Traffic Shaping

Voice Basics

Voice over data includes Voice over IP (VoIP), Voice over Frame Relay (VoFR), and Voice over ATM (VoATM). Each of these three voice over data technologies transports voice, and each is slightly different. Most of the questions you should see on an exam will be related to VoIP, and not VoFR or VoATM, because of the three options, VoIP is the most pervasive. Also calls between Cisco IP Phones use VoIP, not VoFR or VoATM. Imagine a call between the two analog phones in Figure 1-17, extensions 201...

Link Fragmentation and Interleaving

Both types of QoS tools covered in this chapter address bandwidth constraints to some degree. Compression tools directly attack bandwidth constraints by lowering the bandwidth required to forward packets. Link fragmentation and interleaving (LFI) tools directly lower delay by defeating a side effect of a small transmit clock speed, namely serialization delay. A quick review of serialization delay should help you make more sense out of LFI tools. Serialization is the time required to send a...

The Class Selector PHB and DSCP Values

RFC 2475, which defines DiffServ, became an RFC in December 1998. Even today, some QoS features in IOS do not support DiffServ Some QoS features will never support DiffServ, because newer, better tools that can do the same thing may have been introduced. All tools that support Cisco's strategic direction for QoS configuration, using the Modular QoS command-line interface (MQC), support DSCP. However, depending on the tools you need to use, and the IOS revisions you use in your network, you may...

Bandwidth Engineering

To successfully implement CAC mechanisms in your packet network, you must begin with a clear understanding of the bandwidth required by each possible call that can be placed. In Chapter 7, Link-Efficiency Tools, you learned about bandwidth requirements for two of the most popular codecs deployed in converged networks, G.711 and G.729. The G.711 codec specification carries an uncompressed 64-kbps payload stream, known in the traditional telephony world as pulse code modulation (PCM). G.711...

Zone Bandwidth Calculation

The gatekeeper does not have any knowledge of network topology and does not know how much bandwidth is available for calls. Nor does the gatekeeper know how much of the configured bandwidth on the links is currently used by other traffic. The gatekeeper takes a fixed amount of bandwidth, statically configured on the gatekeeper, and subtracts a certain amount of bandwidth for each call that is set up. Bandwidth is returned to the pool when a call is disconnected. If a request for a new call...

VoIP Dial Peer

IOS voice gateways provide many services to connect the packetized, VoIP network to non-packetized, traditional voice services, including analog and digital trunks. IOS gateways perform many tasks, but one of the most important tasks is to convert from packetized voice to nonpacketized voice, and vice versa. In other words, voice traffic entering a router on an analog or digital trunk is not carried inside an IP packet, but the IOS gateway converts the incoming voice to a digital signal (analog...

Committed Access Rate CAR

CAR provides policing functions and marking. Chapter 5, Traffic Policing and Shaping, covers the policing details of CAR and CB policing. However, a quick review of policing before getting into CAR's marking features will help you appreciate why CAR includes marking. Policing, in its most basic form, discards traffic that exceeds a particular traffic contract. The contract has two components a rate, stated either in bits per second or bytes per second and a burst size, stated in either bits or...

Call Rerouting Alternatives

If a call has been rejected by a CAC mechanism due to insufficient network resources, there needs to be some alternate route in place to establish the call. In the absence of an alternate route, the caller will hear a reorder tone. The reorder tone is called a fast-busy tone in North America, and is known as overflow tone or equipment busy in other parts of the world. This tone is often intercepted by Public Switched Telephone Network (PSTN) switches or PBXs with an announcement such as All...

Ip Rtp Priority Configuration

To configure IP RTP Priority, you just add one additional configuration command to an interface that already uses WFQ. There are no additional show commands to list information about IP RTP Priority beyond the commands that apply to the underlying queuing tools (WFQ and CBWFQ). Table 4-15 lists the two configuration commands, and Example 4-9 lists several example configurations. Table 4-15 Command Reference for IP RTP Priority ip rtp priority starting-rtp-port-number port-number-range bandwidth...

Policy Based Routing PBR

PBR enables you to route a packet based on other information, in addition to the destination IP address. In most cases, engineers are happy with the choices of routes made by the routing protocol, with routing occurring based on the destination IP address in each packet. For some specialized cases, however, an engineer may want some packets to take a different path. One path through the network may be more secure, for instance, so some packets could be directed through a longer, but more...

Generic Traffic Shaping Configuration

GTS performs traffic shaping using the same logic and features discussed in the introductory section of this chapter. It can be enabled on a large variety of interfaces. It can also adapt the rate based on BECN signals, and reflect BECNs on a VC after receiving a FECN. It supports a single queuing tool for the shaping queues Weighted Fair Queuing (WFQ). It can also classify traffic, performing shaping on a subset of the traffic on an interface by classifying packets based on access-control...

