The best practice man/wan design (Data Communications and Networking)

Developing best practice recommendations for MAN and WAN design is more difficult than for LANs and backbones because the network designer is buying services from different companies rather than buying products. The relatively stable environment enjoyed by the MAN/WAN common carriers is facing sharp challenges by VPNs at the low end and Ethernet services at the high end. As larger IT and equipment firms begin to enter the VPN and Ethernet services markets, we should see some major changes in the industry and in the available services and costs.

Energy Sciences Network

Figure 9.14 Energy Sciences Network

We also need to point out that the technologies in this topic are primarily used to connect different corporate locations. Technologies primarily used for Internet access (e.g., DSL, cable modem) are discussed in the next topic.

We use the same two factors as we have previously for LANs and backbones (effective data rates and cost), plus add two additional factors: reliability and network integration. Reliability refers to the ability to predictably send messages as expected. Network integration refers to the ease with which the MAN/WAN service can be used to connect LANs and backbones.


Figure 9.15 summarizes the major services available today for the MAN and WAN, grouped by the type of service. A few patterns should emerge from the table. For small MANs and WANs with low data transmission needs, POTS dial-up services are a reasonable alternative. POTS can be more difficult to integrate with LANs and backbones, so this is a good option only if one is willing to use dial-up connections. Since most of this type of network is used for Internet access, we really need to wait until the next topic before drawing conclusions.

Type of Service

Nominal Data Rates

Effective Data Rates

Relative Cost

Reliability

Network Integration

Circuit-Switched Services

POTS


33.6 Kbps to 56 Kbps

33 to 300 Kbps1

Low

High

Difficult

ISDN

128 Kbps to 1.5 Mbps

122 Kbps to 1.3 Mbps

Moderate

Moderate

Difficult

B-ISDN

155 Mbps to 622 Mbps

300 Mbps to 1200 Mbps2

High

Low

Difficult

Dedicated-Circuit

Services

T Carrier

64 Kbps to 274 Mbps

53 Kbps to 218 Mbps

Moderate

High

Moderate

SONET

50 Mbps to 10 Gbps

48 Mbps to 9.1 Gbps

High

High

Moderate

Packet-Switched

Services

X.25

56 Kbps to 2 Mbps

50 Kbps to 1.5 Mbps

Moderate

High

Difficult

ATM

52 Mbps to 10 Gbps

84 Mbps to 16 Gbps3

High

Moderate

Moderate

Frame Relay

56 Kbps to 45 Mbps

56 Kbps to 44 Mbps

Moderate

Moderate

Moderate

Ethernet

1 Mbps to 40 Gbps

900 Kbps to 36 Gbps

Low

High

Simple

VPN Services

VPN

56 Kbps to 40 Gbps

50 Kbps to 36 Gbps

Very Low

Low

Moderate

Notes:

1 Assuming data compression and no noise 2B-ISDN is full duplex 3 ATM is full duplex

Figure 9.15 MAN/WAN services

For networks with moderate data transmission needs (64 Kbps-2 Mbps) there are several distinct choices. If cost is more important than reliability, then a VPN is probably a good choice. If you need flexibility in the location of your network connections and you are not completely sure of the volume of traffic you will have between locations, frame relay is probably a good choice. If you have a mature network with predictable demands, then T carrier services is probably a good choice (Figure 9.16).

For high-traffic networks (2 Mbps-45 Mbps), the new Ethernet services are a dominant choice. Some organizations may prefer the more mature—and therefore proven—T3 or frame relay services, depending on whether the greater flexibility of packet services provides value or a dedicated circuit makes more sense.

For very-high-traffic networks (45Mbps-10 Gbps), Ethernet services again are a dominant choice. And again some organizations may prefer the more mature ATM or SONET services, depending on whether the greater flexibility of packet services provides value or a dedicated circuit makes more sense.

Unless their data needs are stable, network managers often start with more flexible packet-switched services and move to the usually cheaper dedicated-circuit services once their needs have become clear and an investment in dedicated services is safer. Some packet-switched services even permit organizations to establish circuits with a zero-CIR (and rely entirely on the availability of the MAR) so network managers can track their needs and lease only what they need.

Network managers often add a packet network service as an overlay network on top of a network built with dedicated circuits to handle peak data needs; data usually travels over the dedicated-circuit network, but when it becomes overloaded with traffic, the extra traffic is routed to the packet network.

Network Needs

Recommendation

Low Traffic Needs (64 Kbps or less)

POTS if dial-up is acceptable VPN if reliability is less important Frame relay otherwise

Moderate Traffic Needs (64 Kbps to 2 Mbps)

VPN if reliability is less important

T1 if network volume is stable and predictable

Frame relay otherwise

High Traffic Needs (2 Mbps to 45 Mbps)

Ethernet if available

T3 if network volume is stable and predictable Frame relay otherwise

Very High Traffic Needs (45 Mbps to 10 Gbps)

Ethernet if available

SONET if network volume is stable and predictable ATM otherwise

Figure 9.16 Best practice MAN/WAN recommendations

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