Purpose of WANs
LANs and WANs
Whether at work or at home, we all use Local Area Networks (LANs). However, LANs are limited to a small geographical area.
A Wide Area Network (WAN) is required to connect beyond the boundary of the LAN. A WAN is a telecommunications network that spans over a relatively large geographical area. A WAN operates beyond the geographic scope of a LAN.
In the figure, WAN services are required to interconnect an enterprise campus network to remote LANs at branch sites, telecommuter sites, and remote users.
The table highlights differences between LANs and WANs.
|Local Area Networks (LANs)||Wide Area Networks (WANs)|
|LANs provide networking services within a small geographic area (i.e., home network, office network, building network, or campus network).||WANs provide networking services over large geographical areas (i.e., in and between cities, countries, and continents).|
|LANs are used to interconnect local computers, peripherals, and other devices.||WANs are used to interconnect remote users, networks, and sites.|
|A LAN is owned and managed by an organization or home user.||WANs are owned and managed by internet service, telephone, cable, and satellite providers.|
|Other than the network infrastructure costs, there is no fee to use a LAN.||WAN services are provided for a fee.|
|LANs provide high bandwidth speeds using wired Ethernet and Wi-Fi services.||WANs providers offer low to high bandwidth speeds, over long distances using complex physical networks.|
Private and Public WANs
WANs may be built by a variety of different types of organizations, as follows:
- An organization that wants to connect users in different locations
- An ISP that wants to connect customers to the internet
- An ISP or telecommunications that wants to interconnect ISPs
A private WAN is a connection that is dedicated to a single customer. This provides for the following:
- Guaranteed service level
- Consistent bandwidth
A public WAN connection is typically provided by an ISP or telecommunications service provider using the internet. In this case, the service levels and bandwidth may vary, and the shared connections do not guarantee security.
Physical topologies describe the physical network infrastructure used by data when it is travelling from a source to a destination. The physical WAN topology used in WANs is complex and for the most part, unknown to users. Consider a user in New York establishing a video conference call with a user in Tokyo, Japan. Other than the user’s internet connection in New York, it would not be feasible to identify the all of the actual physical connections that are needed to support the video call.
Instead, WAN topologies are described using a logical topology. Logical topologies describe the virtual connection between the source and destination. For example, the video conference call between the user in New York and Japan would be a logical point-to-point connection.
WANs are implemented using the following logical topology designs:
- Point-to-Point Topology
- Hub-and-Spoke Topology
- Dual-homed Topology
- Fully Meshed Topology
- Partially Meshed Topology
Note: Large networks usually deploy a combination of these topologies.
A point-to-point topology, as shown in the figure, employs a point-to-point circuit between two endpoints.
Point-to-point links often involve dedicated, leased-line connections from the corporate edge point to the provider networks. A point-to-point connection involves a Layer 2 transport service through the service provider network. Packets sent from one site are delivered to the other site and vice versa. A point-to-point connection is transparent to the customer network. It seems as if there is a direct physical link between two endpoints.
It can become expensive if many point-to-point connections are required.
A hub-and-spoke topology enables a single interface on the hub router to be shared by all spoke circuits. Spoke routers can be interconnected through the hub router using virtual circuits and routed subinterfaces. The figure displays a sample hub-and-spoke topology consisting of three spoke routers connecting to a hub router across a WAN cloud.
A hub-and-spoke topology is a single-homed topology. There is only one hub router and all communication must go through it. Therefore, spoke routers can only communicate with each other through the hub router. Consequently, the hub router represents a single point of failure. If it fails, inter-spoke communication also fails.
A dual-homed topology provides redundancy. As shown in in the figure, two hub routers are dual-homed and redundantly attached to three spoke routers across a WAN cloud.
The advantage of dual-homed topologies is that they offer enhanced network redundancy, load balancing, distributed computing and processing, and the ability to implement backup service provider connections.
