Designing WANs
WAN communication occurs between geographically separated areas. In enterprise networks, WANs connect campuses. When a local end station wants to communicate with a remote end station (an end station located at a different site), information must be sent over one or more WAN links. Routers within enterprise networks represent the LAN/WAN junction points of a network. These routers determine the most appropriate path through the network for the required data streams.
WAN links are connected by switches, which are devices that relay information through the WAN and dictate the service provided by the WAN. WAN communication is often called a service because the network provider often charges users for the services provided by the WAN (called tariffs). WAN services are provided through the following three primary switching technologies:
- Circuit switching
- Packet switching
- Cell switching
Each switching technique has advantages and disadvantages. For example, circuit-switched networks offer users dedicated bandwidth that cannot be infringed upon by other users. In contrast, packet-switched networks have traditionally offered more flexibility and used network bandwidth more efficiently than circuit-switched networks. Cell switching, however, combines some aspects of circuit and packet switching to produce networks with low latency and high throughput. Cell switching is rapidly gaining in popularity. ATM is currently the most prominent cell-switched technology. For more information on switching technology for WANs and LANs, see Chapter 2, "Network Design Basics."
Trends in WAN Design
Traditionally, WAN communication has been characterized by relatively low throughput, high delay, and high error rates. WAN connections are mostly characterized by the cost of renting media (wire) from a service provider to connect two or more campuses together. Because the WAN infrastructure is often rented from a service provider, WAN network designs must optimize the cost of bandwidth and bandwidth efficiency. For example, all technologies and features used to connect campuses over a WAN are developed to meet the following design requirements:
Optimize WAN bandwidth
Minimize the tariff cost
Maximize the effective service to the end users
Recently, traditional shared-media networks are being overtaxed because of the following new network requirements:
Necessity to connect to remote sites
Growing need for users to have remote access to their networks
Explosive growth of the corporate intranets
Increased use of enterprise servers
Network designers are turning to WAN technology to support these new requirements. WAN connections generally handle mission-critical information and are optimized for price/performance bandwidth. The routers connecting the campuses, for example, generally apply traffic optimization, multiple paths for redundancy, dial backup for disaster recovery, and QoS for critical applications.
Table 1-2 summarizes the various WAN technologies that support such large-scale network requirements.
Table 1-2 Summary of WAN Technologies
|
WAN Technology |
Typical Uses |
|
Asymmetric Digital Subscriber Line |
A new modem technology. Converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. ADSL transmits more than 6 Mbps to a subscriber, and as much as 640 kbps more in both directions. |
|
Analog modem |
Analog modems can be used by telecommuters and mobile users who access the network less than 2 hours per day, or for backup for another type of link. |
|
Leased line |
Leased lines can be used for Point-to-Point Protocol (PPP) networks and hub-and-spoke topologies, or for backup for another type of link. |
|
Integrated Services Digital Network (ISDN) |
ISDN can be used for cost-effective remote access to corporate networks. It provides support for voice and video as well as a backup for another type of link. |
|
Frame Relay |
Frame Relay provides a cost-effective, high-speed, low-latency mesh topology between remote sites. It can be used in both private and carrier-provided networks. |
|
Switched Multimegabit Data Service (SMDS) |
SMDS provides high-speed, high-performance connections across public data networks. It can also be deployed in metropolitan-area networks (MANs). |
|
X.25 |
X.25 can provide a reliable WAN circuit or backbone. It also provides support for legacy applications. |
|
WAN ATM |
WAN ATM can be used to accelerate bandwidth requirements. It also provides support for multiple QoS classes for differing application requirements for delay and loss. |