qos

Quality of Service in Integrated Voice, Video, and Data Networks

A Memotec Communications White Paper

Introduction

Advances in todays technology enabling integrated provisioning of Voice, Video, andData services offer much promise to enterprise customers in enabling them to both savemoney on their telecommunications costs and offer new services on their networkenhancing productivity. Digital Signal Processing (DSP) technology and VoiceCompression algorithms have advanced to the point that toll quality voice can be offeredover these new integrated infrastructures, with no noticeable degradation of voice qualityto users.

These advances, though, are useless if an effective mechanism cannot be implemented forassuring Quality of Service (QoS) for these integrated applications. Todays backboneintegrated infrastructures may be Frame Relay based, ATM based, IP based, satellitebased, or ISDN based, or any combination of these, and an effective mechanism forassuring QoS must work regardless of how the backbone infrastructure is implemented.

Memotec offers a complete, working solution for these QoS issues in todaysheterogeneous networks. These mechanisms are built upon field proven techniques,which enable high quality voice, video, and data to be delivered for todays demandingbusiness applications.

Issues Involved in Assuring QoS

There are many issues that may affect the QoS delivered for network based integratedservices. Essentially, these issues arise from the fact that different traffic types requiredifferent levels of service from the network. Some examples of these are:

Packetized Voice Traffic. Packetized Voice traffic is characterized as relatively lowbandwidth (typically 8 Kbps), but requiring a low latency delivery to ensure highquality audio.

Video Traffic. Video Traffic is generally higher bandwidth (128 Kbps to 384 Kbpsor more), but still requiring low latency for high quality video images.

File Transfers. File transfers require high bandwidth, but can be allowed to sufferlatency through the network.

E-Mail. E-mail is typically low bandwidth, and can also be allowed to suffer somelatency through the network.

Legacy applications. Legacy applications such as SNA may require moderatebandwidth, with moderate latency requirements.

Quality of Service in Voice, Video, and Data Networks 2000 Memotec Communications, Inc.

If one analyzes these requirements, one can see that a mechanism for ensuring QoS in anetwork must be capable of: identifying traffic types (even specific applications runningover IP); prioritizing these traffic types; and then delivering them over the network insuch a way that the QoS requirements for the service type are met.

What are some of the issues that may affect the QoS offered to these services?

1. Large packets delivered from lower priority, high bandwidth applications may affectthe latency for higher priority, latency intolerant applications (such as voice). Forexample, a 1500 byte packet delivered as part of a file transfer over a 64Kbps linkwill take 187 ms to be transmitted. This means a voice packet cannot be transmittedduring this interval. As a result, voice cuts or delays will be heard for voice trafficqueued behind this large packet.

2. Different speed links in the network may mean that packets can get queued internallyin the network. When packets queue internally in the backbone of a network, latencyand therefore quality can be affected.

3. Network based IP applications may not respect the QoS policy set up for the network.In a network of hundreds of PCs, it may be impossible to adequately police QoSpolicies on each desktop, thus resulting in policy violations which can affect QoS.

Data PacketVoicePacket

187 ms Delay

64 Kbps2 Mbps

Packets will queueinternally in thenetwork waiting forslower link


A flexible Quality of Service mechanism must be able to handle all these traffic typeswithout affecting the quality offered to other services. In addition, the QoS mechanismsmust be designed to operate over a reasonably large set of network topologies andpotential congestion conditions.

Requirements for Effective QoS

In order to effectively manage QoS, therefore, based upon the above requirements,several requirements must be met:

1. Quality of Service at Link Level must be employed. If the user is running IP over aFrame Relay network, then if the device employed cannot ensure QoS over FrameRelay, then the QoS mechanism employed at the IP layer will not be effective.Therefore, a solution that effectively implements a QoS mechanism must be able todeliver the QoS at the Frame Relay or ATM layer.

2. Mechanisms for Identifying Traffic Types must be employed. If the mechanismemployed cannot tell if an IP packet is important (perhaps Voice) or less important(perhaps FTP), then the mechanism will not be effective.

3. Mechanisms for Implementing a QoS Policy must be employed. If the deviceemployed cannot police the network and ensure that a rogue user cannot override aquality policy, then QoS cannot be adequately ensured. Packets should be capable ofbeing identified as to specified quality policy, and then enforced.

4. Mechanisms used to implement the QoS mechanism must cooperatively interact withother, third party, network elements. If the QoS mechanism only works with a singlevendors equipment, then the network may become unmanageable if third partydevices are added later.

Memotec has implemented a QoS mechanism within its CX- series which meets thesecriteria, and enables real world management of QoS in real world networks under realworld conditions.