The disadvantage is that they are more expensive to implement than single-homed topologies. This is because they require additional networking hardware, such as additional routers and switches. Dual-homed topologies are also more difficult to implement because they require additional, and more complex, configurations.
Fully Meshed Topology
A fully meshed topology uses multiple virtual circuits to connect all sites, as shown in the figure.
This is the most fault-tolerant topology of the five shown. For instance, if site B lost connectivity to site A, it could send the data through either site C or site D.
Partially Meshed Topology
A partially meshed topology connects many but not all sites. For example, in the figure sites A, B, C are still fully meshed. Site D must connect to site A to reach sites B and C.
Another aspect of WAN design is how an organization connects to the internet. An organization usually signs a service level agreement (SLA) with a service provider. The SLA outlines the expected services relating to the reliability and availability of the connection. The service provider may or may not be the actual carrier. A carrier owns and maintains the physical connection and equipment between the provider and the customer. Typically, an organization will choose either a single-carrier or dual-carrier WAN connection.
Single-Carrier WAN Connection
A single-carrier connection is when an organization connects to only one service provider, as shown in the figure. An SLA is negotiated between the organization and the service provider. The disadvantage of this design is the carrier connection and service provider are both single points of failure. Connectivity to the internet would be lost if the carrier link or the provider router failed.
Dual-Carrier WAN Connection
A dual-carrier connection provides redundancy and increases network availability, as shown in the figure. The organization negotiates separate SLAs with two different service providers. The organization should ensure that the two providers each use a different carrier. Although more expensive to implement, the second connection can be used for redundancy as a backup link. It could also be used to improve network performance and load balance internet traffic.
Network requirements of a company can change dramatically as the company grows over time. Distributing employees saves costs in many ways, but it puts increased demands on the network. Not only must a network meet the day-to-day operational needs of the business, but it must be able to adapt and grow as the company changes. Network designers and administrators meet these challenges by carefully choosing network technologies, protocols, and service providers. They must also optimize their networks by using a variety of network design techniques and architectures.
To illustrate differences between network size, we will use a fictitious company called SPAN Engineering as it grows from a small, local, business into a global enterprise. SPAN Engineering, an environmental consulting firm, has developed a special process for converting household waste into electricity and is developing a small pilot project for a municipal government in its local area.
The company initially consisted of 15 employees working in a small office, as shown in the figure.
They used a single LAN that was connected to a wireless router for sharing data and peripherals. The connection to the internet is through a common broadband service called Digital Subscriber Line (DSL), which is supplied by their local telephone service provider. To support their IT requirements, they contracted services from the DSL provider.
Within a few years, the company grew and required several floors of a building, as shown in the figure.
The company now required a Campus Area Network (CAN). A CAN interconnects several LANs within a limited geographical area. Multiple LANs are required to segment the various departments that are connecting to multiple switches in a campus network environment.
The network includes dedicated servers for email, data transfer, and file storage, and web-based productivity tools and applications. A firewall secures internet access to corporate users. The business now requires in-house IT staff to support and maintain the network.
A few years later, the company expanded and added a branch site in the city, and remote and regional sites in other cities, as shown in the figure.
The company now required a metropolitan area network (MAN) to interconnect sites within the city. A MAN is larger than a LAN, but smaller than a WAN.
To connect to the central office, branch offices in nearby cities used private dedicated lines through their local service provider. Offices in other cities and countries require the services of a WAN or may use internet services to connect distant locations. However, the internet introduces security and privacy issues that the IT team must address.
SPAN Engineering has now been in business for 20 years and has grown to thousands of employees distributed in offices worldwide, as shown in the figure.
To reduce network costs, SPAN encouraged teleworking and virtual teams using web-based applications, including web-conferencing, e-learning, and online collaboration tools to increase productivity and reduce costs. Site-to-site and remote access Virtual Private Networks (VPNs) enable the company to use the internet to connect easily and securely with employees and facilities around the world.