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LTE Network Architecture

3 Mar

The high-level network architecture of LTE is comprised of following three main components:

  • The User Equipment (UE).
  • The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
  • The Evolved Packet Core (EPC).

The evolved packet core communicates with packet data networks in the outside world such as the internet, private corporate networks or the IP multimedia subsystem. The interfaces between the different parts of the system are denoted Uu, S1 and SGi as shown below:
LTE Architecture

The User Equipment (UE)

The internal architecture of the user equipment for LTE is identical to the one used by UMTS and GSM which is actually a Mobile Equipment (ME). The mobile equipment comprised of the following important modules:

  • Mobile Termination (MT) : This handles all the communication functions.
  • Terminal Equipment (TE) : This terminates the data streams.
  • Universal Integrated Circuit Card (UICC) : This is also known as the SIM card for LTE equipments. It runs an application known as the Universal Subscriber Identity Module (USIM).

A USIM stores user-specific data very similar to 3G SIM card. This keeps information about the user’s phone number, home network identity and security keys etc.

The E-UTRAN (The access network)

The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN) has been illustrated below.
LTE E-UTRANThe E-UTRAN handles the radio communications between the mobile and the evolved packet core and just has one component, the evolved base stations, called eNodeB or eNB. Each eNB is a base station that controls the mobiles in one or more cells. The base station that is communicating with a mobile is known as its serving eNB.
LTE Mobile communicates with just one base station and one cell at a time and there are following two main functions supported by eNB:

  • The eBN sends and receives radio transmissions to all the mobiles using the analogue and digital signal processing functions of the LTE air interface.
  • The eNB controls the low-level operation of all its mobiles, by sending them signalling messages such as handover commands.

Each eBN connects with the EPC by means of the S1 interface and it can also be connected to nearby base stations by the X2 interface, which is mainly used for signalling and packet forwarding during handover.
A home eNB (HeNB) is a base station that has been purchased by a user to provide femtocell coverage within the home. A home eNB belongs to a closed subscriber group (CSG) and can only be accessed by mobiles with a USIM that also belongs to the closed subscriber group.

The Evolved Packet Core (EPC) (The core network)

The architecture of Evolved Packet Core (EPC) has been illustrated below. There are few more components which have not been shown in the diagram to keep it simple. These components are like the Earthquake and Tsunami Warning System (ETWS), the Equipment Identity Register (EIR) and Policy Control and Charging Rules Function (PCRF).
LTE EPCBelow is a brief description of each of the components shown in the above architecture:

  • The Home Subscriber Server (HSS) component has been carried forward from UMTS and GSM and is a central database that contains information about all the network operator’s subscribers.
  • The Packet Data Network (PDN) Gateway (P-GW) communicates with the outside world ie. packet data networks PDN, using SGi interface. Each packet data network is identified by an access point name (APN). The PDN gateway has the same role as the GPRS support node (GGSN) and the serving GPRS support node (SGSN) with UMTS and GSM.
  • The serving gateway (S-GW) acts as a router, and forwards data between the base station and the PDN gateway.
  • The mobility management entity (MME) controls the high-level operation of the mobile by means of signalling messages and Home Subscriber Server (HSS).
  • The Policy Control and Charging Rules Function (PCRF) is a component which is not shown in the above diagram but it is responsible for policy control decision-making, as well as for controlling the flow-based charging functionalities in the Policy Control Enforcement Function (PCEF), which resides in the P-GW.

The interface between the serving and PDN gateways is known as S5/S8. This has two slightly different implementations, namely S5 if the two devices are in the same network, and S8 if they are in different networks.

Functional split between the E-UTRAN and the EPC

Following diagram shows the functional split between the E-UTRAN and the EPC for an LTE network:

2G/3G Versus LTE

Following table compares various important Network Elements & Signaling protocols used in 2G/3G abd LTE.

DiameterGTPc-v0 and v1 GTPc-v2


LTE S1-interface handover between eNodeBs

20 Feb

LTE networks prefer using the X2 interface for performing inter eNodeB handovers. An S1 handover is a fallback for scenarios where X2 interface is not available.

As the name suggests, S1 handovers take place over the S1-interface. The MME and the SGW are involved during the handover procedure.

An interesting part of LTE S1 handovers is the indirect tunnel that is established to carry the downlink data during the handover process. Refer to the S1 handover call flow for a detailed signaling flow.

