UMB Network Architecture

Rao Yallapgada

(Qualcomm Incorporated, 5775 Morehouse Drive, San Diego CA 92121, USA)

Abstract:Ultra Mobile Broadband(UMB)radio-access technology enables efficient wireless transfer of IP packets at very high data rates while providing seamless mobility and best Quality of Service(QoS),even at the cell edges,without lowering frequency re-use.UMB systems benefit from a highly innovative flat network architecture that simplifies the core network and network interfaces,making it easy to scale the network.One of the key principles for UMB architecture is seamless mobility.A major emphasis is placed on the design of network architecture to facilitate seamless handoffs both within the UMB network and across different technologies.Innovative concepts enable fast switching between base stations while minimizing overhead and offering simpler network interfaces.New tunneling mechanisms provide signaling exchange at the data-link layer(layer 2)and IP layer(layer 3)to enable faster mobility across the base stations.This paper discusses key features of UMB network architecture,and provides insight into various architecture design choices.

U ltra Mobile Broadband(UMB)radio-access technology uses advanced innovations in wireless communications to deliver high-speed data throughputs,low latencies and high Quality of Service(QoS)for an enhanced mobile-broadband experience.It is optimally designed from the ground up,at both the physical air-interface and the upper layers,to support bandwidth-intensive mobile service and concurrent VoIPand data services with greater flexibility.The main objectives of the UMB network design are:

·Seamless mobility

·QoSfor a wide range of applications,including latency-sensitive traffic

·Efficient frequency re-use deployment

·Interoperability of equipment from different vendors by simplifying the interface between network elements

·Ease of network management and a reduced number of network elements

·Network scalability and flexibility in service deployment

All of the above objectives are accomplished simultaneously in the UMB design.With UMB,networks can be easily scaled to serve a myriad of base stations for different deployment scenarios,such as environments with varying coverage and capacity requirements.The distributed network architecture allows operators to spread the processing load across the network elements,thus simplifying the overall design.Leveraging standard IP components,operators should be able to scale their networks based on system usage at reduced time and costs.The main features of UMB network can be broadly summarized as follows:

·A flat network architecture that simplifies the core network design and eliminates the need for centralized Base Station Controllers(BSCs)

·A Converged-Access Network(CAN)design that enables seamless mobility

·A multi-route feature that enables fast switching between base stations and provides requisite support for latency-sensitive applications

·Layer 2 and layer 3 tunneling mechanisms to simplify the network interfaces

·Decoupling of layer 3 handoffs with layer 1 handoffs,ensuring fast and efficient mobility across base stations

1 UMB: Flat Network Architecture

The UMB solution,based on flattened network architecture,is a marked departure from the traditional hierarchical architectures defined with multiple layers of control and interconnected platforms.In a traditional hierarchical architecture,the Access Terminal(AT)maintains a single air-interface protocolstack for communication with multiple Base Stations(BSs).A centralized control entity,called Base Station Controller(BSC),keeps the protocol state coordinated between multiple BSs.The BSC coordinates fast-changing state information across multiple BSs and maintains a common state,such as layer 2 buffers,across multiple BSs.

A UMB network using a flat architecture does not require a centralized entity such as a BSC,as shown in Figure 1.In this scheme,there is no need to coordinate the connection state across the UMB's BSequivalent,the Evolved Base Station(eBS).This flat-architecture eBScombines the functions of a traditional BS,a BSC,and some functions of the Packet Data Serving Node(PDSN)into a single node,making the deployment of the networks simpler.As the number of elements required to build a network are reduced,the networks become more reliable,more flexible,easier to deploy and less costly to operate.There is a significant reduction in inter-eBSinteraction,leading to a vastly simpler inter-eBS interface,which thus facilitates multi-vendor interoperability.

The flat architecture BScan be connected directly to the Internet.Traditional hierarchical mobile-broadband radio access networks serve user traffic in multiple types of nodes.For example,in legacy networks,the BS,BSC,PDSNand mobile IPhome agent all cooperate to serve user traffic.Combining these functions into fewer nodes reduces latency,decreases capitaland maintenance costs,and reduces the complexity of interactions between the nodes to deliver end-to-end QoS.

◀Figure 1.The UMB flat network architecture depicting access points connected to their backhauls with a single IP attachment point.

2 UMB and the Converged Access Network

One of the core principles of the UMB network architecture is a CAN that seamlessly integrates UMB with existing 3G core networks.In a converged network,an operator can gain additional capacity via UMBwherever necessary while using existing 3G for mobile-broadband access in the majority of the network.An example of a UMB CAN with appropriate interfaces in interoperation with a CDMA2000 1x EV-DO network is depicted in Figure 2.

