An introduction to the LTE MAC Scheduler

LTE brought a completely new network architecture and managed to revolutionize the data capabilities ever achieved on a mobile network. LTE also brought a new type of radio network, much simpler in its organization. In a previous post we discussed about OFDM being the main reason behind LTE’s high data speed. Today we look into an essential component of the LTE radio network: the MAC Scheduler.

Sitting just above the Physical layer, the MAC Scheduler assigns bandwidth resources to user equipment and is responsible for deciding on how uplink and downlink channels are used by the eNodeB and the UEs of a cell. It also enforces the necessary Quality of Service for UE connections. QoS is a set of rules that come from the Policy and Charging Rules Function (PCRF) in the core network. These rules define priority, bit rate and latency requirements for different connections to the UE. They is usually based on the types of applications using the UE connection. For example, the QoS requirements for a VoLTE call are different from those for checking the e-mail.

As seen in the image below, the MAC scheduler has control over the OFDM modulation in the sense that it decides, according to information received from other LTE network components, how much bandwidth each UE receives at any given moment. In this figure, the resource element (sub-carrier) is represented on the frequency axis, while the sub-frames are represented on the time axis.

mac_scheduler1This figure shows downlink scheduling, but the MAC Scheduler controls uplink scheduling in a similar way.

In order to take its resource allocation decisions, the MAC Scheduler receives information such as:

  • QoS data from the PCRF: minimum guaranteed bandwidth, maximum allowed bandwidth, packet loss rates, relative priority of users, etc.
  • messages from the UEs regarding the radio channel quality, the strength or weakness of the signal, etc.
  • measurements from the radio receiver regarding radio channel quality, noise and interference, etc.
  • buffer status from the upper layers about how much data is queued up waiting for transmission

mac_scheduler2

Typically, a MAC Scheduler can be programmed to support one scheduling algorithm with many parameters.

Here are some examples of scheduling algorithms:

  • Round Robin – used for testing purposes and uses equal bandwidth for all UEs without accounting for channel conditions
  • Proportional Fairness – tries to balance between the QoS priorities and total throughput, usually preferred in commercial networks
  • Scheduling for Delay-Limited Capacity –  guarantees that the MAC Scheduler will always prioritize applications with specific latency requirements
  • Maximum C/I – guarantees that the Mac Scheduler will always assign resource blocks to the UE with the best channel quality

One of the key features of LTE is the ability to control and prioritize bandwidth across users. It is the MAC scheduler that gives LTE this capability.

IoT management at the network’s edge

IoT has enabled users to access control over a multitude of “smart” devices while also unlocking unlimited possibilities for operators in new markets, such as farming, utilities and transportation. A Gartner study claimed that by 2020 there will be around 26 billion IoT connected devices. Imagine the data they collect and the necessary technology required to process it.

Until recently, cloud computing was the answer for storing and processing data collections from IoT applications. However, despite being a cost-effective model in appearance, the handling data in a centralized cloud site is facing new capacity, data management and security challenges. Analysts at Gartner have also raised the alarm on the inefficiency, from both a technical and economical standpoint, of sending all of the gathered data to a single site for processing.

Fog Computing is a new technical solution that allows data to be aggregated in larger number of smaller remote data centers for the initial analysis, and only afterwards sent for storage into the cloud. The term “Fog Computing” is recent and refers to a technology that is an extension of cloud computing. It’s main characteristics are: the geographical distribution of a large number of processing nodes (application servers), its extended mobility, a low latency and location recognition, wireless access and the predominance of real-time applications.

Fog Computing is a virtualized layer between the IoT devices and the conventional data centers in the cloud, that delivers processing, networking and storage services. It is also known as edge computing, because it is usually located at the edge of the network. It allows for a new set of applications and services solely dedicated to routing, managing and analyzing IoT data, relieving data centers from processing and storing the large volume of measurements collected from IoT devices and sensors.

This is where our SDMN YateBTS-powered solution responds to the current needs of IoT data management. To deploy a fully functional Fog Computing ecosystem, operators can install scalable application servers distributed in each cell site for data analysis and monitoring, without the traffic cluttering the core network. They are geographically distributed and connect to each other to perform a “close to the ground” intermediary layer between IoT devices and the cloud, providing security, low latency and high resilience.

SatSite base stations can redirect the traffic locally to the application server, based on the IMSI specific to the device.

fog_computing

Main features:

  • geographical distribution – Fog Computing nodes in application servers are located in each cell site and cover a wide portion of the field.
  • large number of nodes – closely connected to the geographical distribution
  • real-time connectivity – all the Fog application servers communicate directly with the SatSites located in their proximity, ensuring that they interact with client devices without passing through the core network for each IP data session

Our simplified mobile network architecture allows an easy deployment of Fog platforms to deliver real-time analytics, localization services and resilient applications. It reduces the processing burden in cloud data centers without overcharging the core network, making it ideal solution for IoT networks.

