3Com Nbx 100 And Ss3 Nbx Administration Guide
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Connection Considerations431 Most people notice latency when the end-to-end delay is above 200 ms. (The round-trip delay is typically no greater than 400 ms.) Conversations sound most natural when latency is below this range. Network latency can be measured by “pinging” the network connection, but the network connection is only part of the delay. The entire end-to-end delay also includes the H.323 gateway, firewall or router, and the LAN itself. System administrators can control some local device delays by controlling the system load and by upgrading system components as needed. Jitter Momentary transmission delays can affect the pace of a conversation and, if severe, cause the voice to “break up.” This is known as “jitter.” All voice-over-internet devices have a “jitter buffer” at the receiving end whose purpose is to absorb jitter. It does this by delaying the first packets that arrive by some significant amount (from 50 to 200 ms). This delay creates a window of time for receiving the next group of related samples which are then forwarded to a callee at a regular rate. However, if some packets are too late, and exceed the jitter buffer capacity, those packets are lost and there are gaps in the audio. Quality of Service ControlNBX systems address Quality of Service (QoS) issues using methods that are discussed in this section. Silence Suppression NBX Business and Basic Telephones generate voice frames at regular intervals for the duration of a connection. These frames normally continue when no one is speaking, which creates unnecessary LAN traffic. The NBX system has a “silence suppression” mode that sends a “silence indicator” when the NBX Telephone senses the start of a silent period. When another NBX device receives this indicator, it inserts “white noise” until it receives the next frame that contains real voice. All subsequent “voiceless” frames are suppressed during the silent period. This type of silence suppression applies to Layer 2 Ethernet transfers. At Layer 3, the ConneXtions software achieves a similar result by not sending empty packets during a silent period. The receiving ConneXtions gateway generates a silence indicator or sends frames filled with silence, depending on the silence suppression mode.
432APPENDIX A: CONNEXTIONS H.323 GATEWAY Adaptive Jitter Buffering All IP network devices use buffers to retime the packets that they receive from a network. Retiming allows these devices to compensate for the variable delays that occur as the packets pass through an IP network. H.323 calls take different paths through a network so the ConneXtions gateway uses an adaptive “jitter buffer” to minimize delay variability. Initially, the jitter buffer delays the entire packet stream by 50 ms, an amount that is too small to be noticed in conversation, but large enough to account for the variability. If the packet delays are too variable, packets may not arrive in time to be useful. This can result in lost packets and gaps in the conversation. When ConneXtions detects the gaps caused by late-arriving packets, it automatically extends the jitter buffer delay to match the delay so similar packets are not lost. ConneXtions can extend the jitter buffer delay up to its 200 ms limit. Reconstruction NBX Business and Basic Telephones expect to receive voice packets at regular intervals, but unanticipated network delays can cause lost packets and gaps in the conversation. Reconstruction makes these gaps less noticeable with “best guess” substitutes based on the preceding and following samples. If your network is not optimized for voice, the quality of voice can be affected. Priority Schemes Packet-based voice systems depend on the speedy and consistent delivery of voice packets for good voice quality. This dependency presents an obstacle to H.323 communication on the Internet because it was designed to treat all packets alike with respect to time. By treating packets that carry e-mail with the same priority as packets that carry real-time voice, the Internet ignores the important differences between these applications.