Central to the UMB radio access network is the eBS,which,in many respects,is comparable to a combination of today's 3G BSs and BSCs.Fitting the flat architectural model,the eBSallows IP to extend to the BS,thereby lowering latency and enabling faster switching of the ATbetween base stations.

The eBSs connect to a common Access Gateway(AGW),which provides a point of IPattachment for the ATto the packet data network.

2.1 UMB Network Elements

2.1.1 Access Terminal The ATis the subscriber device that provides IPdata connectivity to the user,typically a mobile phone,Personal Digital Assistant(PDA)or mobile-enabled laptop.

2.1.2 Evolved Base Station

The eBSprovides Over-the-Air(OTA)signaling and user-data transport that is used by the ATfor connectivity to the radio access network.Additional functions of the eBSinclude:

·Providing a layer 2 attachment point for the AT

·Acting as a layer 1 attachment point for both forward and reverse links

·Encryption/decryption of packets at the Radio-Link Protocol(RLP)level for OTA transmission/reception

·Scheduling of OTAtransmission

·Header compression

The eBSalso provides these important functions:

·Forward-Link Serving eBS(FLSE)—Serving eBSfor the forward-link physical layer

·Reverse-Link Serving eBS(RLSE)—Serving eBSfor the reverse-link physical layer

·Signaling Radio Network Controller(SRNC)—Session anchor eBSthat stores the AT's session information and serves as a permanent route to the AT

·Data Attachment Point(DAP)—Receiving eBSfor all the data packets from the common AGW

Additionally,an eBSis able to view the user's IPpackets and can optimize OTA scheduling or perform other value-added functions.

▲Figure 2. A UMB converged access network depicting the connecting interfaces between a UMB network and an EV-DO network.

2.1.3 Access Gateway

The AGWprovides the user's point of IP connectivity to the network.That is,the AGWis the first-hop router for the mobile terminal.The AGWperforms layer 3 services and above,including hot-lining,policy enforcement,and more.

2.1.4 Signaling Radio Network Controller(SRNC)

The SRNC maintains radio-access-specific information for the ATin the CAN.The SRNC is responsible for maintaining the session reference(session storage point for negotiated air-interface context),supporting idle-state management,and providing paging-control functions when the ATis idle.It is also responsible for access authentication of the AT.The SRNC function may be hosted by an eBS or may be located in a separate(radio-less)entity.

2.1.5 Authentication,Authorization and Accounting(AAA)Function

This functional entity provides authentication,authorization,and accounting functions with respect to the AT's use of the network resources.

2.1.6 Home Agent(HA)

The HAis used to provide mobility solution to the ATin a 3GPP2 packet-data network.In addition,in an evolved network architecture,the HA may also be used for inter-technology mobility.

Figure 3.▶Example depicting a route set of an AT andthe eBS selection of different personalities.

2.1.7 Packet Data Serving Node

The PDSN is the node that provides the user's point of IPconnectivity in the existing EV-DO or CDMA2000 1X packet-data networks.

2.1.8 Policy and Charging Rules Function(PCRF)The PCRFsets rules for the AGW.The purposes of the PCRFrules are to:

·Detect a packet belonging to a service data flow

·Provide policy control for a service data flow

·Provide applicable charging parameters for a service data flow

3 Mobility Management

3.1 Key Functions and Concepts

UMB benefits from a highly innovative network design,particularly with respect to mobility management,in enabling faster handovers,freedom in network scalability and a truly distributed access design.With UMB network architecture,operators can offer full-mobility applications while providing highest QoS.To get an insight into UMB's mobility management,it is useful to review a set of key concepts that are used to handle various mobility scenarios.

3.1.1 Multi-route

Multi-route is at the core of UMB network architecture.The UMBATmaintains an independent air-interface protocol stack associated with each BS.Each of these protocol stacks is called a route.The AT further maintains a route set consisting of all the eBSs that have a route with the AT,and the multi-route is used to signify the multiple routes that the ATmaintains with different eBSs.An important feature is that every eBSis set up to be the serving eBSwhen it is added to the route set.A route set can support a minimum of six routes at any time.When the ATis idle,it maintains one route with the SRNC.

Furthermore,each eBSmaintains a connection state associated with each route.The connection state includes parameter values and the state of algorithms that help maintain the connection between an eBSand an AT,such as transmit/receive buffers,sequence numbers in an RLPthat provide reliable delivery of upper-layer packets,granted QoSfor various flows and granted MAC resources.

Since the ATmaintains a different route with each eBSand the connection state is localto an eBS,as the AThands off from one eBSto another,no connection state is transferred between the eBSs.This significantly reduces the complexity of inter-eBSsignaling.

3.1.2 Common Session

Although each eBShas an independent route,all eBSs share a common session with an AT.Acommon session defines sets of protocol types and protocol attributes negotiated and stored by the ATand eBS.

