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Both wireless LAN (WLAN) and VoIP technologies are growing in popularity in today’s enterprise networks.
Interestingly, these diverse technologies can be used in tandem to provide VoIP services for wireless clients. This section considers the design of Voice over Wireless LAN (VoWLAN) networks, including such topics as the need for VoWLANs, performing site surveys, and core infrastructure requirements.
Introduction to VoWLAN Technologies
A WLAN contains access points (with which wireless devices communicate), antennas (which help determine the wire-less coverage areas), and wirewire-less endpoints (such as a laptop containing a wirewire-less network interface card). Cisco offers a suite of wireless technologies that fall under the umbrella of the Cisco Unified Wireless Network. An example of a wire-less network, demonstrating various wirewire-less bridging methods, is illustrated in Figure 11-1.
Elements of a Cisco Unified Wireless Network include mobility services, network management services, network unifica-tion, access points, and client devices. Motivation to offer VoWLAN services include the widespread deployment of WLANs in enterprise networks, the enhanced communication features offered by VoIP, in addition to productivity and cost benefits. Although some might argue that cell phones provide an alternative solution to mobile communications, VoWLAN services offer access to a wider range of enterprise voice applications (for example, access to a corporate phone directory).
However, a VoWLAN designer must understand the stringent requirements of VoIP. Specifically, if VoIP packets experi-ence excessive packet drops, jitter (that is, a variation in interpacket arrival times), and delay, the voice quality will be considered unacceptable by the end users. The G.114 recommendation offers one example of a VoIP design guideline.
Specifically, the G.114 recommendation states that the maximum one-way delay for a VoIP packet should not exceed 150 ms. Fortunately, Cisco offers an array of quality of service (QoS) solutions that can help minimize packet loss, jitter, and overall delay for voice traffic.
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Cisco uses the terminology of Cisco voice-ready architecture to describe their end-to-end solution for WLANs that can transmit VoIP traffic, while maintaining voice quality. The four primary components of the Cisco voice-ready architecture are as follows:
n VoWLAN clients: For example, wireless IP phones
n Voice-ready WLAN: A WLAN capable of prioritizing voice traffic
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FIGURE 11-1 Cisco Unified Wireless Network Example
CCDP ARCH Quick Reference
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Designing Voice over WLAN Networks
n Unified wired/wireless LAN infrastructure: The combination of wireless and wired network components that provide end-to-end connectivity for VoWLAN clients
n Cisco Unified Communications and mobility applications: A collection of Cisco software and hardware products that offer a feature-rich IP telephony environment
Provisioning for VoWLAN Coverage
Wireless LANs need seamless coverage through the areas where VoWLAN clients might roam. Fortunately, Cisco Unified Wireless Network offers a variety of products for ensuring appropriate coverage.
As a VoWLAN client roams from one cell of coverage to another, the signal quality might vary. To maintain a more consistent call quality, Cisco recommends the following radio frequency (RF) parameters:
n Wireless signal stream of –67 dBm or greater n A maximum packet error rate of 1 percent n A minimum signal-to-noise ratio (SNR) of 25 dB
A wireless access point shares bandwidth among its clients. Additional bandwidth per client can be achieved by adding access points. However, to prevent RF interference, adjacent wireless access points should use different frequencies (that is, channels). These channels should be nonoverlapping channels. Nonoverlapping channels extend coverage while main-taining available bandwidth. The three nonoverlapping channels commonly used in North America are channels 1, 6, and 11. To provide continuous coverage, as wireless devices roam from one cell to another cell, Cisco recommends a 15 percent to 20 percent cell coverage overlap.
Although multiple IEEE 802.11 implementations exist for wireless networking (for example, 802.11a, 802.11b, and 802.11g), 802.11a often serves as an appropriate choice for VoWLANs. Specifically, 802.11a suffers from less RF inter-ference from other sources, such as cordless phones, and 802.11a supports and as many as 14 simultaneous voice calls per wireless access point.
