4.1 Mobile Backhaul Application

4.1.1 Backhaul Service

Radio network (together with the core network) offers a service that is accessible for users: a mobile voice call, or internet connectivity for a smartphone, as examples. For a functioning mobile network, a transport service is needed.

Transport service to the radio network is modeled in Figure 4.1 as:

• an integrated transport, which is implemented within a mobile network node, and
• a backhaul service, providing a service between the peer mobile network elements via the external physical interfaces of the mobile network elements and via the intermediate backhaul network.

Both integrated transport, and the backhaul service are required for serving the mobile network application.

Figure 4.1 Transport service.

In Figure 4.1, a transport service consisting of the integrated transport, and the backhaul service, is offered for the radio network layer. The access to the transport service is only available in the mobile elements and not in the intermediate transport network nodes. This is e.g. UDP/IP connectivity for radio network layer Frame Protocol packets with the specified delay, loss, etc. (3G example). Frame protocol over UDP/IP is then terminated at the peer mobile network element, not within the transport network nodes.

At the external interface of a mobile network node (e.g. a NodeB), a backhaul service provides connectivity to the peer mobile network node (e.g. a RNC) at the IP layer. The access to the backhaul service is provided by the physical interface of the mobile network node. Intermediate transport nodes may operate at the IP layer or at L2 or L1, and the backhaul service may consist of multiple legs.

The backhaul service may be a physical layer service, L2 service, or IP layer service. What is required between the peer entities, is an IP layer connectivity. The backhaul transport service is provided by intermediate transport network nodes, either as self-deployed, or as a service of a third party service provider.

The main topic is the backhaul service. Anyway, a bigger part of characteristics of the transport service depend on the backhaul service than on the integrated transport. The integrated transport at a minimum is a termination function and a mapping of the radio network layer PDUs into UDP/IP or similar transport bearers.

The model allows the part of functionality that is implemented into the mobile network node to be discussed. This functionality is also essential. As an example, QoS mapping from the radio network layer attributes to the Differentiated Services code points takes place within the mobile network nodes – it would in fact not be possible in the intermediate transport nodes, since the radio network layer information is not available in the transport nodes.

Some of the aspects for the backhaul service are influenced by the radio network layer requirements. This depends on the radio access technology used (2G, 3G, LTE): every radio access technology has its own characteristics. Another part of the requirements originate from the end user service. Both are important.

In addition to user traffic, network control and management traffic is carried in the mobile backhaul. This includes radio network layer signaling, transport layer control protocols, O&M and potentially synchronization. These protocols are critical, as without a service for these functions, user plane services cannot be maintained either.

Figure 4.2 shows requirements for a transport service – originating from the end user service, radio network layers, transport network control, synchronization and O&M.

Figure 4.2 Transport service.

As discussed in Chapter 03, currently three generations of 3GPP mobile systems exist: 2G, 3G, and LTE. Among these systems there are various enhancements introduced along the way. Mobile packet backhaul supports all these systems, offering different types of services, such as:

• Pseudowire emulation service for 2G BTSs (which is natively TDM).
• Pseudowire emulation service for pre- Rel5 3G NodeBs (which is natively ATM).
• Service for native IP 3G NodeBs (Rel-5 onwards).
• Service for LTE IP eNodeBs (all-IP from the start).
• Control plane connectivity.
• Transport control protocols.
• O&M.
• Synchronization.

The basic service is simply that of information transfer: transfer of higher layer protocol data units between the service access points, timely and without impairments. The service as a whole can be abstracted with the help of a Service Level Agreement and Specifications, which help to describe the externally visible characteristics of the service (delay, packet loss, availability, security, etc.).

The service can be leased from a third party service provider. It can equally be supplied in-house by a transport network department. It is often a combination of the two. In both cases it is useful to model the backhaul as a service. It also allows separating the service from the underlying networking technology.

4.1.2 Access, Aggregation and Core

The single line model of transport between the mobile network elements can be replaced by dividing the backhaul into areas of access, aggregation and core. In Figure 4.3, access and aggregation tiers are shown since they are the most relevant for the mobile backhaul.

