AT&T, having conducted POCs and demonstrations on CORD architecture at its lab since last year, is said to have begun field tests from August this year. This obviously means that the operator has already completed a rough sketch and is now ready to take action. 

 

We will discuss CORD in detail through a series of posts.

 

Figure 1 below shows the architecture of the gigabit Internet access network currently being run by AT&T.

 

Figure 1. Current Gigabit Internet Access Network Architecture of AT&T

 

As you can see in the figure above, L3 CPE (RG) is deployed at home while the central office has the following installed: i) GPON OLTs that aggregate subscriber traffic, ii) Ethernet switches that aggregate OLT uplinks, and iii) BNGs, routers that perform subscriber authentication.

 

After two years of providing the gigabit Internet service called GigaPower, AT&T is now planning to attract over 2 million subscribers by the end of 2016.

While trying to win more gigabit Internet subscribers, the operator has inevitably experienced the following challenges:

• Increased CAPEX mainly caused by OLT (Expensive Alcatel 7342 - Chassis-type OLT) and
• Growing OPEX due to L3 CPE (performing complicated features like authentication, IP address allocation to home devices, NAT, and VoIP. required frequent home visits by AT&T service engineers)

Needing a completely new network architecture to keep CAPEX/OPEX down, the operator came up with CORD, whose key technological components are disaggregation, SDN and NFV. 

 

CORD (Central Office Re-architected as a Datacenter) Goal: To become vendor-independent, reduce CAPEX/OPEX, and enable agile service creation by applying disaggregation, SDN and NFV to the access network consisting of CPEs at home, and GPON OLT, Aggregation Switch and BNG at CO. Specifically speaking, 1) Deploy a network using Open H/W (White BoX SDN Switch, X86 server, Open OLT blade, Open ROADM     blade) and Open S/W (ONOS, Openstack, XoS) to:     - Avoid dependency on telecom equipment vendors like Cisco, Nokia/ALU, etc.     - Minimize CAPEX 2) Apply SDN and NFV to:     - Minimize OPEX     - Emulate cloud service providers like Google, Facebook, Amazon in agile service creation

 

To this end, a completely new OLT architecture was defined where vendor-dependency is eliminated through disaggregation of OLT equipment.

 

First of all, the OLT control plane (ONT attachment, authentication, subscriber VLAN allocation/management, IGMP, OAM, etc.), which used to be processed by the CPU in the control board of a legacy OLT, was removed and placed onto the X86 server located in the network (vOLT app in Figure 2). 

 

The role of the control board, which is to control, configure and manage the switching board and GPON line card through the vendor-proprietary interfaces, is replaced by the SDN Controller.

 

The role of the switching board is replaced by the leaf switch in the CO.  

 

Finally, the GPON line cards in the OLT are replaced by Open GPON line card blades which take the form of 1 RU Pizzabox that only have the PON MAC feature.

 

These blades are not parts of a multi-line card chassis OLT system, but are standalone devices in and of itself.

Each open GPON line card blade is connected to a TOR switch through 40Gbps uplink ports. 

 

Figure 2. OLT Disaggregation

 

 

Aggregation switches and BNGs are also disaggregated, transforming the operator's access network as seen in Figure 3.

 

Once the foregoing steps are completed, all network equipment from vendors (like Alcatel, Cisco, Juniper, etc.) in the CO is eliminated, allowing the network to transition to a universal X86 server, White BoX SDN Switch and Open OLT line card blades, all of which can be purchased from the open market.

 

Functions which used to be performed by OLT control plane and BNG's control plane in the legacy system are now handled by the vOLT app. 

 

The vOLT app interworks with i) Radius server (for authentication), ii) SDN controller (for per-subscriber VLAN allocation at OLT line card blade), and iii) SF control application (for path configuration at switch fabric in CO).  

 

 

  

Figure 3. Disaggregation of OLT, Aggregation Switch and BNG at CO

 

L3 CPE (also known as RG, Residential Gateway) installed in subscriber homes is in charge of functions like DHCP and NAT. AT&T virtualized L3 CPE and moved all of its software functions, except for CPE H/W which stays at home, onto the container in the VM of in the server located in the CO. The operator calls this the virtual Subscriber Gateway (vSG). 

 

Figure 4. L3 CPE Disaggregation and Virtualization: vSG (Virtual Subscriber Gateway) 

 

With the foregoing changes, the legacy access network now has a completely innovated, SDN and NFV-based architecture.

 

The data plane in the access network is processed by low-priced open H/W and VNF (vSG). The open H/W are controlled by the SDN controller and NFV orchestrator, which are also open source S/W like ONOS, XOS and Openstack. 

 

Figure 5. Ultimate Architecture (Residential CORD)

 

The idea behind AT&T's CORD was to transform the legacy network, which has been highly telecom equipment vendor-oriented in terms of its deployment and operation, to a completely redesigned, open H/W and open SW-based model.

 

The idea is still considered as one of the most disruptive innovations made in the operators' network architecture over the past few decades.

 

The innovations achieved by AT&T will certainly have a significant impact on the telco ecosystem, including network operators and telecom equipment vendors. It would be worthwhile to look into how they will cope with the changing needs and environment in the industry.