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Consider the following design guidelines for a VoWLAN.
n Determine the required coverage area and number of wireless phones to be supported.
n Use at least two wireless access points (operating on nonoverlapping channels).
n The percentage of time that an access point uses a particular channel (as defined by the QoS basis service set [QBSS]) should be less than 45 percent.
n The percentage of packets transmitted error-free should be at least 99 percent.
n Antenna diversity, which reduces the number of missed or retried packets, should be used on all access points.
n Do not oversubscribe an access point with too many calls. 802.11b and 802.11g access points support a maximum of seven simultaneous G.711 calls or eight G.729 calls, whereas 802.11a access points can support a maximum of four-teen G.711 calls.
Conducting a site survey is an initial step to designing a VoWLAN. Performing an effective site survey involves the following steps:
1. Determine what type of devices the customer needs to support, the number of devices, the service levels of those devices, and the location of the devices to be supported.
2. Review potential structural elements (walls, stairwells, or elevator shafts) that will impede the propagation of the wireless signal.
3. Identify initial access point locations.
4. With the access points in place, conduct the site survey (which identifies the coverage areas and signal strengths that result from the access point placement).
5. Record the results of the site survey.
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Designing Voice over WLAN Networks
VoWLAN Design Requirements
A VoWLAN design requires the designer to consider the following:
n Roaming: Because VoWLAN clients need to maintain connectivity and good voice quality as they roam from one wireless coverage cell to anther, the VoWLAN network should support roaming. Cisco wireless devices support various types of roaming, as illustrated in Figure 11-2.
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LWAPP
n Intracluster roaming: A wireless client changes its association from one wireless access point to another wire-less access point, where both access points are associated with the same wirewire-less LAN controller.
n Layer 2 intercontroller roaming: A wireless client changes its association from one wireless access point to another wireless access point, where the access points are associated with different wireless LAN controllers in the same subnet.
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n Layer 3 intercontroller roaming: A wireless client changes its association from one wireless access point to another wireless access point, where the access points are associated with different wireless LAN controllers in different subnets.
Cisco recommends that voice traffic and data traffic be placed in separate VLANs. This VLAN separation enables the use of various security features and also aids in the prioritization of voice traffic.
n Quality of service (QoS): The IEEE and the Wi-Fi Alliance each have a standard for prioritizing WLAN traffic, specifically, the IEEE 802.1e and the Wi-Fi Multimedia (WMM) standards. Whereas the 802.1e standard specifies eight levels of priority, the WMM standard specifies four levels of priority (Platinum [typically used for voice], Gold, Silver, and Bronze).
n Security: VoWLAN security recommendations include the following:
Use Extensible Authentication Protocol-Flexible Authentication via Secured Tunnel (EAP-FAST) to provide timely authentication for roaming wireless clients.
Use Temporal Key Integrity (TKIP) to encrypt both voice payload (that is, Real-time Transport Protocol [RTP]) and signaling (that is, Skinny Client Control Protocol [SCCP]) traffic.
Use Message Integrity Check (MIC) to verify the integrity of wireless packets.
n Intelligent clients: The Cisco 7921G IP Phone is an example of an intelligent VoWLAN client. The 7921G is flexi-ble in terms of supported radio frequencies (that is, IEEE 802.11a/b/g), and has a long battery life, enhanced secu-rity, and QoS mechanisms.
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Cisco IOS Software Network Management Capabilities
Chapter 12: Cisco IOS Software Network Management Capabilities
Performance, scalability, and availability all can be achieved through the rich set of embedded management functionality found in the Cisco IOS Software. We discuss the implementation of the Cisco IOS Software management instrumentation functionality as part of overall enterprise design.
Built-In Management Capabilities
Large enterprises rely on WAN links, but there are several issues with these, including the following:
n High cost, leading to implementation of low-speed lower-cost links
n Speed mismatches between LAN and WAN links leading to congestion, packet loss, and so on n Combination of real-time applications competing for bandwidth with general data transfer
Cisco IOS software includes management capabilities through offering a broad range of show commands, and Simple Network Management Protocol (SNMP) access to information. Tools such as Security Device Manager (SDM), Adaptive SDM (ASDM), and web tools for managing single devices are also offered, as are embedded management subsystems such as syslog, NetFlow, Network Based Application Recognition (NBAR), and IP Service Level Agreement (IP SLA).