Committed Access Rate CAR Configuration

CAR has more similarities than differences when compared to CB policing. Both perform policing on all traffic on either an interface or subinterface. Both can classify traffic to police a subset of traffic as well. Both use the same units when configuring policing parameters bits per second for the policing rate, bytes for the normal and Be values, with the configured Be value actually representing Bc + Be. CAR differs from CB policing regarding four main features. The most obvious is that CAR...

WRED Configuration

WRED requires very little configuration if you want to take the IOS defaults for the various tunable settings, such as per-precedence and per-DSCP thresholds. If you want to change the defaults, the configuration details can become quite large. This section begins with a table of configuration commands (Table 6-6) and show commands (Table 6-7), followed by three separate examples. Table 6-6 Command Reference for WRED random-detect dscp-based prec-based Interface or class configuration mode...

Cisco Propagation Delay Serialization Delay

All packets in a network experience some delay between when the packet is first sent and when it arrives at its destination. Most of the concepts behind QoS mechanisms relate in some way to delay. Therefore, a deeper look into delay is useful. Take a look at Figure 1-5 this sample network is used often in this book. Figure 1-5 Sample Network for Discussion ofDelay Figure 1-5 Sample Network for Discussion ofDelay At what points will delay occur in this network Well, at all points, in actuality....

How Shaping Works

Shaping only makes sense when the physical clock rate of a transmission medium exceeds a traffic contract. The most typical case for shaping involves a router connected to a Frame Relay or ATM network. More often today, however, connections to ISPs use a point-to-point serial link or an Ethernet link between an enterprise and the ISP, with a traffic contract defining lower traffic volumes than the physical link. Starting with a physical installation using a higher-speed link allows for simple...

Network Based Application Recognition NBAR

CB marking, and other MQC-based tools, can use NBAR to help classify traffic. By using the match protocol class-map subcommand, MQC can match protocols recognized by NBAR. This section describes NBAR, and includes examples of CB marking with NBAR. NBAR classifies packets that are normally difficult to classify. For instance, some applications use dynamic port numbers, so a statically configured match command, looking for a particular UDP or TCP port number, just could not classify the traffic....

Voice over Frame Relay Voice Bandwidth

In a Voice over Frame Relay (VoFR) network, the frame-relay voice-bandwidth command is used in a Frame Relay map class to set aside the bandwidth required to successfully transport the desired number of calls. This method of bandwidth provisioning operates in much the same way as IP RTP Priority and Low Latency Queuing features that reserve bandwidth for traffic flows. Unlike LLQ or RTP Priority, the frame-relay voice-bandwidth command has the capability to provide CAC. Because VoFR operates at...

Foundation Topics Payload and Header Compression

Compression involves mathematical algorithms that encode the original packet into a smaller string of bytes. After sending the smaller encoded string to the other end of a link, the compression algorithm on the other end of the link reverses the process, reverting the packet back to its original state. Over the years, many mathematicians and computer scientists have developed new compression algorithms that behave better or worse under particular conditions. For instance, each algorithm takes...

Call Admission Control and RSVP

Call admission control (CAC) protects network bandwidth by preventing more concurrent voice and video flows than the network can support. By doing so, it not only protects data traffic, but it also protects the quality of voice and video calls that are already set up. If a network engineer designs a network to support 3 concurrent G.729 calls, for instance, which take roughly 85 kbps, depending on the data links used, but if 10 concurrent calls occur, taking roughly 285 kbps, many bad things...

Classification for Voice Packets into LLQ

As you learned in previous chapters, LLQ is one of the most important Cisco QoS mechanisms to ensure quality for voice conversations, because it prioritizes voice packets over data packets at the router egress interface. For this to work, voice packets must be classified such that they are placed in the priority queue (PQ) portion of LLQ. Cisco IOS provides the service levels that RSVP accepts by working in conjunction with IOS queuing tools. As a general Cisco IOS feature, RSVP has its own set...

Diff Serv Specifications and Terminology

DiffServ is defined by the RFCs listed in Table 2-11. Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers Contains the details of the 6-bit DSCP field in IP header. An Architecture for Differentiated Service This is the core DiffServ conceptual document. Defines a set of 12 DSCP values and a convention for their use. Table 2-11 DiffServ RFCs (Continued) Table 2-11 DiffServ RFCs (Continued) Defines a single DSCP value as a convention for use as a low-latency...

TCP and UDP Reactions to Packet Loss

UDP and TCP behave very differently when packets are lost. UDP, by itself, does not react to packet loss, because UDP does not include any mechanism with which to know whether a packet was lost. TCP senders, however, slow down the rate at which they send after recognizing that a packet was lost. Unlike UDP, TCP includes a field in the TCP header to number each TCP segment (sequence number), and another field used by the receiver to confirm receipt of the packets (acknowledgment number). When a...

Tail Drop Global Synchronization and TCP Starvation

Tail drop occurs when a packet needs to be added to a queue, but the queue is full. Yes, tail drop is indeed that simple. However, tail drop results in some interesting behavior in real networks, particularly when most traffic is TCP based, but with some UDP traffic. Of course, the Internet today delivers mostly TCP traffic, because web traffic uses HTTP, and HTTP uses TCP. The preceding section described the behavior of a single TCP connection after a single packet loss. Now imagine an...