Extending LTE networks the easy way

We’ve often stated that YateUCN, our unified 2G/4G core network solution, can be used to extend existing LTE networks or upgrade GSM deployments to 4G LTE. And that’s correct. In this post we will take a closer look at how that happens and why YateUCN is more profitable than current solutions for operators moving towards LTE networks.

YateUCN is designed as a unified equipment that replaces all the functions performed by separate hardware components with one software application running on commodity hardware. This has the advantages of reducing the infrastructure costs, minimizing the equipment’s time to market, and increasing network resiliency due to a simplified management of software.

Let’s look at two scenarios where YateUCN can be integrated in existing networks.

Extend 4G LTE networks

For operators looking to increase access to 4G services YateUCN is a flexible, cost-effective solution. It drastically reduces initial equipment investment, allowing them to roll out more networks in a shorter time, to better serve growing consumer needs.

This can be done easily because YateUCN integrates all the LTE-specific functions and protocols, so that it interconnects with any existing operator setup. Every hardware component in the EPC – the MME, SGW, PGW, PCRF, and PCEF – is replaced with software running on a single piece of equipment.

The MME function handles UEs trying to connect to the network. It is responsible for subscriber authentication and uses S6a interface (Diameter) to connect to the operator’s HSS. The MME is also in charge of mobility management, allowing UEs continuous connectivity and active sessions as they move through the network.

YateUCN is fully compatible with any eNodeB, using S1-AP interface to manage inter-MME handover.

The SGW function allows YateUCN to manage data traffic routing over S1-U interface, ensuring communication between the eNodeB and the PGW, which establishes and maintains the IP session. The PGW interconnects with the charging solution of the operator using Diameter and Radius interface, allowing AAA management for wireless access.

The PCRF in YateUCN maintains QoS levels and charging policies, enabling mobile operators to control bandwidth usage while their subscribers are roaming. The Policy and Control Enforcement Function, PCEF, performs policy enforcement and service data flow detection, making sure the data flow through from the PGW is accessible.

The unified nature of YateUCN leads to large equipment savings, and makes it easy to manage the network capacities with a software upgrade.

Upgrade networks to 4G LTE

2G/3G networks can be upgraded to 4G LTE using YateUCN core network and SatSite for the radio network. A new LTE network with YateUCN and YateENB SatSite significantly reduces overall network roll-out costs. SatSite operating on YateENB is an eNodeB communicating with the MME in YateUCN over S1 interface and with any other eNodeB over X2 interface.

Since YateUCN also unifies all the layers of the GSM/GPRS core alongside the EPC, it also acts as an extension of existing 2G networks, achieved at no costs for additional 2G core equipment.

SatSite can run on YateBTS and YateENB at the same time, so each cell will act as a mixed 2G/4G site. As a result, operators can choose to use SatSite in mixed 2G/4G networks, without needing a new 2G core. What’s more, since in 2G mode YateBTS SatSite unifies both the BTS and the BSC layer, it communicates directly with YateUCN core network, using the SIP/GTP protocols.

The MSC contained in YateUCN allows subscribers to be handed over from the YateUCN – SatSite network to the operator’s current 2G deployment in the case of CS services mobility. Subscribers can roam from the YateUCN/SatSite network to any existing MSC serving the roaming area to ensure voice services continuity.

YateUCN can be integrated in any system already deployed by the operator. Used together with SatSite, it serves to build complete 4G LTE or mixed 2G/LTE networks with a low infrastructure and operations investment, ensuring consumers consistent access to both voice and data services.

Rethinking redundancy: a new approach to core networks

Mobile communications must provide uninterrupted mobile service at all times, but the costs to create network redundancy with current conventional equipment are restrictive. YateUCN unified core is a profitable and flexible solution for redundancy in 2G and 4G mobile networks.

Network redundancy ensures that as technology advances, the capacity of network infrastructures to support more subscribers without blackouts adapts accordingly. YateUCN is a unified core network allowing resiliency in 2G/LTE mobile networks using YateBTS and YateENB SatSite. SatSite acts as a BTS/BSC communicating directly with the MSC/VLR/SGSN/GGSN and EPC in YateUCN.

As a software-defined core solution, YateUCN replaces the heavy, expensive core equipment used in conventional networks with smaller, affordable, and easy-to-manage equipment. It is a software implementation of 2G and LTE core network layers, operating on commodity hardware.

yucn_redund_2015-7-16_draft4.2_pic2

In typical networks, redundancy is achieved by supplying an additional core server for any given core server, causing costs to more than double, since supplementary costs for the configuration of back-up servers add up to the capital expenses.

