Data-Plane and Control-Plane Functions in Base Stations and Mobile Stations

Figure below shows the user data processing path at the BS and MS. As shown in the figure, the user data traverses the path from network layer to physical layer and vice versa. In the transmitter side, a network layer packet is processed by the convergence sub-layer, the ARQ function (if enabled), the fragmentation/packing function, and the MAC PDU formation function, to form the MAC PDU to be sent to the physical layer for processing. In the receiver side, a physical layer SDU is processed by MAC PDU formation function, the fragmentation/packing function, the ARQ function (if enabled), and the convergence sub-layer function, to form the network layer packets. The control primitives between the MAC CPS functions and between the MAC CPS and PHY that are related to the processing of user traffic data are also shown below

Signal flow graph in data- and control-planes

The control-plane signaling and processing flow graph at the BS and the MS. In the transmitter side, the flow of control primitives from control-plane functions to data-plane functions and processing of control-plane signals by data-plane functions in order to construct MAC management messages and MAC header/sub-headers, to be transmitted over the air interface, are illustrated. In the receiver side, the arrows show the processing of the MAC control messages through data-plane functions and the reception of the corresponding control-plane signals by control-plane functions. The dotted arrows show the control primitives between MAC CPS functions and between MAC CPS and physical layer functions that are related to the processing of control-plane signaling. The control primitives to/from M-SAP/C-SAP define the network related functionalities, such as inter-BS interference management, inter/intra RAT mobility management, etc., as well as management-related functionalities, such as location management, system configuration, etc. 

The IEEE 802.16m Protocol Structure

In this section, we further examine the functional elements of each protocol layer and their interactions. The 802.16m MAC common part sub-layer functions are classified into radio resource control and management functional group and medium access control functional group. The control-plane functions and data-plane functions are also separately classified. This would allow more organized, efficient, and structured method for specifying the MAC services in the IEEE 802.16m standard specification. The radio resource control and management functional group comprises several functional blocks including:  

• Radio resource management block adjusts radio network parameters related to the traffic load, and also includes the functions of load control (load balancing), admission control, and interference control; 

• Mobility management block scans neighbor BSs and decides whether MS should perform handover operation; 

• Network-entry management block controls initialization and access procedures and generates management messages during initialization and access procedures; 

• Location management block supports location based service (LBS), generates messages including the LBS information, and manages location update operation during idle mode; 

• Idle mode management block controls idle mode operation, and generates the paging advertisement message based on paging message from paging controller in the core network; 

• Security management block performs key management for secure communication. Using managed key, traffic encryption/decryption and authentication are performed; 

• System configuration management block manages system configuration parameters, and generates broadcast control messages such as superframe headers; 

• Multicast and broadcast service (MBS) block controls and generates management messages and data associated with MBS; 

• Service flow and connection management block allocates Station Identifier (STID) and Flow Identifiers (FIDs) during access/handover service flow creation procedures. 

The MS and BS Interface

The MAC management PDUs that are exchanged over the primary management connectioniii can trigger, or are triggered by, primitives that are exchanged over either the C-SAP or the M-SAP, depending on the particular management or control operation. The messages that are exchanged over the secondary management connection can trigger or are triggered by primitives that are exchanged over the M-SAP. This interface is a set of SAP between an IEEE 802.16 entity and NCMS as shown in Figure below. 

It consists of two parts: the M-SAP is used for delay-tolerant management-plane primitives; and the C-SAP is used for delay-sensitive control-plane primitives that support handovers, security context management, radio resource management, and low power operations such as idle mode and paging functions.