Monitoring and Troubleshooting RSVP

This section covers a few details about the commands used to troubleshoot RSVP installations. Table 8-24 lists other RSVP commands that can be useful in monitoring and troubleshooting RSVP. Table 8-24 RSVP Monitoring and Troubleshooting Commands Table 8-24 RSVP Monitoring and Troubleshooting Commands show ip rsvp neighbor interface-type interface-number show ip rsvp request interface-type interface-number Displays RSVP-related request information being requested upstream show ip rsvp...

Trunk Conditioning

Cisco IOS supports a function called a permanent trunk connection, sometimes called a connection trunk. A connection trunk creates a permanent trunk connection across the VoIP part of the network. To accomplish this, the connection trunk command is configured on a voice port to emulate a permanent connection across a packet network. The bandwidth required by the connection trunk is allocated at the creation of the trunk and remains reserved until the trunk is torn down. Figure 8-11 illustrates...

Cisco Qualified Specialist CQS Program

For any networker in any networking job, it helps to have knowledge and skills. Networkers can benefit from having proof' that they know a set of technologies. Having the right certification on your resume can help you land a job, both at another firm and inside the same company. For those networkers who work with customers and clients, having the right credentials, in the form of certifications, can help convince the salesperson to convince the customer to hire your company for the consulting...

Max Connections

Similar to physical DS0 limitation, Max-Connections uses the concept of limiting the number of simultaneous calls to help protect the quality of voice conversations. Unlike physical DS0 limitation, the max-conn command is a physical gateway configuration applied on a per-dial peer basis. The first advantage that Max-Connections offers over DS0 limitation is the capability to provision for the oversubscription of TDM interfaces on an originating gateway without compromising the quality of the...

Considering the Effects of Packetization and Codec Delay

You need to consider packetization delay and codec delay together, because they do overlap. For instance, packetization delay consumes 20 ms while waiting on 20 ms of voice to occur. But what else happens in the first 20 ms Consider Table 1-18, with a timeline of what happens, beginning with the first uttered sound in the voice call. Table 1-18 Typical Packetization and Codec Delay Timeline G.729 Table 1-18 Typical Packetization and Codec Delay Timeline G.729 Begin collecting voice samples for...

DeJitter Buffer Delay

De-jitter buffer delay is the third voice delay component. Jitter happens in data networks. You can control it, and minimize it for jitter-sensitive traffic, but you cannot eliminate it. Buy why talk about jitter in the section on delay Because a key tool in defeating the effects of jitter, the de-jitter buffer (sometimes called the jitter buffer) actually increases delay. The de-jitter buffer collects voice packets and delays playing out the voice to the listener, to have several ms of voice...

Custom Queuing

Custom Queuing (CQ) followed PQ as the next IOS queuing tool added to IOS. CQ addresses the biggest drawback of PQ by providing a queuing tool that does service all queues, even during times of congestion. It has 16 queues available, implying 16 classification categories, which is plenty for most applications. The negative part of CQ, as compared to PQ, is that CQ's scheduler does not have an option to always service one queue first like PQ's High queue so CQ does not provide great service for...

Summary of Classification and Marking QoS Features

Classification and marking tools can be easily compared based on three general categories. First, some classification and marking tools are specialized, and some are more general. CB marking, CAR, and PBR all perform the same basic function of matching packets based on fields inside the packet header, and marking based on those fields, so these three tools are the more generalized classification and marking tools. Dial peers perform specialized classification and marking functions. This tool is...

Priority Queuing

Priority Queuing's most distinctive feature is its scheduler. PQ schedules traffic such that the higher-priority queues always get serviced, with the side effect of starving the lower-priority queues. With a maximum of four queues, called High, Medium, Normal, and Low, the complete logic of the scheduler can be easily represented, as is shown in Figure 4-9. As seen in Figure 4-9, if the High queue always has a packet waiting, the scheduler will always take the packets in the High queue. If the...

RSVP Configuration

The following three tasks are performed on a gateway to originate or terminate voice traffic using RSVP 1 Turn on the synchronization feature between RSVP and H.323. This is a global command and is turned on by default when Cisco IOS Release 12.1(5)T or later is loaded. 2 Configure RSVP on both the originating and terminating sides of the VoIP dial peers. Configure both the requested QoS (req-qos) and the acceptable QoS (acc-qos) commands. The guaranteed-delay option must be chosen for RSVP to...

Foundation Summary

The Foundation Summary is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final prep before the exam, these tables and figures are a convenient way to review the day before the exam. Table 2-21 lists...

Local Voice Busyout

Several CAC mechanisms generate a busy signal to the originating PBX to indicate that an alternate route must be found to successfully place a call. The preceding section discussed trunk conditioning, which operates on connection trunk networks only. Similar functionality is needed for switched networks. Local Voice Busy-Out (LVBO) is the first of two features that achieve this. LVBO enables you to take a PBX trunk connection to the attached gateway completely out of service when WAN conditions...

