For the mobile networks, the end-to-end IP transport tunnels are established by the EPC as discussed in Virtual EPC for Mobile Broadband Internet Access. The IP transport bearers are used to deliver IMS services over it. The conceptual architecture assumes an enterprise or B2B business model delivering IMS capabilities to its MVNO partners with full multi-tenancy and SLA controls. This section discusses components and interfaces that are relevant for the design.

Access Point Name

The APN is a representation of an IP packet data network and service type in the mobile network. This can be perceived as a basic concept of network slicing core to 5G mobile networks. In our example, multiple APNs are defined for each tenant consumer of IMS services. Dedicated traffic bearers are set up for the Gm and RTP (media) control and data plane interfaces respectively. Additionally, the QoS Class Identifier (QCI) for LTE is set to 1 (Conversational Voice) or 2 (Conversations Video) priority for Guaranteed Bit Rate Bearer (GBR).

Backbone Transport

The backbone transport is typically established over MPLS, which is owned and operated by the CSPs. Which in turn connects to other metro area and super backbone network providers.

PE and Core Routers

The PE and Core routers implement the label-VRF-VLAN translations, routing, and peering. The MPLS class of service can be set to apply a packet classifier priority to packets in case of congestion. For voice the CoS EXP bits are typically set to “assured-forwarding” classification. Dedicated VRF’s are defined for the Gm and RTP traffic streams.

ISP Router

The ISP routers are typically Network-to-Network Interfaces (NNI) providing connectivity to external networks, including operator to operator for voice services.

Perimeter Edge Router

The Perimeter router is an NSX Edge which bridges the data center interconnect with the NFV infrastructure. The PE router terminates the VRF’s which map to different VLANs and routed to the NSX T0 Edge Node perimeter router.

Tenant Edge Router

The Tenant router is an NSX Edge which provides network and resource isolation and management for each tenant consuming the IMS services.

Workloads

While there are other workloads involved to realize the virtual IMS, the following core network functions are discussed:

  • Call Session Control Functions (CSCF). CSCF are part of the session and control layer of the IMS core that are based on the Session Initiation Protocol (SIP). The proxy, interrogating, and serving CSCF functions together provide the call management, session management, routing, and security capabilities.

  • Media and Break Out Gateway Control Functions (M/BGCF) Used to bridge voice sessions between IMS and a circuit switched phone in the PSTN or PLMN domain.

  • Telephony Application Server (TAS). It provides a control plane function extension to integrate third-party value-added services such as call forwarding, multi-party calling, caller-id, presence, messaging, and so on.

  • ENUM. A system that translates standard phone number into internet address such as SIP and H.323. This allows for a user to be reachable in other ways (such as SIP) instead of circuit switched phone call.

  • HSS. The primary database for a user. It is the entity containing the subscription-related information to support the network entities actually handling calls or sessions.