An E-Transit is an OVC-based Carrier Ethernet service in which all OVC End Points are at ENNIs.
Transit E-Line service which is a Point-to-Point OVC (exactly 2 OVC End Points) associating two ENNIs.
Transit E-LAN service which is a Multipoint-to-Multipoint OVC (two1 or more OVC End Points), associating only ENNIs.
An E-Line, E-LAN, or E-Tree service may require access to remote UNIs through an intermediate Carrier Ethernet Network (CEN) that does not contain any of the UNIs in the EVC. If the OVC in the intermediate CEN only requires two end points, a Transit E-Line service can be purchased from an Operator. Figure 1 at right shows two Transit E-Lines within a CEN, each connecting two OVC End Points at two ENNIs. These OVCs are used by the Service Provider to create two EPL services.
An E-Line, E-LAN, or E-Tree service may require access to remote UNIs through an intermediate Carrier Ethernet Network (CEN) that does not contain any of the UNIs in the EVC. If the OVC in the intermediate CEN requires two1 or more OVC endpoints a Transit E-LAN service can be purchased from an Operator. Figure 2 at right shows a Transit E-LAN service within a CEN providing connectivity for an EVP-LAN service associating UNIs in three CENs.
In Figure 3 at right, both remote UNIs are in the E-LAN service are in CEN C. CEN C can only provide point-to-point services to access the UNIs (Access E-Line services), so a Transit E-LAN service with hairpin switching is used in CEN A.
1 The difference between a Transit E-LAN service with two OVC endpoints and a Transit E-LINE service is that the Transmit E-LAN can associate additional OVC endpoints in the future whereas the Transit E-LINE cannot.