Inter eNodeB S1 handover in LTE



LTE EPS Architecture Overview

30 Dec

Excellent video describing the Evolved Packet Core used in LTE networks. The video describes the roles played by the major LTE components:

  • eNodeB
  • MME
  • S-GW
  • P-GW
  • HSS

LTE Topology with X2

9 Dec
What is X2?

In all cellular networks, base stations cooperate to provide certain services to subscribers. Think abouthandover. As you ride in your car talking on your mobile phone (as a passenger, of course), you will eventually pass out of the coverage of your current base station. In order not to lose the call, your base station will cooperate with the rest of the cellular network to automatically find the next base station that can pick up your conversation. This handover from base station to base station is performed so seamlessly that you don’t even notice.




In 2G and 3G networks, handover often requires the involvement of the network core. In the 2G case, when handover is between base stations that are not under the same BSC, the core gets involved. In the 3G case (pictured above-left), RNCs can handle the handover, but when they do not interface with each other, the handover will be processed at the core.

In LTE, the handover intelligence does not necessarily reside in the core. LTE’s intelligent eNodeBs (base stations, in LTE parlance) can execute handovers themselves without the core’s direct involvement. This method is far more efficient and subscriber-friendly as it reduces the considerable network traffic flow from the base stations to the core and the consequential latency. With X2 protocol running between eNodeBs themselves (picture above-right), handover takes less time and requires less network traffic.

The benefits of the X2 interface are not limited to handovers. With the distributed intelligence and X2 communications between them, eNodeBs, can perform load management between themselves (“Hey, I am suddenly busy. Here, you take over this function for me, okay?”). They can report statuses and errors, and perform other cooperative functions as well.



Network Topology Effects

With X2 traffic running between eNodeBs, the network topology changes. Traditional (2G and 3G) cellular networks are built in a kind of hierarchy or tree. Base stations are the leaves while the backhaul network, leading to aggregation points, represents the twigs. Aggregation points are the places where the twigs connect to larger twigs and eventually to the limbs. These limbs eventually combine and connect up to the trunk, the backbone, that leads to the roots, the core network. Information for decision-making flows from the leaves to the trunk where the decisions are made, and commands for execution flow back the other way.

That’s an awful lot of network traffic and time, isn’t it?

In the LTE world, this end-to-end flow gives way to a flatter network where more of the intelligence is distributed around the network so that decisions—like our handover example—can take place way up at the level of the leaves. In this way, traffic that used to flow end-to-end now engages only the eNodeBs who, in many cases, can make their own decisions locally.

In order for X2 to work in all of its glory, eNodeBs must have the means to communicate with each other. As additional LTE advantages require X2 to be as fast as possible (check out ICIC and CoMP for two such examples), implementing X2 with as little latency as possible is mandatory.

There are three methods for implementing X2.  In the direct method, eNodeBs can be connected via fiber or wireless in E/V-band or licensed-band. The direct method is the best since it is the most direct and subject to the least latency. Due to expense, distance and other reasons, however, many network operators have so far shied away from implementing such direct links between eNodeBs.




There are two other ways to implement X2. In a “fast” X2 implementation, eNodeBs communicate with each other via their BSC or RNC which acts as a router for this purpose. This cuts deployment cost considerably, but introduces more latency in the arrangement.

In a “slow” implementation, the X2 messaging can travel all the way back to an aggregation point or even the core. Slow X2 implementation is relatively straightforward since this is similar to the way it works in 3G networks.  However, as its name indicates, the “slow” implementation is fraught with the highest latency.


HetNet Opportunities

With the distribution of intelligence in their networks all the way to the endpoints (leaves) and a method for the endpoints to communicate and make decisions between themselves, operators can consider many new sorts of deployments. Suddenly, efficient small cell deployments are possible. Think about a macro base station (eNodeB) on the roof of a building with lots of small cells under its control deployed at the street level below every 50-100 meters. Subscribers can walk down the street, smartphones in hand, with uninterrupted coverage while their calls and Internet sessions continue, bouncing, in a very coordinated way, among the small cells and between the small cells and the base station.

X2 is the eNodeB communication protocol for all of that to occur.


LTE Layer 2 user plane protocol stack in detail

19 Sep

LTE user Layer 2 Protocol stack-the plan is composed of three substrates, as shown in the figure.

