Archive for June 3, 2011

Before delving into VoIP configurations, a brief introduction with terminology is necessary:


  • Public Switched Telephone Network (PSTN) —PSTN is the world’s collection of interconnected public voice telephone networks. It is also known as the Plain Old Telephone Service (POTS). It is set up and managed by the government and commercial organizations. It has evolved from the early days of Alexander Graham Bell to mostly digital, circuit-switched telephone network.
  • Private branch exchange (PBX) —This is a device located within an organization that routes telephone calls to internal extensions or to the PSTN. It provides additional features such as voicemail and call-forwarding. A PBX is less expensive than connecting an external line to every telephone. Numbers within the PBX (internal numbers) can be dialed using the last few numbers of the entire phone number and without going through the PSTN. A PBX usually has more than 125 ports.
  • Key telephone system —This is used like a PBX in small offices where far fewer phones are required. Each key telephone system supports up to a hundred ports.
  • Software IP phones —These consist of a headset that plugs into the USB or serial interface of a PC. The PC needs client software that supports IP telephony.
  • Hardware IP phones —These look like regular telephone sets, but they are plugged into a LAN switch. Most IP phones get power from the switch (power over Ethernet or PoE) and encapsulate voice data into IP frames for transmission over the LAN.
  • H.323—This was approved by the International Telecommunications Union (ITU) in 1996 as a standard for multimedia and audiovisual transmission across disparate networks. In 1998, it was followed by version 2. It also includes several functions such as bandwidth management, call control, multimedia management, and interoperability between different network types. H.323 has come to be the most popular protocol for VoIP.
  • Session initiation protocol (SIP)—SIP is IETF’s standard for multimedia communication over IP networks. It is an application-layer control protocol that initiates, manages, and terminates calls between two or more terminals. It is picking up as an alternative to H.323.

Blade computing introduces a new data center paradigm where various thin compute blades share centralized resources in a single chassis. Ablade server is a single circuit board populated with components such as memory, processors, I/O adapters, and network connections that are often found on multiple boards. Server blades are built to slide into existing servers. They are smaller, need less power, and are more cost-efficient than traditional box-based servers.


Managing these servers requires the following:

  • A virtualized view of the servers and resources it uses (such as storage)
  • A high level of security within the server and on the network devices
  • Dynamic resource provisioning that is automated as much as possible
  • A layout that is easy to scale to meet ever-increasing user demands

Data centers will realize a shift from box-based servers to densely packed racks of blade-based servers.


System area networks (SANs) represent an area of computer architecture that has evolved quickly. The term SAN in this section refers to “system” (not “storage”) area networks. After various competing standardization efforts starting in the late 1990s, the state of the SAN field became temporarily unclear. However, the technology has emerged with a richer set of

features that promise to impact the server and clustering arena.


A SAN uses high-speed connections to attach high-performance computers in a cluster configuration. The configuration delivers very high bandwidth of 1+ GB/sec with very low latency. They are switched, with a typical hub What’s Next 579 supporting 4 to 8 nodes. Larger SANs are built with cascading hubs with cable length limitations that vary from a few meters to a few kilometers.


Interconnections in a SAN differ from other existing high-performance media (such as gigabit Ethernet and ATM) in several ways. SAN adapters implement reliable transport services that are similar to TCP or SPX, but directly in hardware. SANs have very low error rates. SANs are often made highly available by deploying redundant interconnect fabrics.


SANs provide bulk data transfer through a remote direct memory access (RDMA) mechanism. The performance within a SAN resembles more that of a memory subsystem than a traditional network (such as an Ethernet LAN). The initiator specifies a buffer on the local system and a buffer on the remote system. Data is then transferred directly between the local and remote systems by the network adapters without involving either of the host CPUs. Both read and write operations are supported in this manner.