LAN WAN

If this FTP packet is tagged ashigh priority, other voice trafficmay be affected


Memotecs Quality of Service Mechanisms

As outlined in the previous section, the Quality of Service delivered to IP basedapplications is only as good as the Quality of Service mechanism implemented at the linklayer. Memotecs primary objective is to enable QoS for IP, Frame Relay, or ATMbased applications, and to use link layer (Frame Relay and ATM) QoS mechanismsto ensure IP based applications receive their requested QoS. This is an extremelykey point, and the foundation upon which Memotecs QoS mechanisms are built. Forexample, since ATM natively provides the ability to offer different QoS for differentVirtual Circuits, an effective QoS mechanism will permit higher priority IP traffic to besent over higher priority, traffic tuned PVCs, while lower priority IP traffic is sent over adifferent PVC with different traffic parameters. The advantage of this approach, that is,using link layer QoS mechanisms to ensure IP QoS means that IP can take advantage ofthe sophisticated QoS mechanisms built in to ATM to assure QoS for IP applications.

Frame Relay based Quality of Service Mechanisms

Memotec has implemented a sophisticated, field proven mechanism for ensuring QoSover Frame Relay networks. The mechanism is built upon four basic foundations:

1. Fragmentation. As outlined previously, larger low priority packets when transmittedcan delay smaller, high priority packets. Fragmentation ensures that the larger packetis broken into smaller pieces to make the delay not perceptible. However, excessivefragmentation may result in poor data throughput. It is important to ask your vendorhow much fragmentation is required to assure no impact to higher priorityapplications.

2. Prioritization. Memotec implements four priority queues internally which can beused to sort traffic. Voice and Video traffic, for example, may use the highest priorityqueue, while FTP traffic may use the lowest queues. Users may decide, based upontheir desired QoS policy, what traffic goes into what priority queue. In addition, theuser may control the relative priority between these queues in order to fine tune thedesired QoS policy.

3. Transmission Scheduling. This is an important mechanism which is not implementedby many vendors. Packets to be transmitted over a link are not transmitted basedupon an as soon as possible mechanism, but are transmitted on a as soon as theremote side can receive it mechanism. This mechanism is designed to avoid queuingin the backbone network which can cause impact to high priority traffic. This is asubtle issue, but quite important once the network is operational in a real worldsituation.

4. Congestion Management. The Frame Relay congestion management mechanisms,such as Discard Eligible (DE), Backward Explicit Congestion Notification (BECN),and Forwards Explicit Congestion Notification (FECN) are implemented in such away that in times of network congestion or transmission in excess of the CommittedInformation Rate (CIR), lower priority packets are tagged to be discarded if necessaryby the network rather than the higher priority voice packets.


Memotec employs techniques that enable a user to specify which protocol uses whichpriority queue, which means that he can effectively tailor the mechanisms to his desiredquality policy.

These four mechanisms, taken together, form a sound foundation for delivery of QoS atthe Frame Relay level. As you will see later, they also support the delivery of QoS at theIP level. In addition, they are interoperable with public or private Frame Relaybackbones, which means that they will work even if the backbone of the network is notbased upon Memotec equipment.

Quality of Service for H.320 Video over Frame Relay

When utilizing H.320 Video traffic over a Frame Relay network, the above QoSmechanisms are used to ensure that the video traffic is treated appropriately. MemotecsVideoFramer converts the H.320 video stream into a Frame Relay stream, which can thenbe internally prioritized as above to ensure that it is treated as high priority traffic.

ATM Based Quality of Service Mechanisms

For ATM based networks, Memotec has an equally strong set of mechanisms to supportQoS. These mechanisms take advantage of some of the inherent mechanisms that ATMhas to support QoS, but extend beyond them as well.

Fundamentally, ATM enables Virtual Circuits to be defined with a set of trafficparameters that control the flow of data through the ATM network. Once a set of theseparameters has been established for a virtual circuit, these parameters can essentially beguaranteed through the network. Thus, a strong building block exists for delivery ofQuality of Service in ATM network.

Memotec implements several standard ATM traffic types (such as Constant Bit Rate(CBR), Variable Bit Rate (VBR), Variable Bit Rate Real Time (VBR-rt), andUnspecified Bit Rate (UBR). These traffic types enable a user to match a circuit with thedesired type of traffic that flows inside the circuit. For example, since video traffic isgenerally constant rate, a CBR circuit can be defined to carry the video traffic. LANtraffic may use a UBR circuit to take whatever bandwidth is left after higher priorityapplications transmit.

In addition to this, Memotec implements eight (8) priority levels for these circuits. Thesepriority levels are used to prioritize the transmission of information for different circuittypes. This enables a user to have a high degree of control over how QoS can beeffectively implemented in an ATM network.

