 different VLANs. Each VLAN is identified by a Q-tag (also called a VLAN ID), which has a length of 12 bits, resulting in 4096 different values (from which two are reserved for network administration). Such an approach 4.4 QoS in Ethernet and Metro Ethernet works well in a single enterprise. However, when multiple customer networks are connected to the Internet via a shared Ethernet infrastructure by using virtual LANs, the VLANs are separated by using different Q-tags for different VLANs. For example, VLANs can be a convenient solution for virtual separation of the same physical Ethernet infrastructure within a single enterprise. Provider bridges (IEEE 802.1ad). Provider bridges are introduced with IEEE 802. lad [28]. They put an additional tag in the Ethernet frame (besides the Q-tag), called the S-tag (or provider ID). Its length is 12 bits (the same length as the Q-tag). Due to two same-length tags (S-tag, i.e. Provide ID, and Q-tag, i.e. VLAN ID), this approach is also called Q-in-Q. Provider backbone bridges (IEEE 802.1ah). PBBs are standardized with IEEE 802.1ah [29], which adds source and destination MAC addresses in Ethernet frames (at UNI) when they enter the PBB network. For this purpose PBB uses an additional tag, called I-tag (with length of 24 bits), to identify the services (such as Q-in-Q networks being interconnected via the PBB). For the network deployment, only switches on the edge of the PBB network should be PBB-enabled, while other Ethernet switches in the core PBB network can be only provider bridges. Figure 4.7 illustrates the use of S-tags, Q-tags, and I-tags for deployment of Carrier Ethernet networks. IP networks Backbone DA Backbone SA B-Tag I-Tag Customer DA Provider backbone bridges (PBB) network Customer SA S-Tag C-Tag Customer DA Customer SA Payload S-Tag C-Tag Provider network Checksum (Q-in-Q) Ethernet frame Payload Customer DA Checksum Customer SA Ethernet frame Customer C-Tag network Payload Checksum B-tag - Backbone VLAN tag C-Tag - Customer VLAN tag (customer VLAN ID) Etherne