When designing a WAN solution, the requirements typically stem from two goals:
- Service Level Agreement (SLA): This agreement defines the availability of the network, based on what level of availability, downtime and impact are acceptable to the organization.
- Cost and Usage: Consider the budget, expected utilization and usage requirements
Three objectives of effective WAN solution design:
- WAN must support policies and goals of the organization
- WAN technology selected must meet application requirements as well as future growth
- The proposed design must be within the budget allocated
The WAN interfaces with the Enterprise Edge module. There can be multiple connections, commonly used connectivity modules include Internet, DMZ, and site-to-site circuits. ISPs offer many options for Internet and DMZ connectivity as well as inter-site connectivity such as MPLS VPN/WAN. Alternative connection options include DSL/cable with IPSEC VPN.
WAN technology can be point-to-point or point-to-multipoint, such as MPLS or Frame Relay. Public WAN connections over the Internet such as cable/DSL are available as well. Usually Internet connections have a much lower SLA than MPLS/Frame Relay connections.
WAN Transport Technology
When choosing which WAN technology to implement, consideration must be taken for whether public Internet transport or private WAN connections are required. Geography also plays a role in what WAN technologies are available in a given area. Major cities have many options, while rural areas typically have few. Here are some WAN technologies compared/contrasted in terms of bandwidth, reliability, latency and cost:
ISDN: Low bandwidth, medium reliability, medium latency, low cost
DSL: Low/medium bandwidth, low reliability, medium latency, low cost
Cable: Low/medium bandwidth, low reliability, medium latency, low cost
Wireless: Low/medium bandwidth, low reliability, medium latency, medium cost
Frame Relay: Low/medium bandwidth, medium reliability, low latency, medium cost
TDM: Medium bandwidth, high reliability, low latency, medium cost
Metro Ethernet: Medium/high bandwidth, high reliability, low latency, medium cost
SONET/SDH: High bandwidth, high reliability, low latency, high cost
MPLS: High bandwidth, high reliability, low latency, high cost
Dark Fiber: High bandwidth, high reliability, low latency, high cost
DWDM: High bandwidth, high reliability, low latency, high cost
Above technologies explained below:
Integrated Services Digital Network was standardized in the early 1980's. It's an all-digital phone line that carries voice and data. It comes in two flavors: Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
BRI consists of two B channels and one D channel. Both BRI channels operate at 64kbps and carry data. D channel handles signaling/control info and operates at 16kbps. 48kbps is used for synchronization, totalling 192kbps data rate.
PRI consists of 23 B channels and 1 D channel in North America/Japan. Each channel operates at 64kbps, totalling 1.544 Mbps including the overhead. In Europe/Australia the service has 30 B channels and 1 64 kbps D channel.
Digital Subscriber Line (DSL)
DSL provides high speed Internet over plain old copper telephone cable using frequencies not utilized in normal voice calls.
ADSL is the most popular flavor of DSL and most widely available. The upstream/downstream is asymmetric, usually upstream is much slower than downstream. ADSL's main drawback is that it must be deployed geographically close to a digital subscriber line access multiplexer (DSLAM), typically less than 2 km. With DSL, the customer premise equipment (CPE) generally means a DSL modem and PC. An ADSL circuit consists of twisted-pair telephone line containing three info channels:
- Medium-speed downstream channel
- Low-speed upstream channel
- Basic telephone service channel