WAN storage guide -- Chapter 1: WAN basics
The implications of WAN connectivity have had a profound influence on networking. We look at some of the practical WAN issues, such as bandwidth, cost, latency and reliability.
A wide area network (WAN) basically connects two or more local area networks (LAN). While this may sound like a relatively straightforward goal, the implications of WAN connectivity have had a profound influence on networking. Early on, a WAN was typically proprietary, connecting corporate networks in different geographic locations, but WAN connectivity has proliferated with the growth of the Internet. Today, the WAN has evolved into a global network tying remote offices to datacentres, connecting remote backup and disaster recovery sites, and even connecting customers and partners to shared corporate resources.
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Still, the global WAN is hardly universal -- it has evolved as a mishmash of cabling, switching and routing technologies supported by a vast array of service providers that are guided by disparate local and national regulations. It's easy to get WAN connectivity, but selecting the "right" connectivity requires careful consideration of practical issues, such as bandwidth, cost, latency and reliability.
Private vs. public
Traditional WANs connected two or more locations using dedicated infrastructure (aka leased lines) provided by a telecommunications provider. Leased lines are generally expensive, but they provide good overall service levels and reliability. Leased lines also tend to be secure because they are not accessible to users outside of the locations that are connected.
Private WAN connectivity is still available but has been largely overwhelmed by public WANs (e.g., the Internet) where connectivity is shared between multiple users on the same infrastructure supported by Internet Service Providers (ISP). This reduces costs for the individual users and allows network connectivity literally around the world. However, performance declines as more users compete for service. Global access also carries global security risks where users can attempt to access unauthorized or protected data resources.
Bandwidth and cost
Bandwidth is the first consideration for most WAN deployments, denoting the available data transfer rate between points -- usually expressed as bits per second (bps). Higher bandwidth is crucial because it allows more data to be passed in a given time. For example, a 3 Mbps connection can potentially pass twice the data moved by a 1.5 Mbps WAN link.
WAN connectivity is typically organized into bandwidth "classes." Low-speed WAN connections often include frame relay at 56 Kbps or 1.5 Mbps speeds. DSL and cable connections are available in a variety of service levels up to about 3 Mbps. High-speed/high-reliability WAN connectivity is also available for business users, including T1 service to 1.5 Mbps and T3/DS3 service to 45 Mbps. T3 service can be shared between multiple users with lower bandwidth needs (dubbed fractional T3). It's important to note that additional bandwidth costs more money, and WAN service is a recurring cost for organizations. For example, a business cable WAN connection can run up to $250 per month, while a T3 line can run over $10,000 per month. Bandwidth increases can become a significant hit to the monthly budget.
But this tradeoff between bandwidth and cost puts companies into a quagmire. Corporate data volumes are spiraling upward, growing at rates that sometimes reach 100% annually. As the amount of data increases, it takes longer to transfer that data between locations. Thus, companies have to shoulder the cost of higher bandwidth or employ data reduction technologies to shrink the effective data volume.
Latency
Latency is an important factor to consider because electronic data does not reach its destination instantaneously. Not only does it take finite time for network data to traverse the physical distance between two points through its copper/optical transmission medium, the data packets must also travel through an array of hubs, switches, routers and other network traffic management devices -- each device adds a delay, slowing the effective response time. To exacerbate matters, a typical application may require extensive packet handshaking, significantly worsening the effective delay.
Latency can have a detrimental effect on remote application performance, forcing users to wait (sometimes minutes) for a single query response. This can also have an adverse effect on remote mirroring and replication setups. Latency is such an important issue that synchronous (real time) data replication can only be supported within a fairly short physical distance. If real-time replication is not required, asynchronous replication can support global data transfers.
Reliability
Reliability is vital for a WAN, but no WAN connection is 100% reliable. Data can be interrupted at any point due to cable/hardware malfunctions, poor configurations, natural or man-made disasters and so on. Reliability is also affected by the quality of the service. For example, a typical SOHO broadband cable connection is generally less reliable than a T1 or T3 line where service levels are guaranteed.
Any WAN interruption can impact business operations, so it's important to consider the potential effects of various outages and establish contingency plans accordingly. For example, a service interruption can interfere with the corporate remote replication process. If the interruption is brief, data may simply be cached locally and resynchronized when the WAN link returns. Interruptions may also cause users to switch over to local application versions, such as a database, running locally cached data. Longer interruptions may cause the replication process to switch over to a local secondary storage system.
