Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
Of the branch office WAN connectivity methods provided, dense wavelength division multiplexing (DWDM) is the most expensive. DWDM is a leased-line WAN technology used to increase the amount of data signals that a single fiber strand can carry. To accomplish this, DWDM can transfer data of varying light wavelengths on up to 160 channels per single fiber strand. The spacing of DWDM channels is highly compressed, requiring a complex transceiver design and therefore making the technology very expensive to implement.
Metro Ethernet is not as expensive as DWDM. Metro Ethernet is a WAN technology that is commonly used to connect networks in the same metropolitan area. For example, if a company has multiple branch offices within the same city, the company can use Metro Ethernet to connect the branch offices to the corporate headquarters. Metro Ethernet providers typically provide up to 1,000 Mbps of bandwidth.
Multiprotocol Label Switching (MPLS) is not as expensive as DWDM. MPLS is a shared WAN technology that makes routing decisions based on information contained in a fixed-length label. In an MPLS virtual private network (VPN), each customer site is provided with its own label by the service provider. This enables the customer site to use its existing IP addressing scheme internally while allowing the service provider to manage multiple sites that might have conflicting IP address ranges. The service provider then forwards traffic over shared lines between the sites in the VPN according to the routing information that is passed to each provider edge router.
Integrated Services Digital Network (ISDN) is an inexpensive circuit-switched WAN technology. However, ISDN offers less than 2 Mbps of bandwidth, so it is typically used only for backup WAN connectivity, not for branch office connectivity. Circuit-switched WAN technologies rely on dedicated physical paths between nodes in a network. For example, when RouterA needs to contact RouterB, a dedicated path is established between the routers and then data is transmitted. While the circuit is established, RouterA cannot use the WAN link to transmit any data that is not destined for networks accessible through RouterB. When RouterA no longer has data for RouterB, the circuit is torn down until it is needed again.
Reference:
Cisco: Introduction to DWDM Technology (PDF)