QoS Tools That Affect Jitter

Several QoS features can help with jitter issues. You'll find more detail about each of these tools in various chapters throughout this book. For now, however, knowing what each class of QoS tool accomplishes will help you sift through some of the details. The best QoS tool for jitter issues is . . . more bandwidth again More bandwidth helps bandwidth-related problems, and it also helps delay-related problems. If it helps to reduce delay, and because jitter is the variation of delay, jitter...

Traffic Characteristics of Voice Video and Data

So why do you need QoS QoS can affect a network's bandwidth, delay, jitter, and packet loss properties. Applications have different requirements for bandwidth, delay, jitter, and packet loss. With QoS, a network can better provide the right amounts of QoS resources for each application. Table 1-10 lists some major application categories, along with their typical needs for network resources and behavior. Table 1-10 Applications and Their QoS Needs Fragile (For Instance, SNA*) * SNA Systems...

Voice Traffic Characteristics

Voice traffic can degrade quickly in networks without QoS tools. This section explains enough about voice traffic flows to enable the typical reader to understand how each of the QoS tools applies to voice. NOTE This book does not cover voice in depth because the details are not directly related to QoS. For additional information, refer to the following sources Deploying Cisco Voice over IP Solutions, Cisco Press, Davidson and Fox IP Telephony, Hewlett-Packard Professional Books, Douskalis...

Video Traffic Characteristics

Without QoS, video flows typically degrade. The pictures become unclear. Movement is jerky. Movement appears to be in slow motion. Often, the audio becomes unsynchronized with the video. The video can be completely gone, but the audio still works. In short, unless the network has significantly more bandwidth than is needed for all traffic, video quality degrades. Just like the coverage of voice in this chapter, this section breaks down an analysis of video traffic as it relates to the four QoS...

Policing When and Where

Whenever the physical clock rate exceeds the traffic contract, policing may be needed. Suppose, for instance, that ISP1 has 1000 customers, just like PB Tents, each with a 100-Mbps connection, and a contract for support of 2 Mbps. What happens over time Well, without something to prevent it, each customer will send and receive more and more traffic. For a while, all the customers are happy, because their packets make it through the overbuilt ISP1 core. Even if ISP1 has enough capacity to...

The Assured Forwarding PHB and DSCP Values

RFC 2597 defines something called the assured forwarding per-hop behaviors. This RFC suggests that one good DiffServ design choice would be to allow for four different classes for queuing purposes. Within each queue, three levels of drop probability could be implied. The RFC title rightfully suggests that the focus is on the QoS behavior the PHB at each node. Most engineers also think of the RFC as defining the 12 DSCPs that are used in conjunction with the AF PHBs. An individual PHB describes...

Frame Relay Traffic Shaping FRTS Configuration

Frame Relay traffic shaping (FRTS) differs from the other three shaping tools in several significant ways. The most obvious difference is the most important FRTS applies to Frame Relay only. But the basic shaping function is the same, with the same parameters a shaped rate, which is often set to CIR a Tc interval, which defaults to 125 ms and the Bc value is either set, or calculated based on the Tc Bc CIR formula. The first big difference between FRTS and the other shapers has to do with...

Multilink PPP Interleaving Configuration

Before you configure MLP LFI, think about why you would use MLP at all. If you have a point-to-point link, and need to perform LFI, you must migrate from your current Layer 2 protocol to MLP, to use MLP LFI. However, MLP itself has many benefits, and even a few brief thoughts about what MLP does will help you through some of the configuration tasks. MLP enables you to have multiple parallel point-to-point links between a pair of devices, such as routers. The main motivation for MLP was to allow...

Physical DS0 Limitation

Physical DS0 limitation is a design methodology that limits the number of physical DS0 connections into the gateway. This limitation, in conjunction with other queuing methods, ensures that the gateway can successfully provide IP bandwidth across the WAN for each voice conversation originating from the individual DS0 trunks. If you have determined that there is 158.4 kbps of WAN bandwidth available to handle 6 simultaneous G.729 calls, for example, DS0 limitations can be implemented by allowing...

FIFO Queuing

NOTE This section covers FIFO Queuing, which like TX Queues, is not currently covered on the DQOS 9E0-601 exam. This section does cover some useful information for all people interested in QoS. Therefore, for you DQOS exam takers, you might choose to just focus on the concepts about FIFO, and not worry about memorization or configuration. And as always, check the websites listed in the Introduction for any news about changes to the exams. The first reason that a router or switch needs output...

Voice Jitter Considerations

The previous section about delay explained most of the technical details of voice flows relating to jitter. Ifjitter were to cause no degradation in voice call performance, it would not be a problem. However, jitter causes hesitation in the received speech pattern and lost sounds, both when the jitter increases quickly and when it decreases quickly. For instance, consider Figure 1-25, where packets 3 and 4 experience jitter. Figure 1-25 De-Jitter Buffer Underrun Due to Jitter T X - Instant That...

