how to generate barcode in vb.net 2010 Architecture of a SIP Network in Software

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Architecture of a SIP Network
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Application Servers (ASs)
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The application server (AS) is a SIP entity providing services to the network as well as subscribers. The IN used a similar concept, with servers (or big iron processors) providing services such as number portability and calling name display. Many operators have grown tired of big iron requirements for service platforms, so the concept of an off-the-shelf server supporting traditional services as well as 3G/4G services is much more appealing. The application server sits in the core of the network (although this is not a requirement, as the AS can be deployed anywhere in the network). It is capable of generating requests and initiating dialogs acting as a user agent (both UAS and UAC). The AS also has a SIP URI address. The AS also uses service identifiers to address the various services being supported. This allows the network to send a request to the AS that may be supporting multiple services and direct the AS to provide a specific service. The registrar function may contain not only the public and private identities of a subscriber; it may also associate service identities with each identity for a subscriber. This allows the registrar to provide authorization as well as authentication. Some services may require the AS to support event notification. The AS will send the SIP request SUBSCRIBE to the appropriate registrar in the network so that it can be notified of any changes to a subscriber s status (change of location, for example, or availability of the subscriber). Services such as presence or location-based services would require event notification. This is discussed in more detail in subsequent chapters. If the AS is acting as a SIP proxy, requests are routed to the AS, which then will forward the request to the next hop in the network after implementing whatever service the AS is delivering. The AS address may also be included within the request-URI header if strict routing is being used, in which case the AS will remove its address prior to forwarding the request on. Much of the way the AS behaves is not much different than the service control point (SCP) function defined in SS7/IN networks. The switches in the IN query servers to determine how to deliver services as well. The difference is the ability to support multiple services and multiple terminals on the same platform in the same network.
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The Domain Name System (DNS)
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The DNS is what provides SIP networks (and all other IP networks) the ability to support mobility. In the Internet, the DNS is used to provide routing addresses for servers providing Web services in the network, as well as find the physical addresses of subscribers for the delivery of e-mail and other media. This requires converting names to numeric IP addresses. The DNS stores the URI of the subscriber and all other entities in the network with an address. This includes application servers. The URI is then translated into an IP address for routing to the final destination. This means that the registrar in the network must update the DNS when any user agent registers with a new IP address.
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When the user agent registers, it provides its IP address that has been assigned by the network (which is usually the case when dynamic addressing is used). The DNS must be queried by the proxies in the network when they receive a message to be routed and only the URI is provided. To prevent overloading the DNS, the function is distributed throughout the network. This prevents the possibility of too many requests flooding the DNS and causing it to fail, thereby denying service to all those whose address is stored in the DNS. In fact the DNS architecture is quite sophisticated. There are many different levels of DNS, enabling a tiered approach to finding information. This avoids the need to have one central server for all queries. In fact some servers may not contain the information needed to route to a specific subscriber but know the address of the DNS server that would have the information. The DNS queried would then provide the address of another DNS server to be queried. There may be multiple queries to the DNS for a single session, since the address information may not be forwarded between networks. This means that many queries could be possible for a single call. The DNS is the busiest database in the world, receiving millions of queries each day, and changing entries by the millions each day. To better manage DNS entries, domain names are divided into domains such as .com, .org, and .edu, to name just a few. There is a DNS server responsible for each of these domains. All of the domain names are managed by a third party that administers domain names to recipients all over the world. They are then entered into the domain name servers. The authority for managing these domain names, as well for updating the master database (referred to as root ), rests with registrars from other networks. The top level (or root) database then feeds to all other databases worldwide. Notice in Figure 1-5 that there is a DNS for the UK, as well as for each domain (.com, .org, etc.). Some countries choose to maintain their own top-level database, providing addresses for all domains within their country. For addresses outside of the UK, the root DNS is accessed. To further prevent congestion, corporations may also maintain their own DNSs. This eliminates the need to access the DNS of an external network. These DNSs then connect to an external DNS within the hierarchy until the address is resolved to an IP address. For example, an e-mail sent to travisruss@tcg.com could be sent to the DNS maintained by my ISP. If my ISP cannot resolve the address, it would then provide the IP address of the .com DNS, which could then resolve the address. This is an oversimplified example of how the DNS works and is not meant to be an exhaustive study of DNS servers, since that is outside the scope of this book. There are plenty of tutorials on the Internet that provide exhaustive information about the workings of the DNS.
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When VoIP came along, the need to support telephone numbers challenged the DNS. As discussed previously, the DNS resolves names into IP addresses. Telephone numbers are not included as names; therefore, another database is needed to support resolving telephone numbers to IP addresses.
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