YateUCN implements the core network functions and protocols in software, enabling any other YateUCN node to take over extra-traffic in case of failure of a node, or if the network capacity needs to be increased.

While conventional MSC/VLR in data centers are limited to serving a given number of BSCs in a defined geographical area, in a YateUCN – SatSite network the base station allows a device to connect to any YateUCN node in the network, irrespectively of the geographical location of the device/BTS and of whether the network is 2G or 4G. A list of available YateUCN units is configured in each YateBTS/YateENB SatSite.

Core equipment is usually designed to allocate specific core network functions (authentication, mobility, call setup, data routing) to separate nodes. Such equipment is heavy due to the large number of components, increases lead time, and requires separate back-up equipment for each node.

YateUCN unifies both GSM and LTE core layers, meaning that a single alternate YateUCN server provides full redundancy for any other server in the network. If a failover should occur in a YateUCN node, a device can register to a different YateUCN, remaining attached to the same base station, as shown below.

yucn_redund_2015-7-15_draft4.1

A new YateUCN is chosen from the list of YateUCN units held in the base station. If a mobile device remains connected to the same BTS, registration to the MSC/VLR in the new YateUCN is performed whenever the device communicates with the network to perform an action. Registration to the new YateUCN is updated in the HSS/HLR.

If the device roams to an area served by a different BTS, they will connect to the new SatSite, but will remain connected to the YateUCN currently serving it, and a new query in the HLR is not required. This reduces the load on the HLR and allows it to support a higher subscriber capacity. This can be seen below:

MS connecting to a new YateUCN

MS connecting to a new YateUCN

Increasing traffic to a YateUCN core server is easily performed because YateUCN communicates with 2G base stations using SIP and GTP, and with eNodeBs over SIP/S1AP/GTP. SIP and GTP signalling protocols have the advantage of scalability and interoperability, allowing different service requirements to be served at the same time and with the same quality standards.

Because YateUCN uses commodity hardware, operation and servicing can be managed remotely, with minimal external support, significantly driving operational costs down. YateUCN provides simplicity and cost-effectiveness to building redundancy in mobile networks so that operators can provide high-quality service at all times.

Software-defined radio for frequency reuse in LTE

The expansion of 4G LTE challenges operators who have limited spectrum; as some decide to take down existing 2G (and even 3G) deployments in favor of 4G, bandwidth allocation in an area must be carefully planned to match the quality requirements of LTE.

In 4G LTE, spectrum is a crucial resource. Performance is dependent on the proximity between the radio network and the devices. The closer the radio tower, the higher the data throughput. This means that the more cell towers operators build, the better they can cover the area.

Frequency reuse is a widely adopted solution for LTE; essentially, a given area is served by more cell towers using the same frequency. An easier and more efficient approach to this is software-defined radio.

Cell edge interference management using YateENB

Cell edge interference management using YateENB

Frequency reuse means splitting an area in several new, smaller cells. In LTE, to maintain a high throughput, the same frequency is allocated to all the new cells, at the expense of higher interference at the cell edges. Since all the new cells have equal power, two or more cells meeting causes interference around the cells edges.

Apart from that, building and maintaining additional infrastructure required by frequency reuse results in high capital and operational expenses.

SDR in the LTE base station (eNodeB) can be a solution to these limitations. The fact that SDR implements the communication protocols in software and uses general-purpose hardware has several benefits.

The most important one is the effect on infrastructure costs. Base stations built on special-purpose hardware need heavier equipment and hence larger towers, which are expensive to install and operate. An eNodeB using general purpose hardware relies on more lightweight equipment, meaning that smaller towers can be deployed more densely in an area and provide better coverage. A lower power consumption associated with SDR-based BTS equipment also contributes to reducing the overall RAN costs.

Another major benefit of SDR is flexibility. SDR-based eNodeBs can be configured more easily to manage spectrum use at the edges of the cells, and thus minimize interference. Frequency sub-carriers can be selected at two cell edges in such a way that they do not overlap as in the case of conventional systems.

What’s more, SDR permits an adaptable power management so that different services can be assigned optimal QoS depending on the context.

Another aspect of SDR is the ability to build mixed networks. Base station equipment can be programmed to support different technologies at the same time and using the same hardware, serving more users with virtually no infrastructure investment. You can read more about this topic in this previous blog post.

Cable operators becoming MVNO: a win-win scenario

In our previous blog post we opened up the discussion about cable companies staying relevant in a day and time where the subscriber trend is to become more mobile, at the expense of home-based data consumption. As a 2014 report from Adobe already showed, more than 50% of browsing on smartphones and 93% of browsing on tablet comes from WiFi.

Even as they expand their offer, cable providers still see usage limited to home or office hours. On their side, mobile operators continue to upgrade their networks to 4G (and future 5G) technology to deliver more high-quality media content; this makes them competitive in terms of service quality but also results in rising infrastructure investment.