The IEEE 802.16m Reference Model

Figure below illustrates the IEEE 802.16 reference model. The data link layer of IEEE 802.16 standard comprises three sub-layers. The service-specific convergence sub-layer (CS) provides any transformation or mapping of network-layer data packets into MAC SDUs. On the transmitter side, the CS receives the data packets through the CS Service Access Point (SAP) and delivers MAC SDUs to the MAC Common Part Sub-layer (MAC CPS) through the MAC SAP. This includes classifying network-layer SDUs and associating them with the proper MAC Service Flow Identifiers (SFID) and Connection Identifiers (CID). The convergence sub-layer also includes payload header suppression function to compress the higher-layer protocol headers. Multiple CS specifications are provided for interfacing with various network-layer protocols such as Asynchronous Transfer Mode (ATM)i and packet-switched protocols such as IP or Ethernet. The internal format of the CS payload is unique to the CS, and the MAC CPS is not required to understand the format of or parse any information from the CS payload.  

The IEEE 802.16 reference model

The MAC CPS provides the core MAC functionality of system access, bandwidth allocation, connection establishment, and connection maintenance. It can receive data from the various convergence sub-layers, through the MAC SAP classified into particular MAC connections. An example of MAC CPS service definition is given in reference. The Quality of Service (QoS) is further applied to the transmission and scheduling of data over the physical layer. 

The MAC also contains a separate security sub-layer providing authentication, secure key exchange, and encryption. The user data, physical layer control, and statistics are transferred between the MAC CPS and the Physical Layer (PHY) via the PHY SAP which is implementation-specific. The IEEE 802.16 physical layer protocols include multiple specifications, defined through several amendments and revisions, each appropriate for a particular frequency range and application.

The IEEE 802.16 compliant devices include mobile stations or base stations. Given that the IEEE 802.16 devices may be part of a larger network, and therefore would require interfacing with entities for management and control purposes, a Network Control and Management System (NCMS) abstraction has been introduced in the IEEE 802.16 standard as a “black box” containing these entities. The NCMS abstraction allows the physical and MAC layers specified in the IEEE 802.16 standard to be independent of the network architecture, the transport network, and the protocols used in the backhaul, and therefore would allow greater flexibility. The NCMS entity logically exists at both BS and MS sides of the radio interface. Any necessary inter-BS coordination is coordinated through the NCMS entity at the BS. An IEEE 802.16 entity is defined as a logical entity in an MS or BS that comprises the physical and MAC layers on the data, control, and management planes.

The IEEE 802.16f amendment (currently part of IEEE 802.16-2009 standard) provided enhancements to IEEE 802.16-2004 standard, defining a management information base (MIB), for the physical and medium access control layers and the associated management procedures. The management information base originates from the Open Systems Interconnection Network Management Model and is a type of hierarchical database used to manage the devices in a communication network. It comprises a collection of objects in a virtual database used to manage entities such as routers and switches in a network.

The IEEE 802.16 standard describes the use of a Simple Network Management Protocol (SNMP),ii i.e., an IETF protocol suite, as the network management reference model. The standard consists of a Network Management System (NMS), managed nodes, and a service flow database. The BS and MS managed nodes collect and store the managed objects in the form of WirelessMAN Interface MIB and Device MIB that are made available to network management system via management protocols, such as SNMP. A Network Control System contains the service flow and the associated Quality of Service information that have to be provided to BS when an MS enters into the network. The Control SAP (C-SAP) and Management SAP (M-SAP) interface the control and management plane functions with the upper layers. The NCMS entity presents within each MS. The NCMS is a layer-independent entity that may be viewed as a management entity or control entity. Generic system management entities can perform functions through NCMS and standard management protocols can be implemented in the NCMS. If the secondary management connection does not exist, the SNMP messages, or other management protocol messages, may go through another interface in the customer premise or on a transport connection over the air interface. Figure 3-4 describes a simplified network reference model. Multiple mobile stations may be attached to a BS. The MS communicates to the BS over the air interface using a primary management connection, basic connection or a secondary management connection. The latter connection types have been replaced with new connection types in IEEE 802.16m standard

