The business network services many VoIP users and has greater data traffic. New issues arise and new solutions are required. Data and voice converge and are using the same network infrastructure. Quality of Service (QoS) becomes very critical in order to provide stable voice communications.
The critical areas of design for a Business network for traffic and congestion are:
The first step is to separate the voice and data traffic. Two separate networks could be used but that would be expensive and a waste of resources. A better solution is to VLAN the network. Layer 2 Ethernet switches are used to divide the physical network into Virtual LANs (VLANs). These VLANs can span across many switches and many floors of a building. This physically isolates the data traffic from the voice traffic.
Instead of routers, Layer 3 switches can be used to route between VLANs. A layer 3 switch has limited routing capabilities and unlike a router, it doesn't support WAN protocols. Layer 3 switches support VLANs by their nature.
3 Layer Hierarchial Model
As the network grows, a redundant mesh topology can be used by dividing the network into 3 distinct layers. Cisco developed the 3 Layered Hierarchial Model to address the needs of a large business network.
3 Layer Hierachial Model
The 3 Layer Hierarchial model provides redundancy through the network by providing a mesh network. Loops are prevented by using the Spanning Tree Protocol to automatically detect loops and block redundant paths. It is a dynamic protocol and detects when links fail and auto fall-over to the backup paths.
Quality of Service
Network layer Quality of Service (QoS) can be implemented by assigning priority to devices through the IP header's ToS/DiffServ field. ToS stands for Type of Service and consists of 3 bits. This gives a range of priorities from 0 (default) to 7 (highest priority. Typically, data has a priority of 0 and voice traffic has a priority of 5.
DiffServ is an improved version of ToS. It stands for Differentiated Services and consists of the ToS's 3 bits plus 3 more. You either use ToS or DiffServ but not both. For this simplified explanation, DiffServ has a priority range of 0 (default) to 64 with data having a priority of 0 and voice a priority of 46.
In this manner, voice traffic will have priority over data traffic as it is transmitted throughout the network.
Quality of Service can be implemented at the Data Link layer by using the IEEE802.1Q (DOT1Q) VLAN frame tagging protocol. VLANs can be given priority by using the Classification of Service (CoS) field in the DOT1Q tagged frame. The CoS field consists of 3 bits with a priority of 0 (default) to 7. This leads directly to MultiProtocol Label Switching (MPLS) which is a complete book in itself.
Congestion on the WAN Link
As a network administrator, you have complete control over your LAN based network. But you have limited control over the WAN link. You can control the QoS of traffic leaving your network through sound QoS principles discussed previously. You can also give priority to voice protocols such as SIP, IAX, RTP, SCCP, H.323, etc.. by incorporating an Application Layer Gateway (ALG). An ALG examines contents of the packets leaving the network and can give priority to protocols related to the voice traffic.
You do not have control of the priority of the traffic entering your network unless you have a Service Level Agreement (SLA) with your service provider. An SLA will usually implement a connection to the service provider's MPLS network to control priority.
Quality of Service will give voice priority over data but the question that needs to be asked is "How much bandwidth is needed for voice traffic?". The WAN page will aid in determining how many voice channels are required for a trunk and how much bandwidth is needed through the WAN link.
There are many options available for the WAN link but be aware that some provide asynchronous bandwidth between uploads and downloads. Cable modems and ADSL are examples. Typically, the upload bandwidth is a fraction of the download bandwidth. For example, basic ADSL provides 3 Mbps download but only 500 kbps upload! A better solution is Synchronous DSL which has equal upload and download bandwidths.
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