Connection Considerations433 NBX systems use the latest developments to address voice packet priority concerns at the Layer 2 Ethernet level and at the Layer 3 IP network level. Layer 2NBX systems address Layer 2 priority concerns through the 802.1(p and q) standards. These standards have two parts. The first part addresses the way Ethernet frames get onto the local “wire.” The NBX system uses a special “back-off” algorithm that gives voice frames a higher priority when both voice frames and data frames try to access the Ethernet wire at the same time. The second part of the 802.1(p and q) standards addresses the way LAN switches prioritize different packets that are competing to enter a different LAN segment. This scheme is based on a 3-bit priority field within the Ethernet header. NBX ConneXtions does not support the Layer 2 (Ethernet) 802.1 (p and q) priority field. However, it is usually possible for IP routers to use these priority schemes if they are configured to prioritize H.323 packets. Layer 3NBX systems address Layer 3 priority concerns through a packet priority scheme called “IP/DS” (for differentiated services). Many routers support this scheme, which replaces an earlier scheme (TOS), which uses a 6-bit priority field within the IP header of every packet. Most routers examine this field and base their pass-through priorities on it. NBX systems are designed to use the default values that come with 3Com switches. If you use other routers, you may need to reprogram their diff-serv settings. The 3Com default is 101110xx. This setting must be consistent at both ends of the connection. Note that some routers overwrite the TOS field (diff-serv priority field) and eliminate the priority distinctions between packets. NBX ConneXtions does not support the Layer 3 (IP) 6-bit TOS/DS priority field. However, it is usually possible for IP routers to use these priority schemes if they are configured to prioritize H.323 packets. Selection and configuration of routers is important to the establishment and maintenance of a high performance network.
434APPENDIX A: CONNEXTIONS H.323 GATEWAY Special IssuesThis section describes issues related to H.323 telephony in general and to ConneXtions gateways in particular. These include: ■Firewall Security ■Gateway Load ■Remote Access ■PBX Connections ■Class of Service ■IP Type of Service and Differentiated Services ■Alternate Gatekeepers Firewall SecurityFirewalls determine which packets can cross the boundary between a protected network (intranet) and the public internet. The network administrator specifies crossing privileges according to network needs and policies. Control criteria consists of direction of transfer, source and destination address, packet type, and access ports. Firewalls affect, and are affected by, H.323 gateways. For example, firewall processing increases packet delay while the complexity of the H.323 protocol complicates the firewall programming. The only way to safely avoid firewall delays is to exclude outside internet access. This means calls can only be made within the secure intranet. In some business applications, it is possible to eliminate the firewall delay by setting up a dedicated physical connection between the H.323 gateway and the router. This approach, which requires a second NIC in the ConneXtions PC system, bypasses the firewall and puts the burden of discriminating against non-H.323 packets on the gateway. The PC system that runs the ConneXtions software must be secure. Systems that must conform to very conservative firewall policies can use a Virtual Private Network (VPN) if they need to filter incoming H.323 calls from the public Internet. An alternative is to use a firewall with H.323 proxy support. While the operating system that runs the H.323 gateway can be programmed to serve both as an H.323 gateway and as an IP router, such arrangements are usually impractical because the gateway needs so much processing power just to handle audio conversions.
Special Issues435 3Com recommends that a high-performance PC be dedicated to the ConneXtions software. The question of whether an operating system is adequately “secure” is a subject of debate. The concern is that Windows has many IP ports of its own. One way to deal with these ports is to set up a firewall that limits the range of externally accessible ports. However, some organizations connect the ConneXtions gateway directly to the Internet through a second NIC that bypasses the firewall protecting the rest of the local network. ConneXtions supports either configuration. Organizations that want to completely bypass firewall delays can research the large volume of security information on the subject. These descriptions focus on the firewall-protected approach, and offer guidelines for programming a firewall that can be used to support H.323 connections that are accessible to the public internet. Outbound Calls Most firewalls do not restrict outbound packets or IP packets that respond to outbound initiatives. They are configured for unrestricted outbound packets with unrestricted reply packets. They do not have to be changed to support outbound H.323 calls from an NBX system. Inbound Calls Firewalls usually discriminate against incoming packets. The network administrator configures a list of acceptable sources for each destination address within a protected network. The configuration list includes a list of entries that the firewall compares to the IP address of the local H.323 gateway and the IP address of an external caller. The configuration list also discriminates for or against specific types of packets. IP addresses and packet types must match for packets to pass. The H.323 protocol uses TCP packets for call setup, and UDP packets to carry the voice payload. Each type of packet includes an array of port addresses that are used during the connection. Ports 1720 negotiates which of the other available ports is used to carry the connection.
