What is M-CORD?
M-CORD is a CORD use-case for mobile edge cloud. The architecture allows Service Providers to disaggregate both the RAN and the core, as well as to virtualize their components, either as VNFs or as SDN applications. Moreover, the architecture enables programmatic control of the RAN, as well as core services chaining. M-CORD-based edge clouds are also capable of hosting MEC services.
As for any other CORD flavor, the services management of both VNFs and SDN applications is orchestrated by XOS.
M-CORD is a powerful platform that allows to rapidly innovate cellular networks, towards 5G. As such, it features some 5G specific functionalities, such as split-RAN (C-RAN), RAN user plane and control plane separation (xRAN), programmable network slicing (ProgRAN), and MME-disaggregation. These features have already been demonstrated.
The first release (4.1) of M-CORD ships with the basic CORD building blocks (ONOS, XOS, Docker, and Open Stack), as well as with a number of VNFs to support 3GPP LTE connectivity. These VNFs include a CUPS compliant open source SPGW (in the form of two VNFs: SPGW-u and SPGW-c), as well as a VNF emulating an MME with an integrated HSS, eNBs and UEs. The emulator is not open source. It comes as a binary, courtesy of ng4T (http://www.ng4t.com), who provides free trial licenses for limited use.
Following releases of M-CORD will complete the open source EPC suite, by offering an MME, an HSS, as well as a PCRF VNF. The inclusion of these additional VNFs is targeted for the 6.0 release.
The picture below shows a diagram representing a generic M-CORD POD.
As shown, M-CORD provides connectivity for wireless User Equipments (UEs) to Packet Data Networks (PDNs). The PDNs are Service Provider specific networks, such as VoLTE networks, and public networks, or such as the Internet. Connectivity wise, at a high level M-CORD uses two networks: a Radio Access Network (RAN) and a core (EPC in LTE and NG core in 5G) network. The RAN is composed of a number of base stations (eNBs in LTE / gNBs in 5G) that provide wireless connectivity to UEs while they move. eNBs are the M-CORD peripherals. Both non-disaggregated and split eNBs architectures are supported. In case of a split-RAN solution, the eNBs are split in two components: a Distributed Unit (DU) and a Centralized Unit (CU). The CU is virtualized and implemented as a VNF, exposed as a service (CUaaS) that can be onboarded, configured and instantiated in XOS.
In the RAN, both the non-disaggregated eNB and the RU, are connected to the M-CORD POD through a physical connection to one of the leaf switches. The traffic generated by the UEs first travels wirelessly to the eNBs, which are connected to the M-CORD fabric.
The 3GPP cellular connectivity requires a number of core network components. The eNBs need to pass the UEs’ traffic to the SPGW-u VNF through the fabric and the soft-switch, e.g. OVS, running on the server on which the VNF itself is instantiated. 3GPP has its own control plane, which is responsible for mobility and session management, authentication, QoS policy enforcement, billing, charging, etc. For this reason, in addition to the SPGW-u, the eNB needs also connectivity to the MME VNF to exchange 3GPP control messages. The 3GPP control plane service graph also requires connectivity between the MME, the HSS, the SPGW-c, the SPGW-u, and the PCRF VNFs. It’s the SPGW-u VNF that carries the UE traffic out of the M-CORD POD through a leaf switch, towards an external PDN.
M-CORD is a powerful edge cloud solution: it allows Service Providers to push all the core functionalities to the edge, while they distribute CORD across their edge and central clouds, thereby extending core services across multiple clouds.
EXPERTS! Maybe we’re saying something obvious here, but we want to make sure we’ve a common understanding on the basic terminology, before going through the rest of the guide. If you already know all this, just skip this section.
Following is a list of basic terms used in M-CORD
- Base Station: a radio receiver/transmitter of a wireless communications station.
- eNodeB (Evolved Node B): the base station used in 4G/LTE network.
- EPC (Evolved Packet Core): it’s the core network of an LTE system. It allows user mobility, wireless data connections, routing, and authentication
- HSS (Home Subscriber Server): a central database that contains user-related and subscription-related information interacting with the MME.