CSN-anchored Mobility

The CSN-anchored mobility refers to mobility across different ASNs alternatively to mobility across different IP subnets, and thereby requires network layer mobility management. The mobile IP protocols are used to manage mobility across IP subnets, and to enable CSN-anchored mobility. This section describes mobile IP based macro-mobility between the ASN and CSN across R3 reference point. In the case of IPv4, this implies re-anchoring of the current FA to a new FA, and the consequent binding updates (or MIP re-registration) to update the upstream and downstream data forwarding paths. In CSN-anchored mobility, the anchor mobile IP FA of the MS is changed. The new FA and CSN exchange messages to establish a data forwarding path. The CSN-anchored mobility management is established between ASN and CSN that are in the same or different administrative domains. The mobility management may further extend to handovers across ASNs in the same administrative domain. The procedures for CSN-anchored mobility management and the change of MS point of attachment to the ASN may not be synchronized. In this case, the procedures may be delayed relative to the completion of link layer handover by the MS. 

In an intra-NAP R3 mobility scenario, an MS is moving between FAs within a single NAP domain. The R3 mobility event results in a handover between two FAs, thereby relocating the ASN R3 reference anchor point in the NAP. Note that R3 mobility does not automatically terminate or otherwise interfere with idle/sleep operation of the MS. The CSN-anchored mobility accommodates the scenario in which the MS remains in idle state or sleep mode until it is ready to transmit uplink traffic or is notified of downlink traffic by the serving BS. In all non-roaming scenarios, the HA is located in the CSN of H-NSP. For roaming scenarios, the HA is located in the CSN of either the H-NSP or V-NSP, depending on roaming agreement between H-NSP and V-NSP, user subscription profile and policy in H-NSP. The CSN-anchored mobility within a single NAP administrative domain does not introduce significant latency and packet loss. A make-before-break handover operation (i.e., when a data path is established between the MS and target BS before the data path with the serving BS is broken) is feasible within the same NAP administrative domain. To accomplish this procedure, the previous anchor FA maintains data flow continuity while signaling to establish the data path to a new anchor FA. The PMIP procedures do not require additional signaling over-the-air or additional data headers to perform CSN-anchored mobility. The CSN-anchored mobility activities are transparent to the MS. The MS uses Dynamic Host Configuration Protocol (DHCP) for IP address assignment and host configuration. DHCP is a network application protocol used by devices to obtain configuration information for operation in an IP network. This protocol reduces system administration workload, allowing devices to be added to the network with minimal user intervention 

WIMAX Mobility Management

The WiMAX network architecture supports two types of mobility: ASN-anchored mobility (intra-ASN) and CSN-anchored mobility. ASN-anchored mobility refers to a scenario where a mobile terminal moves between two base stations belonging to the same ASN while maintaining the same foreign agent at the ASN. The handover in this case utilizes R6 and R8 reference points. The CSN-anchored mobility refers to an inter-ASN mobility scenario where the mobile station moves to a new anchor foreign agent and the new FA and CSN exchange signaling messages to establish data forwarding paths. The handover in this case is performed via R3 reference point with tunneling over R4 to transfer undelivered packets. 

Figure below illustrates three different mobility scenarios supported in WiMAX networks. When the mobile station moves from positions 1 to 2, or 1 to 3, an ASN-anchored mobility through R8 or R6 reference points, respectively, is implied, whereas moving from position 1 to 4 involves a CSN-anchored mobility scheme though R3 reference point.