436APPENDIX A: CONNEXTIONS H.323 GATEWAY The ConneXtions gateway uses these default port assignments: ■For UDP traffic, ConneXtions uses ports 8000-8099 by default. Calls require four UDP ports each. ■ConneXtions uses ports 1025-5000 for TCP traffic. You can configure TCP ports during installation. During ConneXtions installation, you can configure the TCP ports that are used for incoming calls. For outgoing calls, no control is possible. Port 1720 must be preserved. You must configure a firewall to accommodate both TCP and UDP ports on the same system. Gateway LoadIf the gateway system NIC is attached to a LAN with heavy packet traffic (more than 700 non-H.323 packets per second), the extra address processing burden, which requires processing power, can slow down the gateway. This occurs because the ConneXtions software makes H.323 ports look like hardware line cards to a Call Processor. To emulate a group of simulated line cards, the gateway system must read the destination address of every frame that is presented to its Network Interface Card, instead of responding to only one hard-coded Ethernet address. The gateway system is able to examine every Ethernet frame because its NIC does not discriminate between frames. The NIC passes every frame that it sees to the software for address evaluation. To reduce the load on an H.323 gateway, you can connect it to an existing multi-port switch. For optimum performance, use switches that support 802.1(p and q). The 802.1(p and q) standard offers priority enhancement which NBX systems exploit. Most 3Com switches support this feature. Remote AccessBusiness people who travel can make routine calls without long distance line charges by using an internet-ready laptop with Microsoft NetMeeting to make H.323 calls, and a Virtual Private Network (VPN) connection to the NBX system LAN. Microsoft NetMeeting software works with Windows 9x, Windows 2000 and Windows NT, and it can be downloaded for free from www.microsoft.com.
Special Issues437 You can use Microsoft’s VPN Dial Up Networking (version 1.3) to establish a virtual private network connection between a roaming laptop and the NBX system LAN. One end of the VPN connection is in the laptop while the other end must be located in a VPN server between the router and firewall. The VPN server provides caller authentication and a secure (encrypted) channel across the internet. After a caller has been authenticated, the connection is passed to the firewall, which sees the VPN connection as coming from a recognizable (and therefore firewall-configurable) IP address. VPN allows a business person to establish an IP connection into the NBX LAN from a hotel room with internet service. After an Internet connection has been established, you must change your automatic call forwarding number: 1Log in to the NBX NetSet utility as a user. 2On the User Information tab, click Call Forward. 3Click the telephone number radio button. 4Enter the number to which you want to forward the call and click OK. The caller is now ready to use NetMeeting to place an H.323 to the NBX system at the office. Configure NetMeeting with the IP address of the ConneXtions gateway as the gateway in Advanced Calling options. Dial the NBX extension to place the call. The call passes through the Auto Attendant to your extension and forwards the call to your previously specified number. After the call, return to the NBX NetSet utility and remove the forwarded number so that work-related calls to your extension are not forwarded to your home, or to wherever you placed your last H.323 call. PBX ConnectionsH.323 gateways allow NBX systems to establish IP connections to other H.323-equipped PBXs as well as to similarly equipped NBX systems. Although H.323 standards describe a universally accepted interface for interconnecting similar systems, each of the 20 or 30 PBX manufacturers brings its own PBX solution to the marketplace. This complexity is further increased by the diversity of products and release levels that are associated with each manufacturer. Because any implementation differences can affect connectivity, this manual can only offer guidelines for connecting NBX and PBX systems.