- MME (Mobility Management Entity): it’s the key control node used in LTE access networks. An MME provides mobility management, session establishment, and authentication.
- PCRF (Policy and Charging Rules Function): a network function defined in the 4G/LTE standard. It computes in real-time the network resources to allocate for an end-user and the related charging policies.
- RAN (Radio Access Network): it describes a technology, a set of devices, to connect UEs to other parts of a network through radio connections
- SP-GW-C (Serving Gateway and PDN Gateway Control plane): a control plane node, responsible for signaling termination, IP address allocation, maintaining UE’s contexts, charging.
- SP-GW-U (Serving Gateway and PDN Gateway User plane): a user plane node connecting the EPC to the external IP networks and to non-3GPP services
- UE (User Equipment): any device used directly by an end-user to communicate to the base station, for example a cell phone.
The current release of M-CORD includes:
- An open source EPC, providing an SPGW control plane and an SPGW user plane (respectively represented in the system by two VMs deployed on the compute nodes). The current release of the EPC doesn’t yet provide MME, HSS and PCRF services.
- A closed source test suite, emulating UEs, eNodeBs, a minimal version of an MME with an integrated HSS, and an application server (used to emulate the upstream connectivity).
With customizations the system is able to use real hardware base stations, but the released version supports just simple tests with emulated traffic.
At high level, a UE emulator generates some traffic, that passes through the EPC, goes to an emulated application server, and then goes back to the traffic generator again.
Looking at the system from another perspective, three VMs are provided (some of which implement multiple services:
- mysite_venb-X: a test suite that emulates the RAN components (UEs, eNodeBs), an application server, and some of the EPC components (MME and HSS)
- mysite_vspgwc-X: a component of the EPC implementing the S-GW and the P-GW control plane functionalities
- mysite_vspgwu-X: a component of the EPC implementing the S-GW and the P-GW user plane functionalities
NOTE: in the list above, X is a number that varies, and that is automatically generated by the system. More information on how to get the list of VMs can be found in the installation and troubleshooting guides, below.
The picture below describes how the VMs are connected.
Following, is a list of the networks between the VMs:
- S11_net: used to exchange control plane traffic between MME and SPGW (i.e. tunnel and IP address allocation)
- S1U_net: used to exchange user traffic. This specific network is used to exchange data between the RAN components (UEs, eNodeBs) and the EPC
- SGI_net: used to exchange user traffic. This specific network is used to exchange data between the EPC and the Application Server, running on the mysite_venb-X VM
- spgw_net: network dedicated to the communication of the vSPGW components (control plane and user plane)
User traffic is generated on the mysite_venb-X VM. It goes uplink via S1U_net, reaches the EPC, flows through the SGI_net to the application server, emulating Internet. The application server replies, and the answer flows back in downlink, via SGI_net, through the EPC, via S1U_net, back to the emulated RAN (eNB and UEs).
Evolved Packet Core (EPC)
The EPC shipped with M-CORD is called “Next Generation Infrastructure Core” (NGIC). It’s provided as an open source reference implementation by Intel. In the current release it includes two services, implemented in separate VMs: the vSPGW-C and the vSPGW-U.
The vSPGW-C and the vSPGW-U are the Control User Plane Separated (CUPS) implementation of the conventional SAE-GW (S-GW and P-GW) which deals with converged voice and data services on Long Term Evolution (LTE) networks. The NGIC CUPS architecture is aligned with the 3GPP 5G direction. It has been developed using data plane development kit (DPDK) version optimized for Intel Architecture.
If you’re interested to know more and explore the EPC code, see here.
NG40 vTester software
Within M-CORD, ng4T provides for free a limited version of its NG40 software.
The free version:
- Expires on the April 1st 2018
- Emulates up to 1 eNB
- Emulates up to 10 UE
- Has 1 MME with an integrated HSS
- Can connect up to 1 SPGW-C/U
- Allows users to attach, detach and send user plane traffic
- Support only standard Linux interfaces, no DPDK
In order to use the NG40, the Operator will need to apply for a free NG40 M-CORD license with ng4T at the beginning of the setup. Detailed steps can be found in the Installation section below in this guide.