  1. Packet layer packet data convergence Protocol (PDCP) : this layer processes messages resource control (RRC) in the control plan and packages of Internet Protocol (IP) addresses in the user’s plan. According to the spokesman, the main functions of the PDCP are header compression, security (encryption and integrity protection) and support for reordering and retransmission during teaching. For radio bearers which are configured to use the PDCP, there is only one entity per PDCP radio spokesman.
  2. The Radio Link Control (RLC) : the main functions of the layer are segmentation and reassembly of RLC top layer of packages in order to adapt them to the size that can be effectively transmitted over the radio interface. For radio bearers which are in need of transmission errors, the RLC is relayed to discover how to recover from packet losses. In addition, the RLC Reordering performed to compensate for out-of-order Receipt due to hybrid automatic repeat request (HARQ) Layer below. There is only one entity per RLC radio spokesman.
  3. Medium-MAC (Access Control) layer: This layer performs the multiplexing of data for a variety of radio carriers. Therefore, it is not only one of the MAC for the UE. Determining the amount of data that can be transmitted from each radio bearer layer, the size of the packet, RLC and instructing on the MAC layer to achieve negotiated Quality of Service (QoS) for each radio bearer.Uplink, this process involves communication of the amount of data to transfer the eNodeB.

overview of user-plane architecture

On the sending side, each layer receives a data service unit (SDU) from the higher layers, layer provides services and outputs the Protocol data unit (PDU) to the lower layer. A layer of RLC receives packets from PDCP layers. These packages are called PDUs with the PDCP point of view, and represent the PDCP RLC SDUs perspective, RLC. A layer of RLC creates packages that are scheduled for the layer below, i.e., the MAC layer.

Packets are the RLC RLC MAC layer PDUs from the RLC SDUs and MAC MAC perspective. On the receiving side, facing the process, with each layer by layer to the SDUs above, if they are received as PDUs.

An important feature of the LTE protocol stack is that all PDUs and SDUs byte aligned. This is To facilitate handling of microprocessors, which are usually defined to handle packages in units of bytes. To further reduce the processing requirements of the user plane Protocol Fund in LTE, the head, who created each of the PDCP, RLC and MAC layer are also byte-aligned. This means that sometimes the unused padding bits are needed in the minds, and thus the cost of design for effective treatment is that it is spent small amounts of potentially available capacity.


Alternatives for Increasing Capacity Coverage

2 Aug

Today we’re discussing a relatively old, yet new, subject which still creates industry debates on its applicability for the majority of the world’s networks. What I would like to do before we dive into the fronthaul definitions, is to take a step back and discuss the relevant alternatives for increasing capacity coverage in a modern mobile network and its impact on operations, access spectrum utilization and the back or front haul – or in short – hauling segment.  Obviously, the main industry discussion is around small cells. When we’re talking about a small cell environment we should expect to have the following four main types which will likely coexist together in the network in the near future:   

The Offload Model – Either WiFi or femto based, running off net on any broadband connection connected to a gateway somewhere in the network where it joins back to the mobile network

The Integrated Model – The integrated model is just as a regular small cell that everybody is referring to.  Basically a macro-cell functionality, but with lower power.  It’s a relatively low cost device.  It’s not really tiny but it also comes with all the requirements from  an LTE-eNodeB.  Meaning all the requirements in timing (1588 etc), capacity (>100 Mbps) and latency (<10 Msec e-t-e) are there.

Small Cells Defined

The Coordinated Model – Coordinated is something that does not exist today.  It will arrive in a year or two with arrival of LTE-Advanced (3GPP rel 10 and 11).  With, eICIC – Enhanced inter cell interface cancellation,  Base stations have higher requirements.  And when we talk about higher requirements it’s higher requirements in capacity and in latency. As these eNodeBs need to coordinate in real time with their peers we will need to target 1-5 m/sec instead of 10msec.  The desired benefit to operators is to only use three Coordinated small cells for every macro compared to ten regular small cells expected ratio and still achieve the same capacity coverage targets.

The Distributed Model – Distributed macro cells takes small cells to a whole different level.  Instead of deploying those independent small cells, operators can  drop radio units or micro/Pico radio units.

The basic idea is to deploy more sectors – instead of three or six, why not 20? Much of that will be for indoor coverage but will also function for outdoor use. The savings can be found in real estate used and truck rolls as radio units are simpler and smaller when compared to the costs of running a complete base station. The way we connect base band units or as we call them, digital units (DU) with the remote radio head or unit (RU) using a  carrying Common Public Radio Interface (CPRI). Originally planned to carry traffic between the base of the tower and the top over fiber for a very short distance, it is easy to see you can extend the distance to few miles easily either using available fiber assets or high capacity wireless solutions.

This connectivity is called fronthaul and we will dedicate more time trying to understand the relation with C-RAN and weigh the alternatives.


Evolved Packet Core Architecture and network element Functionality

24 Jul
In this post and next one, I try to discuss a little on the Evolved Packet Core (EPC) Architecture, functionalities and interfaces. For this post I will focus on the Architecture and functionality of EPC network elements. My literary references are MTN Academy, Telecom Academy documentations,  and a book which is called “SAE and the Evolved Packet Core”.

