network devices and the public network directly using the SGi

interface. The L-GW also communicates with the serving gateway (S-GW) of the core network through

the S5 interface.

The HeNB assigns an IP address to the L-GW and it is possible to assign the same IP address as that of

the HeNB to the L-GW.

The core network is still responsible for registration signaling and session management procedure to

establish the local IP packet data network connection, however, when the connection is set up the LIPA

/ SIPTO functionalities are activated in the L-GW, and the L-GW starts implementing some S-GW and

P-GW functions such as allocating IP address for the UE, packet filtering and buffering downlink data,

and it directs the data path.

Alternative two:” Architecture with breakpoint at/above the radio access network”.

In this alternative breakpoint may be still remained in the operator’s core network. And the offload

mechanism can be implemented in the two forms:

- Bypassing one of the core network nodes, thereby reducing the number of hops on the data path.

In this scenario gateway selection function uses the UE’s location information to route the certain

type of traffic at a network node through the Local PDN-GW (LP-GW) to the radio access

network (RAN) which is closer to the UE current location.

- Selecting a gateway pair (S-GW and P-GW) close to the UE’s point of attachment, so choosing

the most optimal path. In this scenario, both HeNB and L-GW are at the local/

residential/enterprise IP network, but in separate nodes. The UE is able to access a defined IP

SGi

SGi

S5

S-11

S5

S1-u Uu (H)eNB UE

L-GW

S-GW P-GW

MME

Core

Network

RAN

SIP to offload data path through L-GW

Regular data path through P-GW

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network (e.g., Internet) through a (H)eNB without the data traffic traversing the operator’s core

network.

This technique can be used in the future smart home and smart vehicle to reduce the traffic load from the

operator’s IP core network

Regular data traffic through the core network

Uu

S1-U S5

S11 L-S5

UE (H)eNB

L-GW

S-GW

MME

P-GW

Core Network RAN

LIPA/SIPTO Traffic