Again, Memotecs mechanisms are flexible such that a user, based upon his desired QoSpolicy, can control which protocols and traffic types on his network get assigned whichpriority and circuit type. In addition, they are interoperable and work with mechanismsemployed on third party ATM backbone switches to ensure they work over real networks.


IP Based Quality of Service Mechanisms

Memotec has designed and implemented a quite sophisticated mechanism for delivery ofQoS for IP traffic. The mechanism, as described before, builds upon the QoSmechanisms implemented for Frame Relay and ATM, and is also standards basedenabling it to work with third party, standards based routers. The mechanism is basedupon the IETF Differentiated Services RFC (diff-serv), which uses the IP header to signalthe priority of the IP packet between routers. Memotec uses the Type of Service bits(TOS bits) to enable 8 different priorities of IP traffic to be recognized and handledappropriately. The diff-serv RFC, while specifying exactly how to signal priority, doesnot discuss how each router should ensure the priority of the traffic. Memotecimplements mechanisms for routing and tagging packets, as well as mechanisms forensuring that they get treated appropriately.

Memotecs IP based QoS mechanisms work in a three-step process. First, packetsincoming into the box are identified by their IP header as to priority. Next, according tothe QoS policy configured, these priorities may be overridden or set in the IP header.These policies may set the priority based upon traffic type (e.g. FTP traffic, e-mail traffic,Web traffic), source IP address, destination IP address, source port number, destinationport number, etc. Thus, completely customized policies may be set up for a network.Finally, the packet is routed and the link layer QoS mechanisms are used to actuallyimplement the quality policy. As an example, if a packet is determined to be a highpriority IP traffic, it is sent to the high priority Frame Relay queue as described before,where the combination of Frame Relay prioritization, fragmentation, and transmissionscheduling are used to ensure that is appropriately delivered through the network. Theuser can decide which ATM or Frame Relay priorities are appropriate for which IPpacket priorities.

Memotec has further extended these mechanisms by enabling different virtual circuits tohandle these different priority IP packets. For example, if a user has two ATM virtualcircuits, one with a CBR service and one with a UBR service, the user can configure highpriority IP packets (perhaps an IP video session) to use the CBR circuit, while all other IPtraffic uses the UBR circuit. Thus, very fine control of QoS for IP can be attainedthrough the backbone, built upon the ATM QoS mechanisms.

IP traffic with bothVideo and LowPriority Web Traffic

Low priority Webtraffic sent over lowpriority UBR circuit

Video traffic sentover High PriorityCBR Circuit


Quality of Service for H.323 Traffic

Voice over IP (VoIP) poses some special issues for control of Quality of Service. Due tothe nature in which H.323 passes voice traffic, it can be difficult to design a basic policythat can assure all H.323 traffic is treated appropriately. Memotec has designedmechanisms that enable this H.323 traffic to be handled with effective QoS through thenetwork.

Memotec has the ability to internally gateway PSTN traffic to H.323 traffic, as well as toroute external VoIP terminal traffic. Each of these needs to be analyzed separately forQoS.

For internally generated VoIP traffic, Memotec internally recognizes this as VoIP voicetraffic. Since all the traffic is originating from within the Memotec unit, this is relativelyeasy to do. Once identified as voice traffic, it is treated as high priority, and transmittedon the high priority Frame Relay or ATM queues.

For external H.323 traffic, the problem is a bit more complicated. Since it can be hard toidentify this traffic, Memotec has chosen a different mechanism to solve the problem.The mechanism is called proxy H.323, and it enables LAN based H.323 terminals tocommunicate with remote H.323 terminals by utilizing the Memotec unit as anintermediary.

Using this mechanism, which is transparent to the end terminals, QoS can be controlled,since the proxied session will then appear to us as an internally generated H.323 session.There are additional benefits which result form this technique including security andbandwidth management, but these are not the subject of this paper.

H.323

IP Network

Traffic isautomaticallyidentified as H.323because it originatesin our unit


In the example above, the LAN based VoIP terminal places a call to the remote phone.Instead of the connection going end to end directly, the call is effectively terminated andregenerated again by the local unit. By doing this, it is known to be VoIP traffic andtreated as high priority.

Conclusion

Assurance of Quality of Service is critical for proper operation of an integrated voice,video, and data network. The evolution of IP based applications place more stress andrequire more sophistication in equipment designed to support these applications over realworld networks, while delivering services at similar reliability levels to those experiencedover non-integrated traditional networks. The ability to have a flexible mechanism thatenables a user to tailor the QoS policy to his specific needs is a critical component of anoverall integrated network.

ProxyH.323 session

ProxyH.323 session

LAN basedVoIP H.323terminal

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