Diff Serv Classifiers and Traffic Conditioners

DiffServ focuses on scalable service differentiation in the Internet according to RFC 2475. In times past, the Internet provided a best-effort service networks generated and sent traffic, and the Internet forwarded it and everyone hoped that market forces helped the Internet grow to accommodate the loads. With the advent of premium offerings from ISPs, new QoS tools and models were needed, with DiffServ being just one component. Because DiffServ focuses on Internet services, the RFCs focus on...

Class Based Policing Configuration

CB policing performs policing using three separate actions for packets that conform, exceed, or violate the traffic contract. (The exact meanings of each of these three categories were covered in the How Policing Works section earlier in this chapter.) Generally speaking, CB policing considers packets that happen to arrive when enough Bc tokens are available as conforming packets. Packets that arrive when Bc is consumed, but Be is not, are considered exceeding and packets that arrive after Bc...

The Integrated Services QoS Model

Integrated services (IntServ) defines a different model for QoS than does DiffServ. IntServ defines a signaling process by which an individual flow can request that the network reserve the bandwidth and delay needed for the flow. The original work grew out of the experiences of the IETF in multicasting the audio and video for IETF meetings in the early to mid-1990s. To provide guarantees per flow, IntServ RFC 1633 describes two components resource reservation and admission control. Resource...

LAN Class of Service CoS

Many LAN switches today can mark and react to a Layer 2 3-bit field called the Class of Service (CoS) located inside an Ethernet header. The CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link (ISL) trunking is used. You can use the field to set 8 different binary values, which can be used by the classification features of other QoS tools, just like IP precedence and DSCP. Figure 3-3 shows the general location of the CoS field inside ISL and 802.1P headers. The term...

Codec Delay

The time required to process an incoming analog signal and convert it to the correct digital equivalent A feature called look-ahead With the first component of codec delay, which is true for all codecs, the IP Phone or gateway must process the incoming analog voice signal and encode the digital equivalent based on the codec in use. For instance, G.729 processes 10 ms of analog voice at a time. That processing does take time in fact, the actual conversion into G.729 CS-ACELP (Conjugate...

QoS Tuning Bandwidth Delay Jitter and Loss

Different types of end-user traffic require different performance characteristics on a network. A file-transfer application might just need throughput, but the delay a single packet experiences might not matter. Interactive applications might need consistent response time. Voice calls need low, consistent delay, and video conferencing needs low, consistent delay as well as high throughput. Users might legitimately complain about the performance of their applications, and the performance issues...

Congestion Avoidance Concepts and Random Early Detection RED

1 Describe the function of the congestion window in TCP, and how it is changed as a result of packet loss. 2 Identify the two TCP windowing mechanisms, and describe when each is used. 3 Describe the process of TCP slow start, and when it occurs. 4 Describe the process of TCP congestion avoidance, and when it occurs. 5 Describe the meaning of the term global synchronization, and discuss what causes it. 6 Define the meaning of the term tail drop. 7 Define the meaning of the term TCP starvation. 8...

Class Based Shaping Configuration

The underlying processes of class-based (CB) shaping are the same as GTS. Just like GTS, CB shaping can be enabled on a large variety of interfaces. It can also adapt the rate based on BECN signals, and reflect BECNs on a VC after receiving an FECN. It can also perform shaping on a subset of the traffic on an interface. Finally, the configuration is simple, because like all other QoS features starting with the words class based, CB shaping uses the Modular QoS command-line interface (MQC) for...

Low Latency Queuing LLQ

Low Latency Queuing sounds like the best queuing tool possible, just based on the name. What packet wouldn't want to experience low latency As it turns out, for delay (latency) sensitive traffic, LLQ is indeed the queuing tool of choice. LLQ is simple to understand and simple to configure, assuming you already understand CBWFQ. LLQ is not really a separate queuing tool, but rather a simple option of CBWFQ applied to one or more classes. CBWFQ treats these classes as strict-priority queues. In...

LLQ Configuration

LLQ configuration requires one more command in addition to the commands used for CBWFQ configuration. Instead of using the bandwidth command on a class, use the priority command. This single additional command is listed in Table 4-24. The syntax for this command is as follows priority bandwidth-kbps percent percentage burst This class subcommand enables LLQ in this class, reserves bandwidth, and enables the policing function. You can also configure the burst for the policer with this command,...

Advanced Voice Busyout

Whereas LVBO provides for busyout based on local conditions of the originating gateway, AVBO adds the capability to trigger an SAA probe to one or more configured IP destinations. The information returned by the probe, which can be either the explicit loss and delay values, or the ICPIF congestion threshold, is used to trigger a busyout of the TDM trunk connection to the PBX. AVBO therefore introduces the capability to busy out a PBX trunk, or individual voice...

Rsvp Cac Summary

Remember the following factors regarding the use of RSVP as a CAC mechanism In current Cisco IOS Software, H.323 synchronization is initiated by default. RSVP packets (path and resv) travel as best-effort traffic. WFQ must be enabled on an interface PVC as a basis for LLQ. RSVP is a true end-to-end CAC mechanism only if configured on every interface that a call traverses. For the unique capability to serve as both an end-to-end CAC mechanisms, and to guarantee the QoS for the entire duration of...