3.1.3 Personality

The session comprises one or more personalities.Personality defines the protocol types and attributes that are used between the ATand the eBS during communication.While the session is common between the ATand all eBSs in a route set,each eBSindependently negotiates a personality to use for its route.

A personality negotiated by one eBS can be used by another eBSwith no need for any new renegotiation.This significantly reduces the amount of time required to add a new eBSto the route set.The key outcomes are as follows:

·It is faster to add a new eBSto the route set compared to adding a BSto the active set in existing networks.

·Inter-eBSinterface is made simple,with only a minimal need for coordination of the eBSconfigurations.

3.1.4 Forward-Link Serving Entity This serving entity is the eBSthat provides layer 1 connectivity on the forward link.

3.1.5 Reverse-Link Serving Entity

This entity is the eBSthat provides layer 1 connectivity on the reverse link.

Figure 3 illustrates the relationship between the eBSand the ATwith respect to sessions and personalities.The AT and eBSs share a common session,whereas each route can choose its own personality.

3.2 Layer 1 Handoff Mechanism

Given the definition of key functions and concepts,the following explains the mechanics of a layer 1 handoff:

·Based on measurements of channel conditions,the ATdetermines an appropriate eBSin the route set to serve as the layer 1 point of attachment(one for FLSEand one for the RLSE).

·The FLSEand RLSEfunctions may be served by either the same or different eBSs.

·The chosen FLSEand RLSEare communicated by the ATto the network using physical-layer channels that support fast switching of FLSEand RLSE as channel conditions change.

·When the ATswitches from one eBSto another,it uses the route associated with the new serving eBSto transfer packets.

The distinguishing features in UMB network design are as follows:

·Because the target eBSis set up to be the serving eBSwhen it is added to the route set,and because no connection state info is exchanged between eBSs,the inter-eBSinterface is vastly simplified,enabling fast switching of layer 1 FLSEand RLSEentities during a handoff.

·The system provides QoSto latency-sensitive applications without resorting to compromises using a lower frequency reuse.

·The ATmaintains an active set within which it is allowed to switch the FLSEand RLSEwith fast physical-layer signaling.The eBSs are added to the active set ahead of switching time,during which,the connection setup is completed with the new eBS.This allows very fast switching between eBSs,as the target eBSis ready to accept the AT's traffic when the ATswitches to the eBS.

After the ATswitches from the source eBSto the target eBSin a layer 1 handoff,it is likely that there are remnants or fragments of data packets in the route to the source eBSthat still need to be delivered to the AT.A tunneling mechanism ensures zero loss of packets during handoff.

3.2.1 Tunneling

The tunneling mechanism provides zero-loss seamless handoffs in layer 1 eBSswitching,without the need for a BSC or centralized controller.

·A layer 2 tunnel between eBSs is used to deliver fragmented data packets or fully buffered packets(left at the previous source eBS)to the target eBS.

·A layer 3 tunnel between eBSs is used to deliver unfragmented IPpackets buffered at the previous eBSto the target eBS,for OTA delivery to the AT.

·The target route then carries the packets OTA on behalf of the source route and delivers the packets to the AT.

3.3 Essentials of Layer 2 Communication

Once an eBSis in the route set,the eBS will need to exchange messages with the ATto maintain the connected state,even though the current route is not serving.Layer 2 communication facilitates this process.

In traditional networks,if an AThas to communicate with a non-serving BS,the interface is necessary to translate the protocols between the BSs.For instance,the OTA protocol for the serving BSis converted into a network protocol.

In the UMBnetwork design,a layer 2 communication is established between an eBSand the ATas soon as the eBSis added to the route set.Arelationship is established between the new eBSand the ATusing the personality's defining protocols and attributes.All exchanges between the ATand an eBSin the route set are communicated through a layer 2 tunnel between that eBSand the serving eBS.This is also called blind tunneling,where the serving eBSblindly delivers the packets to the ATwithout interpreting the content(e.g.,transactions between the ATand the eBSrelated to reporting radio measurements,QoSrequests and grants,etc.).Thus,the protocol running between the eBSand the ATis independent of the delivery service provided by the serving eBS,and it needs no translation of protocols between eBSs.

3.4 Essentials of Layer 3 Handoffs

As mentioned in section two,an eBScan serve any of the four functions:FLSE,RLSE,SRNC and DAP.The network is designed such that these functions need not be designated to a single eBS,but so that different eBSs in the route set can serve any or all the four functions for a given connection between the ATand the network.

The AGWacts as the layer 3 attachment point for the AT.The AGW sends packets destined to the ATto the eBSserving the DAPfunction.If an eBS is assigned to be the DAPfunction,the DAPeBSand the AGWestablish a Proxy Mobile-IP(PMIP)binding between them.