Cisco application optimization cycle:
1. Create baseline of application traffic.
2. Meet objectives through optimization.
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3. Measure, adjust, and verify effectiveness of techniques.
4. Deploy the new applications.
Cisco IOS System Message Logging (syslog): Syslog allows reporting and archiving of error messages locally or on a remote logging server. Syslog messages always begin with a percentage sign (%) followed by a structure that consists of facility, severity, mnemonic, and message text.
Working with NetFlow
Embedded within Cisco IOS Software, NetFlow is designed to provide network and security monitoring, traffic analysis, and IP accounting, and to assist with network planning.
NetFlow usage: Used both by service providers and enterprise organizations, although their usage of it may differ. For service providers (SP), it can provide assistance with traffic engineering, network planning, accounting and billing, secu-rity monitoring, and information regarding peering arrangements. Enterprises typically use NetFlow for user and Internet access monitoring, application monitoring, charge-back billing for departments, and security monitoring.
Defining a flow: A flow in NetFlow consists of seven fields: IP source address, IP destination address, source port number destination port number, Layer 3 protocol type, type-of-service (ToS) byte, and input logical interface. NetFlow inspects packets for key field values and compares these to existing flows in the cache. If the values are unique, a flow is created in the cache.
By examining flows and caching information about unique values, NetFlow-enabled switching can provide scalability and performance based on flow cache management.
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Cisco IOS Software Network Management Capabilities
NetFlow Versions
There are a number of NetFlow versions. Older versions (1, 5, 7, 8) support statistically defined fields, whereas newer versions (9) support dynamically defined fields:
n Version 1: Original n Version 5: Most popular
n Version 7: Supports Cisco Catalyst 6500 switches with a Multilayer Switch Feature Card (MSFC) on CatOS Release 5.5(7) and later
n Version 8: Provides on router aggregation; choice of 11 aggregation schemes n Version 9: Flexible, extensible file export format
n IPFIX: IETF standard mechanism for information export
NetFlow Version 9: This version has an export format that allows new fields to be easily inserted. It includes a template that describes what is being exported in the export data sets. A matching ID number is then used to associate templates to the data records.
Flexibility: Network managers have the flexibility to configure what key and nonkey fields define each flow. This helps provide enhanced optimization of network infrastructure while reducing costs and improving capacity planning and secu-rity detection.
Deployment of NetFlow: There are a number of Cisco NetFlow products with solutions available on both Windows and Linux platforms. Deployments vary with smaller deployments using a single server for both reporting and collecting, whereas with large-scale deployments, a two-tier architecture that uses collectors at key sites is often used.
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CCDP ARCH Quick Reference
CCDP ARCH Quick Reference By Kevin Wallace, Michael Watkins ISBN: 9781587054990 Publisher: Cisco Press
Prepared for Kevin Kem, Safari ID: [email protected] Licensed by Kevin Kem
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IP IP
IP
Branch IP
IP
IP
Branch
IP IP
IP
Branch Tele-Workers
IP Data Center
Wide Area Network
NetFlow Monitoring
FIGURE 12-1 NetFlow Monitoring
CCDP ARCH Quick Reference
CCDP ARCH Quick Reference By Kevin Wallace, Michael Watkins ISBN: 9781587054990 Publisher: Cisco Press
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Cisco IOS Software Network Management Capabilities
Network Based Application Recognition
NBAR can provide organizations with a means of traffic classification. By adding classification to the network, it can deliver more granular identification and control over multiple applications, which common quality of service (QoS)
NBAR can provide organizations with a means of traffic classification. By adding classification to the network, it can deliver more granular identification and control over multiple applications, which common quality of service (QoS)