Packetization Delay

The IP Phone or voice gateway must collect 20 ms of voice before it can put 20 ms worth of voice payload into a packet. (The defaults for G.711 and G.729 on IP Phones and Cisco IOS Software gateways are to put 20 ms of voice into an RTP packet the value can be changed.) Therefore, for the sake of discussion in this book, we consistently consider packet delay always to be 20 ms in examples. That is, the speaker must talk for 20 ms before a packet containing 20 ms of voice can be created.

The Expedited Forwarding PHB and DSCP Values

RFC 2598 defines the expedited forwarding per-hop behaviors. This RFC defines a very simple PHB (low latency, with a cap on bandwidth), and a single DSCP (EF) to represent it. Expedited forwarding simply states that a packet with the EF DSCP should minimize delay, jitter, and loss, up to a guaranteed bandwidth level for the class. Like AF, the EF RFC suggests two QoS actions be performed to achieve the PHB. First, queuing must be used to minimize the time that EF packets spend in a queue. A...

CBWFQ Configuration

CBWFQ configuration uses the same MQC commands as does CB marking, with a few additional commands added. If you remember how to configure CB marking from Chapter 3, CBWFQ configuration will be easy for you. The commands used for CP marking and CBWFQ configuration are repeated here in Tables 4-11 and 4-12. Table 4-11 Command Reference for CBWFQ Global config names a class map, where classification options are configured. Class map subcommand defines specific classification parameters. match...

RSVP Synchronization Configuration

To configure RSVP for use with H.323, you need to enable RSVP as normal and add a few commands to the H.323 dial peers. By default, RSVP is disabled on each interface to remain backward compatible with systems that do not implement RSVP. To enable RSVP for IP on an interface, use the ip rsvp bandwidth command. This command starts RSVP and sets the maximum bandwidth for all RSVP flows combined, and a single-flow bandwidth limit. The default maximum bandwidth is up to 75 percent of the bandwidth...

TCP and RTP Header Compression Configuration

Unlike payload compression, Cisco IOS Software does not have different variations on the compression algorithms for TCP and RTP header compression. To enable TCP or RTP compression, you just enable it on both sides of a point-to-point link, or on both sides of a Frame Relay VC. Note that when enabling compression, it is best practice to enable the remote side of the WAN link before enabling the local side of the WAN link. This enables the administrator to retain control of WAN connectivity. If...

WFQ Classification

Flow-Based WFQ, or just WFQ, classifies traffic into flows. Flows are identified by at least five items in an IP packet Transport layer protocol (TCP or UDP) as defined by the IP Protocol header field TCP or UDP destination port Depending on what document you read, WFQ also classifies based on the ToS byte. In particular, the CCIP QoS exam expects WFQ to classify based on the ToS byte as well as the five items listed previously. The DQOS course, and presumably the DQOS exam, claims that it...

WRED Summary

WRED provides a valuable tool for managing congestion in queues. Cisco IOS uses defaults that conform to the DiffServ Assured Forwarding conventions, which reduce the likelihood that you will need to configure thresholds for WRED. WRED can be particularly effective when used with MQC-based queuing tools, but when enabled directly on an interface, WRED has the unfortunate side effect of disallowing other queuing tools to be used. Table 6-8 lists some of WRED's key points. Discards packets to...

CB Marking show Commands

CB marking provides only one show command that provides statistical information show policy-map interface. The statistics do provide some good insight to the packet volumes being marked by CB marking. The next sample configuration includes a new configuration and several variations of the show policy-map command. The same network is used for the next example as was used in the other CB marking examples, but with different marking criteria. In this case, traffic is generated so that the show...

Traffic Shaping When and Where

Networks use traffic shaping for two main reasons To shape the traffic at the same rate as policing (if the service provider polices traffic) To avoid the effects of egress blocking For instance, consider Branches 1 and 24 in Figure 5-3. Branch 1 does not shape, whereas Branch 24 does shape to 96 kbps. In both cases, the Frame Relay switches they are connected to police packets at a 96-kbps rate. (The CIR in each case is 64 kbps. Therefore, the service provider is not policing aggressively. The...

LLQ with More Than One Priority Queue

Some Cisco documentation claims that you can only have one low-latency queue inside a single policy map. In other words, only one class can use the priority command, making it a low-latency queue. Other Cisco documentation claims that you can have more than one low-latency queue in a single policy map. As it turns out, you can have multiple low-latency queues in a single policy map. Why would you need more than one low-latency queue in a policy map, and how would it work Well, it's actually...

WFQ Summary

WFQ works well for networks where the most delay-sensitive traffic requires less bandwidth than the average flow. Also flows with higher precedence work well, with low-volume, high-precedence flows receiving exceptional treatment. Best of all, WFQ requires no classification configuration. As a result, WFQ provides a great default queuing choice, particularly when traffic characteristics are unpredictable and difficult to qualify. WFQ works poorly for voice and interactive video traffic, because...

