Much of the progress in high availability applications these days is taking place in the Linux world where the nature of the open-source development paradigm provides a fertile proving ground for many new ideas. Interest in Carrier Grade Linux, in particular, is increasing, and acceptance of the Linux OS in telecommunications equipment designs that require high availability was one of the key areas of growth in 2002. One of the main reasons for the increase in acceptance is the fact that Linux is being optimized to meet the carrier grade requirements of modern telecommunications carriers.
High Availability Defined
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Availability is technically defined as: MTBF means Mean Time Between Failures, and MTTR means Mean Time To Repair.
So, if a system were to offer MTBF of 20,000 hours with an average MTTR of 2 _ hours, then its availability would be 99.9875 percent.
The term 5-nines, or 99.999 percent uptime, literally implies exactly that – 365 days, 23 hours, 54 minutes, and 45 seconds of operational time during a year (or 5 minutes and 15 seconds downtime). However, in practice, that is not a useful number when defining high-availability software because these nines only apply to an overall solution that involves integrated high-availability hardware, software (OS and middleware), and the application.
A complete high-availability solution that demonstrates 5-nines requires close integration of high-availability hardware such as CompactPCI or AdvancedTCA, a robust high-availability Linux solution such as Carrier Grade Linux, high-availability middleware, and application software that can cause failover to redundant systems. Figure 2 illustrates a complete high-availability solution using Carrier Grade Linux.
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Linux in telecommunications
Three main categories of applications for which equipment providers are targeting Linux include gateways, signaling servers, and management servers.
Most applications fall into one of the following categories:
- Gateways are bridges between two different technologies or administration domains. For example, a media gateway performs the critical function of converting voice traffic from a native telecommunications time-division-multiplexed (TDM) network to an Internet Protocol (IP) packet-switched network. A gateway maintains a large number of connections in real-time over a large number of interfaces without losing a frame or packet. Gateways are implemented on dedicated platforms with replicated (rather than clustered) systems for redundancy.
- Signaling servers handle call control, session control, and radio recourse control. A signaling server handles routing and maintains the status of calls over the network. It takes the requests of user agents that want to connect to other user agents and routes these requests with the appropriate signaling. Signaling servers require soft real-time response capabilities of less than 80 msecs and may manage tens of thousands of simultaneous connections. Due to requirements for quick switching and a capacity to manage large numbers of connections, a signaling server application is context-switch and memory intensive.
- Management servers handle traditional network management operations as well as service and customer management. These servers provide services such as home location and visitor location registers for wireless networks, or customer information such as personal preferences or authorized features the customer can use. Typically, management applications are data and communication intensive. Their response time requirements are less stringent by several orders of magnitude compared to those of signaling and gateway applications.
Carrier Grade Linux standards
Open standards are a key reason why equipment providers are moving toward Linux-based solutions. Creating platforms based on open standards ensures interoperability with third-party software and makes maintenance and application development much easier. Therefore, utilizing the standard Linux kernel and adhering to key Linux standards, including the Linux Standards Base (LSB), is essential.
There are many standards-related activities in the industry to define hardware and software high availability:
- The PICMG group is defining them for high availability hardware.
- The Service Availability Forum (SA Forum) is focusing on APIs for hardware platform management and for application failover in the application API.
- The Open Source Development Lab (OSDL) is defining specifications for Carrier Grade Linux.
One of the assumptions in this Carrier Grade Linux specification is that Linux is capable of being an embedded real-time Linux. This is a key reason why Carrier Grade Linux, rather than a typical workstation or server Linux, is the preferred choice for telecommunications solutions.
Linux Standards Base
The Linux Standards Base (LSB) defines a system interface for compiled applications and a minimal environment for support of installation scripts. Its purpose is to enable a uniform, industry-standard environment for high-volume applications conforming to the LSB.
The LSB defines source and binary interfaces for application programs compiled and packaged for LSB-conforming implementations on many different hardware architectures. Since a binary specification must include information specific to the computer processor architecture for which it is intended, it is not possible for a single document to specify the interface for all possible LSB-conforming implementations. Therefore, the LSB is a family of specifications, rather than a single specification.
The LSB is composed of two basic parts: A common specification, generic LSB or gLSB, describing those parts of the interface that remain constant across all implementations of the LSB, and an architecture-specific specification, archLSB, describing the parts of the interface that vary by processor architecture. Together, the generic LSB and the architecture-specific supplement for a single hardware architecture provide a complete interface specification for compiled application programs on systems that share a common hardware architecture.
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