In a well-engineered network, you must be careful to separate functions that occur on the edges of a network from functions that occur in the core or backbone of a network. It is important to separate edge and backbone functions to achieve the best QoS possible.
Cisco offers many tools for implementing QoS. In some scenarios, you can use none of these QoS tools and still achieve the QoS you need for your applications. In general, though, each network has individual problems that you can solve using one or more of Cisco's QoS tools.
This chapter discusses the following tools associated with the edge of a network:
• Additional bandwidth
• Compressed Real-Time Transport Protocol (cRTP)
• Queuing o Weighted Fair Queuing (WFQ) o Custom Queuing (CQ) o Priority Queuing (PQ)
o Class-Based Weighted Fair Queuing (CB-WFQ) o Priority Queuing—Class-Based Weighted Fair Queuing
• Packet classification o IP Precedence o Policy routing o Resource Reservation Protocol (RSVP) o IP Real-Time Transport Protocol Reserve (IP RTP Reserve) o IP RTP Priority
• Shaping traffic flows and policing o Generic Traffic Shaping (GTS) o Frame Relay Traffic Shaping (FRTS) o Committed Access Rate (CAR)
• Fragmentation o Multi-Class Multilink Point-to-Point Protocol (MCML PPP) o Frame Relay Forum 12 (FRF.12) o MTU
o IP Maximum Transmission Unit (IP MTU) This chapter also discusses the following issues associated with the backbone of a network tools:
• High-speed transport o Packet over SONET (POS)
o IP + Asynchronous Transfer Mode (ATM) inter-working
• High-speed queuing o Weighted Random Early Drop/Detect (WRED) o Distributed Weighted Fair Queuing (DWFQ)
Voice over IP (VoIP) comes with its own set of problems. As discussed in Chapter 8, "VoIP: An In-Depth Analysis," QoS can help solve some of these problems—namely, packet loss, jitter, and handling delay. (Serialization delay, or the time it takes to transmit bits onto a physical interface, is not covered in this book.)
Some of the problems QoS cannot solve are propagation delay (no solution to the speed-of-light problem exists as of the printing of this book), codec delay, sampling delay, and digitization delay.
A VoIP phone call can be equivalent to any other large expense you would plan for. Therefore, it is important to know which parts of the budget you cannot change and which parts you might be able to control, as shown in Figure 9-1.
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