A key difference from 2G/3G Core networks is that the EPC is defined to support packet-switched traffic only with IP based protocols’ Interfaces. It means that all services will be delivered through packet connections, including voice; and it means saving for operators, because they use a single network for all services.

There are two principle functionalities in the Evolved Packet Core, one is Packet routing and transfer functions, the other one is mobility management. These functionalities are applied in below main network elements.

  • SGW – Serving Gateway; router, packet marking, anchor for inter-eNodeB handover, some accounting
  • MME – Mobility Management Entity; NAS signalling point, admission control, bearer setup, authentication, roaming functions, selects SGW
  • P-GW – Packet Gateway; date entry/exit point, packet inspection/filtering, IP address allocation, mobility anchor for non-3GPP handover

EPC Network Architecture

There are several functional entities within the core Evolved Core Network. Within the user plane the core network is the gateway between the access network and the PDNs (e.g., the Internet) that support the interfaces, mobility needs and the differentiation of QoS flows. The gateway may be split in two separate nodes with an optional S5 interface. The two logical gateway entities are the Serving Gateway (S-GW) and the PDN Gateway (P-GW).

The Serving Gateway (S-GW) acts as a local mobility anchor, forwarding and receiving packets to and from the eNodeB currently serving the UE.

The PDN Gateway (P-GW) interfaces with the external PDNs, such as the Internet and IMS. It is also responsible for several IP functions, such as address allocation, policy enforcement, packet classification and routing, and it provides mobility anchoring for non-3GPP access networks.

The control plane functions are performed by the MME which is connected to the gateway via the S11 interface.

The PCRF makes policy decisions based on information obtained via the Rx interface. It confirms that the information received is consistent with policies defined in the PCRF. The PCRF will authorize QoS resources and will decide if new resources are required for existing connections. PCRF mechanism is used also in 3G network.

HSS (Home Subscriber Server)

The HSS has a combination function of the HLR (Home Location Register) and the AuC (Authentication Centre).


The SGSN enables:

  • Inter EPC node signalling for mobility between 2G/3G and E-UTRAN 3GPP access networks;
  • PDN and Serving GW selection: the selection of S GW/P GW by the SGSN is as specified for the MME;
  • MME selection for handovers to E-UTRAN 3GPP access network.

Serving Gateway (S-GW)

The S-GW terminates the interface towards the E-UTRAN and is the main packet routing and forwarding node in the EPC. It is able to provide transport level packet marking in the uplink and downlink by setting the Code Point dependent upon the QoS class identifier  (QCI) of the associated EPS bearer which may be used for QoS management by other network elements. The S-GW provides accounting functions based on the user and QCI inter-operator charging and uplink and downlink charging per UE, PDN, and QCI for roaming within home routed traffic. The S-GW acts as a local anchoring point for inter eNodeB handover and assists in the reordering function by sending one or more “end marker” packets to the source eNodeB immediately after switching the path. The S-GW acts as a 3GPP anchoring point for inter-RAT handovers by providing the termination point for the S4 interface and relaying traffic between 2G/3G systems and the PDN-GW (P-GW). It provides idle mode functions such as packet buffering and initiation of network triggered service request.

The S-GW is one of the Lawful Interception points in the network. There are a number of interfaces associated with the S-GW including:

  • S11 interface that connects to the MME
  • S1-Uinterface connection to the eNodeB
  • The interface between the S-GW and P-GW is the S5/S8 interface.

Packet Data Network (PDN) Gateway (P-GW)

The PDN Gateway is the link between the mobile device and the services that reside in an external packet network such as IMS. The P-GW provides an entry and exit point for UE connectivity with external data networks and terminates the SGi interface towards the PDN and acts as a mobility anchor between 3GPP and non-3GPP technologies such as 3GPP2 CDMA2000 and WiMAX. The P-GW is also responsible for the allocation of user IP-address. The P-GW also provides support for charging, packet filtering and lawful interception.