WIMAX Radio Resource Management (RRM)

Efficient utilization of radio resources within an access network is performed by the radio resource management entity. The mobile WiMAX RRM is based on a generic architecture. The RRM defines mechanisms and procedures to share radio resource related information between BS and ASN-GW. The RRM procedures allow different BSs to communicate with each other or with a centralized RRM entity residing in the same or a different ASN to exchange information related to measurement and management of radio resources. Each BS performs radio resource measurement locally based on a distributed RRM mechanism. It is also possible to deploy RRM in an ASN using base stations with RRM function, as well as a centralized RRM entity that does not reside in the BS and collects and updates radio resource indicators such as choice of target BS, admission or rejection of service flows, etc., from several BSs. The RRM procedures facilitate the following WiMAX network functions: 

• MS admission control and connection admission control, i.e., whether the required radio resources are available at a candidate target BS prior to handover; 

• Service flow admission control, i.e., creation or modification of existing/additional service flows for an existing MS in the network, selection of values for admitted and active QoS parameter sets for service flows; 

• Load balancing by managing and monitoring system load and use of counter-measures to enable the system back to normal loading condition; 

• Handover preparation and control for improvement/maintenance of overall performance indicators (for example, the RRM may assist in system load balancing by facilitating selection of the most suitable BS during a handover).  

The RRM is composed of two functional entities, i.e., radio resource agent (RRA) and radio resource control (RRC). The radio resource agent is a functional entity that resides in the BS. Each BS includes a radio resource agent. It maintains a database of collected radio resource indicators. An RRA entity is responsible for assisting local radio resource management, as well as communicating to the RRC to collect and measure radio resource indicators from the BS and from a plurality of mobile terminals served by the BS using MAC management procedures as specified by the IEEE 802.16 specifications. It also communicates RRM control information over the air interface to the MS, as defined by the IEEE 802.16 specifications. An example of such RRM control information is a list of neighbor BSs and their parameters. It further performs signaling with RRC for radio resource management functions, as well as controlling the radio resources of the serving BS, based on the local measurements and reports received by the BS and information received from the RRC functional entity. 

The local resource control includes power control, monitoring the MAC and PHY functions, modifying the contents of the neighbor advertisement message, assisting the local service flow management function and policy management for service flow admission control, making determinations and conducting actions based on radio resource policy, assisting the local handover functions. 

The radio resource control functional entity may reside in BS, in ASN-GW, or as a standalone server in an ASN, and is responsible for collection of radio resource indicators from associated RRAs. The RRC can be collocated with RRA in the BS. The RRC functional entity may communicate with other RRCs in neighboring BSs which may be in the same or different ASN. The RRC may also reside in the ASN-GW and communicate to other RRAs across R6 reference point. When the RRC is located in the ASN, each RRA is associated with exactly one RRC. The RRC relay functional entity may reside in ASN-GW for the purpose of relaying RRM messages. The RRC relay cannot terminate RRM messages, but only relays them to the final destination RRC. Standard RRM procedures are required between RRA and RRC, and between RRCs across network interfaces to ensure interoperability. These procedures are classified into two types: information reporting procedures for delivery of BS radio resource indicators from RRA to RRC; and between RRCs and decision support procedures from RRC to RRA for communicating recommendations on aggregated RRM status (e.g., in neighboring BSs) for various purposes. 

The RRM primitives can be used either to report radio resource indicators (i.e., from RRA to RRC or between RRCs) or to communicate decisions from RRC to RRA. The former type of primitive is called information reporting primitive and the latter is called decision support primitive. The available radio resource information provided by the RRAs to RRC is used by RRC for load balancing. The RRC may interact with the handover controller to ensure load balance. 

WIMAX Authentication, Authorization, and Accounting (AAA)

AAA refers to a framework based on IETF protocols, Remote Authentication Dial-in User Service (RADIUS) or Diameter , which specify the procedures for authentication, authorization, and accounting associated with the user terminal’s subscribed services across different access technologies. As an example, AAA includes mechanisms for secure exchange and distribution of authentication credentials and session keys for data encryption. The AAA protocols provide the following services:  

• Authentication including device, user, or combined device and user authentication; 

• Authorization including delivery of information to configure the session for access, mobility, QoS, and other applications; 

• Accounting including delivery of billing information and other information that can be used to audit session activity by both the H-NSP and V-NSP. 