Gatekeeper Zone Bandwidth

The gatekeeper function is an IOS-based mechanism specific to H.323 networks. Different levels of Cisco IOS Software provide various, specific capabilities within this feature. In Cisco IOS Releases 12.1(5)T and 12.2, the gatekeeper function has the capability to limit the number of simultaneous calls across a WAN link similar to the CallManager location-based CAC discussed in the preceding section. By dividing the network into zones, a gatekeeper can control the allocation of calls in its...

Channel Partner IP Telephony Technology Specializations

The most popular reason for taking the DQOS exam relates to the Cisco Channel Partner program. Cisco calls their resellers and services partners Channel Partners. The way the program works is that Cisco moves more than 90 percent of their product sales, in dollar volumes, through their Channel Partners. Thus, Cisco is motivated to help themselves by working well with their Channel Partner community. Cisco also focuses heavily on customer satisfaction. So Cisco uses both a carrot and a stick to...

The Cisco DQOS Exam 9E0601

The DQOS exam consists of a one-hour exam administered at a proctored exam facility affiliated either with VUE (www.vue.com) or Prometric (www.2test.com). The exam typically includes approximately 70 questions. Cisco lists the topics covered in the DQOS exam on their web site the list is repeated in this introduction. Like many Cisco exams, the DQOS exam covers the topics in the Cisco DQOS course. In fact, the list of exam topics for the exam are actually taken from the list of objectives that...

Goals of This Book

Unquestionably, the primary goal for this book is to help you pass the DQOS certification exam. However, the means by which that goal is accomplished follows the Cisco Press Exam Certification Guide philosophy, which makes a statement about helping a reader pass the test through a deeper understanding of the material, as opposed to simply helping the reader memorize the answers to multiple-choice questions. To accomplish this goal, the book's main chapters cover all the topics on the DQOS exam,...

Voice Bandwidth Considerations

Voice calls create a flow with a fixed data rate, with equally spaced packets. Voice flows can be described as isochronous, which, according to Dictionary.com, means characterized by or occurring at equal intervals of time. Consider Figure 1-19, where a call has been placed between analog phones at extensions 201 and 301. Figure 1-19 Isochronous Packet Flow for Voice Call Figure 1-19 Isochronous Packet Flow for Voice Call R1 creates the IP UDP RTP voice payload packets and sends them, by...

Output Queues TX Rings and TX Queues

NOTE This section covers the concepts behind TX Rings and TX Queues. Interestingly, the DQOS exam 9E0-601 does not mention these topics at all, but the QoS exam does. For a true understanding of queuing inside IOS, however, you must understand these concepts. Therefore, for you DQOS exam takers, you might choose to just focus on the concepts about TX Queues and TX Rings, and not worry about memorization. As always, check the websites listed in the Introduction for any news about changes to the...

Subnet Bandwidth Management

Subnet bandwidth management (SBM) specifies a signaling method and protocol for LAN-based CAC for RSVP flows. SBM allows RSVP-enabled Layer 2 and Layer 3 devices to support reservation of LAN resources for RSVP-enabled data flows. SBM uses the concept of a designated entity elected to control the reservations for devices on the managed LAN. The elected candidate is called the designated subnetwork bandwidth manager (DSBM). It is the DSBM's responsibility to exercise admission control over...

WFQ Scheduling The Process

WFQ gives each flow a weighted percentage of link bandwidth. However, WFQ does not predefine queues like class-based queuing tools do, because WFQ dynamically classifies queues based on the flow details. And although WFQ ends up causing each flow to get some percentage of link bandwidth, the percentage changes, and changes rapidly, because flows come and go frequently. Because each flow may have different precedence values, the percentage of link bandwidth for each flow will change, and it will...

Classification and Marking Tools

A variety of classification and marking tools exist. Classification and marking tools first classify by looking at something inside each packet you can compare these tools by listing the fields the tool can examine. Classification and marking tools mark the frame or packet based on the earlier comparisons you can compare these tools by listing the fields that can be marked. Some classification and marking tools also perform other functions, as noted in Table 2-2. Chapter 3 explains the details...

Queuing Tools

QoS queuing tools provide you with a variety of queuing methods. Queuing tools define a number of queues. Cisco publishes the queue service algorithm in some cases in others, Cisco publishes only the end result (the what), but not the algorithm (the how). Table 2-3 outlines the key features of IOS queuing methods. Table 2-3 Comparison of Queuing Tools Table 2-3 Comparison of Queuing Tools Queue Service Algorithm End Result of Algorithm Strict service always serves higher-priority queue over...

Queuing on Interfaces Versus Subinterfaces and Virtual Circuits VCs

IOS queuing tools create and manage output queues associated with an interface, and then the packets drain into the TX Ring Queue associated with the interface. IOS also supports queuing on subinterfaces and individual VCs when traffic shaping is also enabled. Shaping queues, created by the traffic-shaping feature, drain into the interface output queues, which then drain into the TX Ring Queue. Like the interface output queues, the shaping queues can be managed with IOS queuing tools. The...