Figure 4.3 Access and aggregation tiers.

Networking technologies are often different in these three backhaul segments. In this text the focus is on access and aggregation. For access, it is critical to provide the ‘first mile’ physical connectivity to the BTS: support the traffic mix with the required characteristics. Cost is an issue, due to the amount of access lines needed.

At the aggregation edge, one needs more functions: carrier grade resilience for high availability, QoS mappings, potentially further access control, and multiplexing and mapping of the access lines into the aggregation network.

4.1.3 3GPP Guidance for the Backhaul

3GPP has little guidance on how to build the backhaul: what technology to use and how to use it. The logical interfaces mandate the use of IP (IPv4 or IPv6). At the logical interface, the layer below IP can be Ethernet, PPP, ATM or something else.

In Figure 4.4, mobile elements may connect with any backhaul tier, access, aggregation or backbone. Mobile elements use a service of the backhaul. 3GPP specified protocols are carried transparently over the mobile backhaul. There are no protocol interactions between the radio network layer and the transport network layer in the intermediate backhaul network.

Figure 4.4 3GPP logical interfaces.

Above it was stated that mobile elements use a service of the backhaul. Would a Service Level Agreement (SLA) or other such type of specification with technical requirements for the backhaul be available directly from the 3GPP?

This would be very useful for the design of the backhaul, however, such specification does not exist. Many topics are implementation dependent: radio layer protocol options and timers used, algorithms, and so on. Due to the functionality being split to the mobile network elements, e.g. a delay budget needs to be allocated for every element and for every protocol layer. Also, the service offered by the mobile network to users introduces service-specific requirements to the backhaul. Mapping all these topics into a single generic document would be a difficult task.

4.1.4 Networking and Backhaul

Networking in general uses a model of local area and wide area networks. How do the general networking principles apply to the mobile backhaul?

In the case of an enterprise, a simplified model looks like that of Figure 4.5.

Figure 4.5 A simplified enterprise network. (For enterprise network architectures, see e.g. [1]).

The model is relevant for the mobile backhaul as well: Technologies used in the packet mobile backhaul originate from the IT (Information Technology) and the enterprise world. Protocols are originally designed for this type of application.

In Figure 4.5, hosts connect to a Local Area Network (LAN), which could be a single office building, a floor in that building, or a campus area (with a limited physical distance). For communication between sites, Wide Area Network (WAN) links are needed. IP routers act as gateways to networks on other sites. Each WAN link is treated as a network of its own.

The same networking technologies are used in the mobile backhaul application. A similarity is that the amount of BTSs is rather high even though not fully comparable to the number of hosts. For cost optimization, it is crucial to have low cost ports at the hosts, and a low cost technology for any LAN.

Traffic flow follows a different topology in the mobile backhaul. In the enterprise LAN, not all traffic needs to exit the LAN. Hosts may contact other hosts or servers within the LAN, so direct connectivity via the LAN is a benefit. In the mobile backhaul, all traffic flows from the BTSs are directed towards remote sites. So the emphasis is on arranging site-to-site links in the mobile backhaul. Both applications, the enterprise network and the mobile backhaul, also require wide area connectivity that has high availability, is secure, and supports QoS for the services.

Often the mobile backhaul is not managed commonly with the radio network. Responsibility for the operation of the transport network is separate from the responsibility for the operation of the radio network. This has commonality with enterprise networks: individual sites and local area networks are managed by the enterprise itself. A service provider is often responsible for the wide-area network (WAN) connections. Naturally it is possible to deploy the whole network by the enterprise, or by the mobile operator (a self-deployed backhaul).

In the leased service model, BTSs and other mobile network elements are in the role of customer equipment (CE). At some point in the network, the mobile system interfaces the provider network, through a provider edge (PE) node. As an example, microwave radios may be used as a first mile access technology managed by the mobile operator. Subsequent legs of the transport service may be provided by a service provider.