The AAA framework supports global roaming across operator networks, including support for reuse of credentials and consistent use of authorization and accounting. It further supports roaming between H-NSP and V-NSP. The AAA framework is based on use of RADIUS or Diameter in ASN and CSN. The AAA framework accommodates both Mobile IPv4  and Mobile IPv6  Security Association (SA) management. It further accommodates various network operation scenarios from fixed to full mobility. The AAA framework provides support for deploying MS authorization, user and mutual authentication between MS and the NSP, based on Privacy Key Management (PKMv2). In order to ensure interoperability, the AAA framework supports Extensible Authentication Protocol (EAP)-based authentication mechanisms that include passwords, Subscriber Identity Module, Universal Subscriber Identity Module, Universal Integrated Circuit Card, Removable User Identity Module, and X.509 digital certificates. The AAA framework is capable of providing the V-CSN or ASN with a temporary identifier that represents the user without revealing the user’s identity.

The NAP may deploy an AAA proxy between two NASs in ASN and the AAA in CSN in order to provide security and enhanced manageability. The AAA proxy will also allow the NAP to regulate the AAA attributes received from the visited CSN, and to add additional AAA attributes that may be required by the NASs in the ASN. Note that the CSN hosts the AAA server, whereas the ASN hosts one or more NASs. The PKMv2 protocol is used to perform over-the-air user authentication. The PKMv2 transfers EAP messages over R1 reference point (i.e., the IEEE 802.16-2009 air interface or its evolution) between the MS and the BS in ASN. 

Connectivity Service Network (CSN)

Connectivity Service Network is defined as a set of network functions that provide IP connectivity to user terminals. A CSN can provide the following functions:  

• MS IP address and endpoint parameter allocation for user sessions; 

• Internet access; 

• AAA proxy/server; 

• Policy and admission control based on user profiles; 

• ASN-CSN tunneling support; 

• Subscriber billing; 

• Inter-CSN tunneling for roaming; 

• Inter-ASN mobility management and mobile IP home agent functionality; 

• Network services such as connectivity for peer-to-peer services, provisioning, authorization and/or connectivity to IP multimedia services, and to enable lawful interception; 

• Connectivity to Internet and managed services such as IP Multimedia Subsystem (IMS),vi Location-Based Services (LBS),vii Multicast and Broadcast Services (MBS), etc.; 

• Over-the-air activation and provisioning of mobile WiMAX terminals.  The CSN may further comprise network elements such as routers, AAA proxy/servers, user databases, and interworking functions.

Access Service Network Gateway (ASN-GW)

The base stations within an ASN-GW group (i.e., all base stations connected to the same ASN-GW) are connected through R8 reference point. Furthermore, the base stations within an ASN-GW group are separately connected to the ASN-GW via R6 reference point, as shown. 

The base station is a logical entity that implements a full instance of Medium Access Control (MAC) and Physical Layer (PHY) protocols, as specified by the IEEE 802.16-2009 standard and includes one or more access functions. A BS instance represents one sector with one frequency assignment. It incorporates scheduler functions for uplink and downlink radio resources, which are typically vendor specific.

The ASN-GW is a logical entity that represents an aggregation of control functions that are either paired with a corresponding function in the ASN (e.g., BS instance), a resident function in the CSN, or a aggregation of control functions that are either paired with a corresponding function in the ASN (e.g., BS instance), a resident function in the CSN, or a function in another ASN. The ASN-GW may also perform bearer-plane routing or bridging functions. A BS is associated with a default ASN-GW; however, the ASN-GW functions may be distributed among multiple ASN-GWs located in one or more ASNs. 

The ASN functions in an ASN-GW can be divided into two groups, i.e., the Decision Point (DP) and the Enforcement Point (EP) functions. The EP category includes bearer-plane functions and the DP category includes non-bearer-plane functions. If such functional split is implemented in ASN-GW, the EP and DP functional groups would include bearer-plane and non-bearer-plane functions, respectively, and are interfaced using R7 reference point.