In the event of a layer 1 eBSswitching and handoff,and if the DAPeBSis different from the new FLSEeBS,when the AGWtransmits packets destined to the AT,the network is designed to enable the DAPeBSto forward the packets to the FLSEeBSusing a layer 3 tunnel.

Alayer 1 switch can trigger the ATto send a DAPmove request to the eBS currently serving as FLSE.Although it is preferable to locate DAPand FLSEwith the same eBS,it is not desirable to move the DAPfunctionality too often because the move causes binding update loading between the DAPand the AGWand requires moving the session reference eBS.To avoid this,the eBSguides the AT by specifying attributes in regards to the frequency of DAPmove requests.

The network is designed to decouple the DAPmove or layer 3 handoffs from a layer 1 handoff allowing layer 1 handoffs to happen more frequently than DAP moves.

3.5 Inter-AGW Handoffs

In general,the network is constructed to have a meshed topology,with several eBSs connected to a single AGW.More or less,eBSs in the same route set should be connected with the same AGW.An inter-AGWhandoff occurs when an eBSadded to a route set is linked to a different AGW.

Inter-AGWhandoffs are implemented as elegantly as the inter-eBShandoffs,with no data interruption.There are no interfaces needed between AGWs.Seamless inter-AGWhandoffs are achieved in a make-before-break fashion,by making use of the layer 2 tunneling mechanisms,as shown in Figure 4.

For an inter-AGWhandoff,layer 2 tunneling is established between neighboring eBSs under different AGWs.During the inter-AGWhandoff process,the ATis served by one DAPeBS,and two PMIPtunnels are maintained between the eBS-AGWpairs for a short period.The AT,during this short period,keeps the two IPinterfaces with the two AGWs in handoff alive.Once the communication is firmly established between the ATand the target AGW,the route set will then have a new DAPeBS with a new PMIPbonding with the target AGW.

◀Figure 4.A UMB network in seamless inter-AGW handoffs.

3.6 Inter-System Handoffs

The UMBnetwork allows for seamless inter-system handoffs.For example,with a minimal service interruption,handoffs can occur between the UMBand EV-DO networks.

For an inter-system handoff,a dual-mode AThas to negotiate an EV-DO session,create a PPP connection and install a TFTthrough the UMBby establishing link-layer tunneling support between the ATand the EV-DO radio access network.The dual-mode AT,through a tune-away procedure,monitors the signal on radio interfaces with both the EV-DO and the UMB networks,to implement a layer 1 handoff.The AGWcan establish a connection with the home agent using a PMIPor client mobile-IP,to implement a layer 3 handoff.

3.7 Paging Design and Function

The paging function is simpler and lighter in the UMBnetwork design.Unlike the traditional centralized-node,BSC-based paging,where a paging area is maintained for the AT,the SRNC takes care of this function by tracking the registration of the ATwith a single eBS.The following describes the process:

(1)When the ATtransitions from connected state to idle state,all routes are deleted except the route to the SRNC.The ATthen performs distance-based registration with the SRNC.Distance-based registration involves the ATre-registering if it has moved more than a set distance from the last location it has registered.For this,the ATsets up a route with a local eBSto tunnelthis registration to the Session Reference(SR)eBS.

(2)If a packet arrives while the ATis in idle state,the DAPeBSsends a paging request to the SReBS,and the SReBSin turn sends the paging request to the last registered eBS.The SReBS's page requests the last registered eBSto propagate the paging request to other eBSs in the neighborhood,within a given radius.Upon receiving the page,the AT creates a route and sets up a connection with the local eBS.In other words,the SR eBSalso provides key paging support for the ATby just passing the request to the last registered eBS.Thus,the paging function is made easier by having the SR eBStrack only one eBS,which is the last eBSregistered by the AT.

4 Conclusions

The flat architecture UMB solution is a simplified network design that provides superior mobility-management support for bandwidth-intensive traffic.The flat architecture removes the need for elements such as centralized BSCs,and greatly reduces the number of network nodes needed for interoperation.

The UMB CAN achieves seamless and fast handoffs,whether they are inter-eBS,inter-AGWor inter-system transitions for the AT,all while minimizing overhead.Thus,the system is well designed to provide QoSfor a wide range of applications,including latency-sensitive applications.

The network architecture is designed to keep the interfaces between the radio access network and core network as simple as possible.For example,avoiding the coordination or transfer of connection states between eBSs and avoiding the interpretation of packets bound to another eBShave clearly simplified the inter-eBSinterfaces.

This will help operators deploy and scale UMB networks with ease,and will go a long way in facilitating multi-vendor interoperability.