438APPENDIX A: CONNEXTIONS H.323 GATEWAY Tie-line connections between NBX and PBX systems require technical people from both ends of the connection to collaborate in these major areas, discussed next: ■H.323 Interoperability ■IP Addressing ■Voice Ports ■Extension Dial Plans ■Extension Delay H.323 Interoperability H.323 protocol stacks provide the foundation for H.323 compatibility. Each consists of a collection of engineered software products that implements the H.323 standard. Although PBX manufacturers can develop their own H.323 software stacks, it is more efficient to purchase software licenses from a company that specializes in developing H.323 protocol stacks. The ConneXtions gateway has been tested for compatibility with PBX H.323 gateways that are licensed to use Lucent Elemedia and RADVision H.323 protocol stacks. It has also been tested with these H.323 telephones: ■Siemens HiNet LP 5100 (phone application version 1.1.3) ■ACT Sagitta PH200 ■Microsoft NetMeeting (version 3.0) IP Addressing The main goal of an H.323 gateway is to provide telephone-like service through IP connections. This means each end-to-end connection involves two types of addresses: a normal telephone number (E.164 address) and an intermediate IP address. Some H.323 implementations use a “gatekeeper” to convert the E.164 number into the appropriate IP address at the calling side, and then to reconvert the IP address to the E.164 number at the receiving side (for caller ID purposes). ConneXtions allows you to choose if you want to use a gatekeeper on your network.
Special Issues439 Outgoing IP addresses can be entered: ■As pre-programmed speed dial numbers that forward callers to the Auto Attendant at a remote NBX system. ■By modifying the dial plan. You can configure the speed dial numbers to include an appended extension if a person in one NBX system needs to make frequent calls to someone in another NBX system. Alternatively, you can configure the dial plan to route these calls seamlessly to the caller. NBX system calls to outside numbers must use IP addresses or host names. The ConneXtions software automatically converts host names to their corresponding IP address. Voice Ports Multiple voice ports allow the Auto Attendant to respond to multiple incoming calls at the same time. However, since these ports are also used by the voice mail system, voice mail inquiries can slow down incoming H.323 calls. You may have to increase the number of voice port licenses. On the NBX 100 system, 3Com offers 4 license levels: ■4 ports, 30 minutes ■4 ports, 4 hours ■6 ports, 20 hours ■12 ports, 80 hours On the SuperStack 3 system, 3Com offers 12, 24, 48, 72, and 100-port licenses. The administrator selects the maximum time for voice mail storage. If you choose to increase your existing level of port licenses, you can purchase additional 1-port incremental licenses. Extension Dial Plans PBX systems can use different dial plans. You must consider dial plan differences when setting up calls between systems. Dial plans differ in their use of leading digits, number of digits, and excluded numbers. For more information, see Chapter 2 .
440APPENDIX A: CONNEXTIONS H.323 GATEWAY Extension Delay Call setup times for digital connections, compared to analog connections, are instantaneous so there is no need to include a delay between the IP address and an appended extension. Incoming H.323 calls to an NBX system usually go directly to the Automated Attendant. Although the Auto Attendant can respond with voice instructions, the call does not have to wait until the end of the voice instruction to respond. The Auto Attendant accepts extensions whether they are entered manually or as part of a speed dial number. Class of ServiceThe use of an H.323 gateway affects the Class of Service assignments that are applied to extensions because: ■H.323 calls use IP addresses instead of the familiar numbers that are used for public switched network calls (different dial plan). ■The cost of an H.323 call is distance-independent, so you do not need to limit long distance calling for cost reasons. External Call Control Users of ConneXtions-equipped NBX systems can place H.323 calls to other H.323 systems anywhere in the world without having to pay long distance charges. Since there are no long distance charges for H.323 calls, there is no need to restrict them for cost reasons. IP Type of Service and Differentiated ServicesThe header of each IP packet contains an 8-bit Type of Service (TOS) field that indicates the precedence (relative importance) of the packet. Routers then examine the TOS field and give precedence to packets with a higher TOS setting. Although your telephone system supports prioritization using the TOS field, this facility is not supported for H.323 calls. However, for H.323 calls over the WAN, routers can prioritize voice traffic using alternative means. For example, during installation, you can select a range of UDP or TCP port addresses to help with router setup.