2.2.3 Leased Line
When permanent dedicated connections are required, leased lines are used with capacities ranging up to 2.5 Gbps.
A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are called leased lines. Leased lines are available in different capacities.
These dedicated circuits are generally priced
based on bandwidth required and distance between the two connected points.
Point-to-point links are generally more expensive than shared services such as
Frame Relay. The cost of leased-line solutions can become significant when they
are used to connect many sites. There are times when cost of the leased line is
outweighed by the benefits. The dedicated capacity gives no latency or jitter
between the endpoints. Constant availability is essential for some applications
such as electronic commerce.
A router serial port is required for each leased-line connection. A CSU/DSU and the actual circuit from the service provider are also required.
Leased lines are used extensively for building WANs and give permanent dedicated capacity.
They have been the traditional connection of
choice but have a number of disadvantages. WAN traffic is often variable and
leased lines have a fixed capacity. This results in the bandwidth of the line
seldom being exactly what is needed. In addition, each end point would need an
interface on the router which would increase equipment costs. Any changes to
the leased line generally require a site visit by the carrier to change
capacity.
Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network. Several connections can be multiplexed over a leased line, resulting in shorter links and fewer required interfaces.
2.2.4 X.25
In response to the expense of leased lines, telecommunications providers introduced packet-switched networks using shared lines to reduce costs. The first of these packet-switched networks was standardized as the X.25 group of protocols. X.25 provides a low bit rate shared variable capacity that may be either switched or permanent.
X.25 is a network-layer protocol and subscribers are provided with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection.
Subscribers connect to the X.25 network with either leased lines or dialup connections. X.25 networks can also have pre-established channels between subscribers that provide a PVC.
X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks.
X.25 technology is no longer widely available as a WAN technology in theUS
Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. Some enterprises also use X.25 based value-added networks (VAN) to transfer Electronic Data Interchange (EDI) invoices, bills of lading, and other commercial documents. For these applications, the low bandwidth and high latency are not a concern, because the low cost makes the use of X.25 affordable.
When permanent dedicated connections are required, leased lines are used with capacities ranging up to 2.5 Gbps.
A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are called leased lines. Leased lines are available in different capacities.
These dedicated circuits are generally priced
based on bandwidth required and distance between the two connected points.
Point-to-point links are generally more expensive than shared services such as
Frame Relay. The cost of leased-line solutions can become significant when they
are used to connect many sites. There are times when cost of the leased line is
outweighed by the benefits. The dedicated capacity gives no latency or jitter
between the endpoints. Constant availability is essential for some applications
such as electronic commerce.A router serial port is required for each leased-line connection. A CSU/DSU and the actual circuit from the service provider are also required.
Leased lines are used extensively for building WANs and give permanent dedicated capacity.
They have been the traditional connection of
choice but have a number of disadvantages. WAN traffic is often variable and
leased lines have a fixed capacity. This results in the bandwidth of the line
seldom being exactly what is needed. In addition, each end point would need an
interface on the router which would increase equipment costs. Any changes to
the leased line generally require a site visit by the carrier to change
capacity.Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network. Several connections can be multiplexed over a leased line, resulting in shorter links and fewer required interfaces.
2.2.4 X.25
In response to the expense of leased lines, telecommunications providers introduced packet-switched networks using shared lines to reduce costs. The first of these packet-switched networks was standardized as the X.25 group of protocols. X.25 provides a low bit rate shared variable capacity that may be either switched or permanent.
X.25 is a network-layer protocol and subscribers are provided with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection.
Subscribers connect to the X.25 network with either leased lines or dialup connections. X.25 networks can also have pre-established channels between subscribers that provide a PVC.
X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks.
X.25 technology is no longer widely available as a WAN technology in the
Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. Some enterprises also use X.25 based value-added networks (VAN) to transfer Electronic Data Interchange (EDI) invoices, bills of lading, and other commercial documents. For these applications, the low bandwidth and high latency are not a concern, because the low cost makes the use of X.25 affordable.
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