Traditionally, enterprises have maintained two separate networks: a voice network, based on PBX and PSTN and an IP network for data applications, such as email, Web, VPN, etc. VoIP technology allows the separate voice and data networks to be merged into a single network because voice can be treated as just another application running over the IP (data) network. Traditional PBX and phone extensions can still be supported using hybrid PBXs that can manage traditional (analog) phones as well as VoIP phones connected via Ethernet. Many enterprises have chosen to deploy a single IP network in their new office buildings where there is no need to support existing analog (PSTN) phone extensions. In addition, many enterprises have replaced their analog phone networks with VoIP [9].

Figure 2 shows an example enterprise network with desktop and notebook clients connected to LAN and WLAN segments. The WLAN segment is operated by an IEEE 802.11 Access Point (AP). These segments are connected to the corporate network via a wired backbone. Other components on the corporate network shown are enterprise servers, a hybrid PBX, legacy phones, and VoIP phones. In addition to these components, call managers are responsible for registration and authentication of VoIP clients and for routing VoIP signaling messages to the correct VoIP endpoints. Call managers also manage phone extensions mapped to VoIP phones (soft and hardphones). A media gateway, on the other hand, converts voice samples from VoIP packets on the VoIP network to a modulated analog signal used by the PSTN.

Beyond the AP, the enterprise network comprises a WLAN Distribution System (DS)–typically implemented as an Ethernet-based LAN, Layer 3 routers, switches, IP / Multi-Protocol Label Switching (MPLS)/Differentiated Services (DiffServ) domains, etc. Networks at multiple sites may be interconnected via VPN/MPLS tunnels/frame relay to give the appearance of a single corporate network. A VoIP call initiated from a WLAN client might terminate at an end host that is outside the Extended Basic Service Set (BSS) serviced by the WLAN DS. In order to provide service differentiation across the entire end-to-end VoIP data flow path, the AP and each router/switch on the way must be configured to identify and prioritize VoIP flows. Details of QoS support required in various enterprise network components are given in a later section.

Figure 2: Enterprise VoIP deployment across wired and wireless networks
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VoIP QoS over Wired Network

In order to provide service differentiation over the end-to-end VoIP data flow path, each router/switch on the way must be configured to identify and prioritize VoIP flows. This can be achieved as follows using Differentiated Services Code Point (DSCP), and it is also shown in Figure 2.

1. VoIP client endpoints must mark the VoIP packets that they send out with a pre-configured DSCP value.
2. In a large enterprise network consisting of multiple campuses/sites, when a VoIP packet arrives at a border router in the VoIP sender's network, it is identified as a VoIP packet from the DSCP in the IP header. The border router may use another DSCP value to ensure that this packet is given a prioritized treatment in the core enterprise network or VPN/Frame Relay tunnels that connect multiple sites.
3. When the VoIP packet reaches a border router in the receiver's network, it is identified as a VoIP packet based on the DSCP marking used in the core enterprise network. The border router may change the DSCP to a value that is reserved for VoIP flows in the receiver's network. The packet is then forwarded towards the receiver.

An IP PBX can use DSCP tags on its signaling packets so that these are forwarded with a higher priority over best-effort traffic in the enterprise LAN. If a SIP server is used for signaling, it can also send SIP signaling packets with the right QoS tags. A media gateway acts like another VoIP endpoint as far as the data network is concerned and hence it also tags the VoIP packets it sends with the appropriate DSCP values.