ed on the above, one might be led to believe that the transport network extends from the base station (which in 4G and 5G necessarily has IP routing functionality) and the provider edge (PE) router at the edge of the mobile core. In fact, this is not quite true. Due to the organizational separation between mobile operator and wholesale provider or trans- port division, cell site gateways (CSGs) (BBF TR-221) are required for demarcation at the cell site, and aggregation site gateways (ASGs) at the edge of the mobile core. When trans- port is based on IP or MPLS technologies the CSG is sometimes called a cell site router (CSR), but this term will be avoided here since it is more often used for routers belonging to the mobile operator's network. 3GPP (which, as has been mentioned, is not concerned with transport network issues) does not define CSG or ASG functionality, leaving it to other standards organizations, notably the BBF and MEF. In any case, in this book we define the transport network to extend from the CSGs to the ASGs. Demarcation is a function used in situations where a communication service is provided to an end user, or to a second operator providing an over-the-top service. The purpose of implementing demarcation in a separate gateway, rather than as a function in an existing NE, is to clearly delineate the boundary between the service provider (or transport division) and its client, in order to avoid finger-pointing arguments regarding provisions of SLAs in the case of a wholesaler or service level objectives (SLOs) in the case of a transport division. Thus, the wholesale provider will continuously monitor its QoS parameters using its OAM toolset, and automatically trigger corrective actions when an SLA objective is endangered. Note that the demarcation devices typically belong to the wholesale provider, not to the mobile operator, making it harder to absorb its functionalities into the base station. 6 Enabling Technologies Once in place, a cell site gateway may be used to provide additional functio