In this post we’ll see how cable companies and MNO can start providing data services using a shared infrastructure, with YateHSS/HLR and the YateUCN unified core.

Mobile data offloading can be an opportunity for both operators and cable companies to provide data access to more users without incurring large expenses. Offloading enables operators to reduce the traffic load on their networks and reallocate bandwidth to other users in case of congestion, by assigning part of the traffic to a WiFi network. For cable companies, it becomes possible to serve subscribers in-between existing hotspots, making them rely primarily on the WiFi network, rather than on the cellular one.

YateUCN and YateHSS/HLR in a cable operator setup

This can be done through MVNO agreements between cable operators and one or multiple MNO, so that the cable provider would share the network assets of the operator to provide carrier-class WiFi access.

As MVNO, a cable company will provide its own SIMs, and its customers will register to and receive data traffic from the MNO’s network. Though some MVNO may choose to also operate their own core network, they are usually likely to hold control over billing, subscriber management and policy control functions, in which case they will only deploy an HLR and/or HSS. In fact, reports suggest that it is preferable for MVNO who offer triple or quad-play operating to deploy their own HSS/HLR (to which they can integrate policy control and AAA), because they need to provide a ‘consistent treatment of the user’ across terminals and technologies.

Providing ubiquitous data access between 2G/3G/4G cellular networks and WiFi hotspots requires offloading to be done seamlessly. Most mobile devices today attempt to connect automatically to an available WiFi network, which they will remember after the first connection has been performed. To connect, once the device has detected the SSID, it proceeds to authentication, which must be done instantly and securely.

In a network using the YateUCN core and the YateHSS/HLR, acting as a typical Home Subscriber Server/Home Location Register with an integrated AuC, SIM authentication is performed between the device and the Access Point, enabling the subscriber a one-time registration to the network using the IMSI stored in their SIM and the secure encryption key Ki. YateHSS/HLR and YateUCN support EAP-SIM/EAP-AKA authentication specified in the IETF standard for WiFi inter-working, securing the connection on both user and network ends.

Once the device is known to the core network, YateUCN communicates with the AuC in the YateHSS/HLR using the SS7 or Diameter protocol, depending on the type of services the user has access to. As soon as the SIM is authenticated, the HSS/HLR takes over and manages the SIM and its services.

YateHSS/HLR supports all the interfaces needed to communicate with the SGSN, EPC, and IMS at the same time, and provides advanced subscriber management options. As a combined HSS/HLR, it allows a subscriber to be located in simultaneous networks if, for example, they are registered to 4G LTE and paging for a CS service is required.

Of course, there are also challenges for cable providers who redefine themselves as WiFi access operators. One of the main concerns is related to the use of non-SIM devices such as laptops, which, even if able to authenticate to the AP in the same way as SIM devices, have no way to then connect to a core network.

While such aspects still need to be approached, the possibility of ‘WiFi-first’ networks seems a venture worthwhile for cable companies.

New opportunities for cable companies: the MVNO route

Cable companies have had an increasing interest in tapping new market opportunities, as consumption of voice and data sees unprecedented growth.

For most cable providers, going from triple play to multi-play is the logical move on customer demand, so the MVNO route naturally seems the next step.

New needs and use patterns on subscribers’ end make the integration of home and mobile services anytime and anywhere necessary. But as cable operators opt to provide WiFi hotspots to their customers, they also need to offer mobility options if they want to turn them into dependable subscribers.

That’s why it goes without saying that rolling out small, private WiFi networks is not enough for most cable operators. If they want to stay in the competition and provide a mix of media services, voice, and data, they need access to the mobile network capabilities by associating with an MNO.

quadruple_player_2015-6-30_version1.3

Partnering not only between a cable company and a mobile network operator, but also between cable providers, is becoming more and more common. On their side, cable companies rally up to ensure they secure themselves a share of the market.

Major players on the US cable providers market, for example, part of the CableWiFi alliance, were estimating the deployment, early 2015, of as many as 10 million new WiFi hotspots around the country, in both homes and businesses. Dual-SSID access points in subscribers’ homes allow the provision of a separate signal for outsiders, who can use that home hotspot without slowing down the network or being granted access to the subscriber’ home network. All the members of the CableWiFi alliance allow their subscribers to use each others hotspots, so, between the five, this could act as a self-reliant WIFI network that uses cellular networks to fill in the ‘gaps’.

Major benefits are evident; cable operators will acquire more customers for their new offerings, while subscribers will gain from getting all their services in one place with reduced subscription costs. But the success of this model still depends highly on cable companies getting to partner with MNOs, who have the necessary network resources to make this model work.

As for mobile network operators, more connected subscribers equals more revenue, which for now may be more to work with than nothing.