There are a number of interfaces associated with the P-GW including:

  • SGi
  • Gx
  • S5

Mobility Management Entity (MME)

The Mobility Management Entity (MME) is the primary signalling node in the EPC, Non Access Stratum (NAS) signalling is terminated at this point and responsible for signalling related to bearer establishment and authentication of the UEs through interaction with the Home Subscriber Server (HSS). It is also the decision point for SGW selection, and MME, SGW selection during handover where EPC node change is necessary. The MME handles roaming functions such as allocation of temporary identities, admission control and communication with the home HSS on the S6a interface. There are a number of interfaces associated with the MME including:

  • S1_MME
  • S3
  • S6a
  • S11
  • S10



  1. MTN Academy
  2. Telecoms Academy
  3. SAE and the Evolved Packet Core, Author(s): Magnus Olsson, Shabnam Sultana, Stefan Rommer, Lars Frid and Catherine Mulligan, ISBN: 978-0-12-374826-3


Design and Development of Medium Access Control Scheduler in LTE eNodeB

11 May

Long Term Evolution (LTE) is a major step in mobile radio communications, and is beyond

3G systems and is the next generation cellular system of 3GPP .3GPP’s Long Term Evolution isdefined by the standardization body’s Release 8. LTE uses OFDMA and SC
-FDMA as its radio accesstechnology with advanced antenna technologies such as Multi-Input Multi-Output (MIMO), for bothdownlink and uplink. LTE is a system with complex hardware and software. In case of Long Term Evolution (LTE), the scheduler in the Medium Access Control (MAC) layer of the eNodeB allocates theavailable radio resources among different UEs in a cell through proper handling. LTE schedulers are part of layer 2 protocol stack and are one such module which can dramatically increase or decreasethe performance of the system. In this paper, we are presenting various types of scheduling schemes of  LTE and their advantages. The output conditions such as memory usage and execution time for varying number of users are investigated for three of the scheduling methods: Proportional Fair (PF), Modified-Largest Weighted Delay First (MLWDF) and EXP-Proportional Fair (EXP-PF) scheduling algorithm. Developed algorithms are tested for single-cell/multi-cell with multiple-user scenarios inboth TDD/FDD frame structure.




Telecom Whitepaper Radio Resource Management Radio Admission Control Radio Bearer Control 0113 1

5 Mar

Radio Resource Management- Radio Admission Controland Bearer Control

Radio Resource Management (RRM) is an E-UTRANNode B (eNodeB) application level function thatensures the efficient use of available radio resources.RRM manages the assignment, re-assignment andrelease of radio resources, taking into account singleand multi-cell aspects.Radio Admission Control (RAC) is a sub-function of RRM. The task of RAC is to admit or reject theestablishment requests for new radio bearers. Theestablishment of a bearer is based on the outcome of the RAC Algorithms.Radio Bearer Control (RBC) is also another sub-functionof RRM. The establishment, maintenance and release of Radio Bearers involve the configuration of radioresources associated with them. It is based on theoutcome of RBC Algorithm. This paper primarily focuses on the admission,establishment and maintenance of radio bearers. Wediscuss a strategy for RAC and RBC, including Quality of Service (QoS) requirements, priority levels andprovided QoS of in-progress sessions and QoSrequirements of the new radio bearer request.
White Paper



LTE eNodeB MAC Scheduler Interface

26 Nov


 An interface which enables smart MAC Scheduler solutions to be easily integrated with third party core MAC implementations leading to improved user experience and system performance.
The MAC Scheduler within the eNodeB MAC sublayer determines how the downlink anduplink channels in the LTE air interface are used. Resources are assigned to UEs enabling them to receive data via the downlink and transmit data via the uplink. The scheduler assigns theseresources in such a way as to satisfy QoS requirements and optimise system performance. TheeNodeB MAC Scheduler design and functionality is largely vendor specic. Different schedulersmay result in signicantly different levels of user and system performance. An open interfacebetween the MAC Scheduler and the rest of the eNodeB MAC sublayer enables equipmentvendors, operators and users to benet from advanced, third-party MAC Scheduler solutions.This white paper provides an overview of the LTE eNodeB MAC Scheduler interface denedby Roke.
The MAC sublayers in the LTE base station (eNodeB) and user devices (UE) are responsiblefor multiplexing the user-plane and control-plane trafc over the air interface. Each UE may have multiple active bearers – logical connections within the LTE network. Each bearer is used to transfer user-plane data with a given quality of service (QoS). The MAC sublayers within theeNodeB and UE are responsible for constructing, sending, receiving and processing transportblocks which contain a combination of user-plane data from one or more bearers, control-plane data and MAC control messages.The MAC Scheduler can be viewed as the L2 ‘brains’ within the eNodeB whilst the rest of  the MAC sublayer performs the ‘heavy lifting’ associated with multiplexing, transport block processing, HARQ, subframe construction, etc. Two eNodeBs with the same core MACsublayer can exhibit signicantly different levels of user and system performance as a result of  their MAC Scheduler implementations. These differences can include user throughput, cellcapacity, QoS and UE power consumption.


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