Full versions of the NG40 vTester can do much more than this. This requires
users to apply for additional licenses. In order to apply for licenses, users
will need to contact directly ng4T, using the email address
More details about the NG40 can be found at their website.
XOS Service Graph
XOS is CORD’s default orchestrator. It is described in detail in the XOS guide. XOS lets service developers describe their services in high-level data models. It then translates those models into configurations of system mechanisms, such as VMs, containers, and overlay networks with the assistance of components such as OpenStack and ONOS. Services can be linked together in the form of graphs. In XOS, everything, including low-level system mechanisms, is implemented as a service.
XOS comes with a UI for instantiating and onboarding services, and creating service graphs. In the current implementation of M-CORD, the service graph shown in earlier sections has the following representation in XOS:
Instances of services are connected to each other in the data plane via private networks. On the orchestration side, they are connected via XOS-defined relations called ServiceInstanceLinks. Using ServiceInstanceLinks, services can query other services in a given instance of the service graph, for example to discover each other’s configurations. Besides ServiceInstanceLinks, several other constructs are involved in the construction of the service graph. These are outlined below:
- Services: Services are representations of a deployed VNF software. There is only one instance of a service on a given pod. In M-CORD, there is a single service object each for NG40 vTester (vENBService), SPGWC (vSPGWCService) and SPGWU (vSPGWUService). These service objects are brought up by a TOSCA recipe when an MCORD pod is built.
- ServiceInstances: ServiceInstances are representations of deployed VNFs for a single subscriber or as is the case for M-CORD, a class of subscribers. ServiceInstances are created by another one of MCORD’s services, vEPC-as-a-service (described below).
- Slices: Slices are units of compute and network resource allocations. All VMs and Networks created by XOS are associated with slices.
vEPC-as-a-Service is a special service that only operates in the service control plane, and has no data-plane functionality. Its job is to bring up and help configure instances of the service graph described in this document. The implementation of vEPC-as-a-Service contains a declarative description of the service graph in its config file, https://github.com/opencord/epc-service/blob/cord-4.1/xos/synchronizer/vepc_config.yaml. It is contained in an option called “blueprints.” While there is currently only one such blueprint graph, more may be added in the future.
In the blueprint graph, the network section configures the networks, and the graph section defines ServiceInstances and links them together via those networks.
When a new ServiceInstance of vEPC-as-a-Service is created via the UI, the entire service graph in this blueprint is instantiated and linked to that vEPC-as-a-Service instance. The XOS linkages in the graph ensure that services are instantiated in the correct order based on dependencies between them, while services that have no dependencies are instantiated in parallel.
Note that vEPC-as-a-Service does not perform any operations that cannot be invoked via the UI, via the REST API, or via the TOSCA engine. It creates data model objects via XOS APIs, just as a user could do manually via the UI, REST, or TOSCA. However, it conveniently performs all of the operations in a single step, and in a valid order so that when an object is created, its dependencies are guaranteed to be met.
Once the objects in the XOS data model have been created, it is up to the XOS synchronizers to translate them and make the service operational. Synchronizers are controllers that run in their own containers and react to changes in the data model to communicate those changes to the VNF in question. In the M-CORD setup, there are synchronizers for each of the services: vENB, vSPGWU, vSPGWC as well as vEPC-as-a-service. In addition, there are also synchronizers for the back-end components: OpenStack and ONOS.
There are two parts of a synchronizer:
model policies and
sync steps. Model
Policies operate on the data model. vEPC-as-a-service is a good example of a
synchronizer that only contains model policies, as it does not have any
data-plane functionality. When a vEPC-as-a-service instance is created, it
simply creates the corresponding M-CORD service objects, links them together
and retreats, leaving it up to other service synchronizers, as well as ONOS and
OpenStack to instantiate VMs, configure them, and create networks and network
“Sync steps” operate on the rest of the system, with the data model excluded. A sync step is typically linked to an Ansible playbook that configures a piece of software. Aside from the playbook, it contains Python code that collects configurations from the XOS data model, via its own data model, and those of neighbouring services in the service graph, and having done so, translates it into arguments for the playbook.