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With VoIP, network design must be aimed at minimizing delay, jitter, and packet loss in a full duplex environment. Generally speaking, these concerns dictate a hierarchical network consisting of at most three layers: core, distribution, and access. Some smaller networks can collapse the functions of several layers into one device.
The core layer is the heart of the network. Its purpose is to forward packets as quickly as possible. It needs to be designed with high availability in mind. Generally, these high-availability features include redundant devices, redundant power supplies, redundant servers, and redundant links. Core interconnections increasingly use Gigabit Ethernet.
The distribution layer links the access layer with the core. Quality of Service features and access lists are applied at this layer. Generally, Gigabit Ethernet connects to the core and either Gigabit Ethernet or 100base-TX/FX links connect the access layer. Redundancy is important at this layer, but it is not as crucial as it is in the core.
The access layer connects servers and workstations. Switches at this layer are smaller, usually 24-48 ports. Desktop computers and workstations are usually connected at 10 or 100Mbps, and servers are connected at 100 Mbps or 1 Gbps. The access layer has limited redundancy. Some Quality of Service and security features are implemented in this layer.
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