The Cisco CCIP Certification

The CCIP Certification requires that you pass four exams, with the QOS exam being one of those. The four exams are listed in Table I-5. Table I-5 CCIP Certification Exams Building Scalable Cisco Internetworks Building Scalable Cisco Internetworks (BSCI) 642-801 Border Gateway Protocol (BGP) 642-661 Multiprotocol Label Switching (MPLS) 640-910

Traffic Shaping Excess Burst and Token Buckets

Traffic shaping includes the capability to send more than Bc in some intervals. The idea is simple Data traffic is bursty, so after a period of inactivity, it would be nice if you could send more than Bc in the first interval after traffic occurs again. This extra number of bits is called the burst excess, or Be. Traffic-shaping tools allow Be as an option. The underlying operation of traffic shaping to allow for Be requires a little more insight into how traffic shaping works, and it also...

Random Early Detection RED

Random Early Detection (RED) reduces the congestion in queues by dropping packets so that some of the TCP connections temporarily send fewer packets into the network. Instead of waiting until a queue fills, causing a large number of tail drops, RED purposefully drops a percentage of packets before a queue fills. This action attempts to make the computers sending the traffic reduce the offered load that is sent into the network. The name Random Early Detection itself describes the overall...

Resource Availability Indication

To allow gatekeepers to make intelligent call routing decisions, the terminating gateway uses resource availability indication (RAI) to report resource availability to the gatekeeper. Resources monitored by the terminating gateway include DS0 channels and DSP channels. When a monitored resource falls below a configurable threshold, the gateway sends an RAI message to the gatekeeper indicating that the gateway is almost out of resources. When the available resources then cross above another...

Congestion Avoidance Tools

This book covers three congestion-avoidance tools. One of the tools was never implemented in IOS (Random Early Detection, or RED) but because the other two features are based on RED concepts, Chapter 6, Congestion Avoidance Through Drop Policies, covers the basics of RED as well. All Congestion-avoidance tools consider the queue depth the number of packets in a queue when deciding whether to drop a packet. Some tools weigh the likelihood of dropping a packet based on the IP precedence or IP...

Location Based CAC Operation

The Cisco CallManager centralized call-processing model uses a hub-and-spoke topology. The host site, or hub, is the location of the primary Cisco CallManager controlling the network while the remote sites, containing IP endpoints registered to the primary CallManager, represent the spokes. The Cisco CallManager Administration web page can be used to create locations and assign IP endpoints, such as IP Phones, to these locations. After the locations have been created and devices have been...

Traffic Policing and Traffic Shaping Concepts

Traffic shaping and traffic policing both measure the rate at which data is sent or received. Policing discards excess packets, so that the overall policed rate is not exceeded. Shaping enqueues the excess packets, which are then drained from the queue at the shaping rate. In either case, both policing and shaping prevent the traffic from exceeding the bit rate defined to the policer or shaper. This section covers concepts related to both shaping and policing. It starts with some of the...

Queuing and Traffic Shaping

Shaping tools support a variety of queuing tools that can be applied to the packets waiting in the shaping queue(s). At the same time, IOS supports queuing tools for the interface output queue(s) associated with the physical interface. Deciding when to use queuing tools on shaping queues, when to use them on the interface, and how the configurations differ in each case, can be a little confusing. This section clears up some of that confusion. To begin, Table 5-5 lists the traffic-shaping tools,...

WRED and Queuing

WRED relies on the average queue depth concept, which calculates a rolling average of the queue depth of some queue. But which queue Well, first consider a serial interface on a router, on which Weighted Fair Queuing (WFQ) is enabled by default. In this case, however, WFQ has been disabled, leaving a single first-in, first-out (FIFO) output queue on the interface. Figure 6-9 shows the basic idea. Figure 6-9 FIFO Output Queue and WRED Interaction Figure 6-9 FIFO Output Queue and WRED Interaction...

Zone Bandwidth Configuration

As of Cisco IOS Software Release 12.1(5)T and Release 12.2, the following types of zone bandwidth limitations can be configured on the gatekeeper The maximum bandwidth for all H.323 traffic between the local zone and a specified remote zone. The maximum bandwidth allowed for a single session in the local zone. This configuration is typically used for video applications, not for voice. The maximum bandwidth for all H.323 traffic allowed collectively to all remote zones. Tables 8-16 and 8-17 list...

Single Zone Topology

Figure 8-22 Simple Single-Zone Topology Figure 8-22 Simple Single-Zone Topology Most networks, however, are not as simple as the one shown in Figure 8-22. Figure 8-23 shows a more complex topology however, it is still configured as a single-zone network. In this topology, the legs in the WAN cloud each have separate bandwidth provisioning and therefore separate capabilities of how many voice calls can be carried across each leg. The numbers on the WAN legs in Figure 8-23 show the maximum number...