Network Topologies (How Sites Are Connected)

The term "topology" refers to the geometric shape of the physical connection of the lines in a network. The shape of the network, the configuration in which lines are connected to each other, impacts cost, reliability and accessibility. The following are network configurations:

• Point-to-point - one line connecting two locations


• Multipoint - one line connecting more than two sites together, also referred to as multidrop


• Star (hub and spoke) configuration - all locations connect to, or "hub into," a central site. PBXs and data switches in LANs are configured in star topologies. If the main location in a star configuration goes down, all nodes (locations) on the network are out of services.


• Mesh design - all points on the network, nodes, connect to each other in a flat or nonhierarchical manner. If one link in a mesh network is out of service, traffic can be rerouted over other links. Peer-to-peer networks for music sharing are examples of mesh networks. Most wireless community networks based on 802.11 technology use a form of mesh design called partial mesh in which access points with antennas are connected to each other. In partial mesh designs, not all end-user devised are connected to each other.

Image Source: http://upload.wikimedia.org/wikipedia/commons/9/96/NetworkTopologies.png


For more information about network topologies (and about more topologies) visit: http://en.wikipedia.org/wiki/Network_topologies.

Dedicated Private Lines

In telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between locations. In practice, dedicated, private lines may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.

The number of costly private lines used by commercial organizations is decreasing. They are being replaced by VPNs, which are less costly to maintain and have lower monthly lease rates. However, large enterprises, utilities, and financial services organization still use high-speed private lines for high-speed, secure communications.

Factoids about private line services:

• Pricing - Private, dedicated links are priced at flat monthly fees. The fees are not based on minutes used or the amount of data transmitted

• Fixed routes - Dedicated lines are not flexible. Calls and data can only be sent between the fixed points to which the lines are connected. Thus, communications with a site not on the network is not possible

• Exclusive use - Dedicated, private lines can only be used by the organization that leases them

• Multi-service - Dedicated lines are suitable for transmission of video, voice and data. Voice, video and data can share the same dedicated services, or they can use completely different dedicated lines. Firms often lease T-3 lines that have 672 channels to tie locations together. They can use, for example, 24 of the paths for voice and the rest for data and video

• Fixed capacity - Dedicated services are leased or built with a fixed capacity or bandwidth. These speeds range from low 9,600 bps up to OC-3 (155 mbps) and ATM megabit speeds. They also include T-1 and T-3 and fractional T-1 and T-3 speeds

• Security - Dedicated lines provide secure transmissions. Organizations concerned about security may place encryption devices on both ends of dedicated services. The encryption device scrambles the transmission when it leaves the sending location and unscrambles it when it arrives at the receiving location

• Convenience of services - Private lines provide abbreviated dialing and other convenient features, e.g., 4- or 5-digit dialing between sites, one operator can answer calls for multiple locations, one voice mail system can be shared by multiple locations, one directory across locations

Metropolitan area networks (MANs) consist of private lines that connect buildings within a city or metropolitan area. Large hospitals transmit customer records, research files, and radiology images over MANs. Major univserties also use MANs. Dedicated services are available around the clock. This is cost effective for companies that use the dedicated lines for voice, video, and e-mail during the day and bulk data transmissions after hours.

Voice on Frame Relay

Some customers replace private lines with FR networks to carry voice traffic between sites. Voice is compressed so that it requires less bandwidth. Customers either add separate PVCs (Permanent Virtual Circuit) for voice or upgrade their CIR (Committed Information Rate) for extra capacity. For large organizations, defining separate PVCs are cumbersome. It is also costly. Moreover, it does not guarantee quality on the access line between the customer and the frame network. These frames do not have fields capable of indicating priority levels.

Because of the quality issue, some customers do not use FR for customers' calls, only for employee-to-employee calls. If the FR network becomes congested, voice quality can be degraded because packets are dropped or delayed even with higher CIRs.

The desire to add voice between sites is another factor in customers' migration to MPLS (Multiprotocol Label Switching) because of this extra expense to upgrade FR service and the lack of prioritization on the access line. This is particularly true for larger organizations that want to send customer traffic between sites.

Frame Relay - Permanent Virtual Circuits and Committed Information Rate

Frame relay service is priced at fixed monthly fees based on the following elements, plus the cost of the access line used to connect each site to the carrier's frame relay equipment:

- The permanent virtual circuit (PVC) is a logical, predefined path or link through a carrier's network. If sites at Point A and Point B need to exchange data, the carrier defines a permanent virtual circuit between these two locations. PVCs are priced at fixed monthly fees.

- The frame relay port is the entry point, on a FR provider's switch, to the FR network. Multiple PVCs can use one port. Ports are available to variable speeds such as T-1, 56Kbps, 256Kbps and 512 Kbps.

- The committed information rate (CIR) is the minimum number of bits per second, perhaps half the capacity of the port, that the customer is guaranteed to be able to send from each site. Some customers save money by using low committed information rates. Customers can "burst," send data at the maximum speed of their FR port, if bandwidth is available.

Frame Relay Access to Other Networks

Frame relay is an access technology in which customers' packets are put into frames. In addition to LAN-to-LAN connectivity, it is used to access the following types of networks:

• Frame relay networks that carry traffic on asynchronous transfer mode (ATM) switches
• MPLS (Multiprotocol Label Switching) virtual private networks
• The public Internet

Equipment on customer premises that converts Ethernet local area network packets into frames is called a Frame Relay access device (FRAD). It is often a card within the router. Each frame has bits called the flag, telling the network when the user data (frame) starts and when it ends. There also are addressing and destination bits in the frame for billing and routing purposes so that the FR provider knows where to route and bill each frame.

Customers' frames are sent to ports on the carrier's network. Routers at the carrier's central office convert the customer data to a format compatible with the carrier's network and send it to the core network.

Frame Relay Access Line

The line that connects each customer to the Frame Relay network is called an access line. It provides access from the user's router to the FR network. Each site that uses the FR service leases a circuit, a telephone line, from its equipment to a port on the FR switch. Access line speeds vary from 56 Kbps to subrate T-1 speeds (that is, 128, 156, and 384 Kbps) and T-1 all the way up to T-3 (44 Mbps).

Sites at different locations in the same organizations can be configured with access lines at different speeds. Some FR vendors also offer dial-up (for example, ISDN) access to their networks, most often as a backup to their dedicated access in case the dedicated access lines to the FR network fail. Higher-speed access lines cost more than lower-speed ones.

To save money on access lines, smaller customers share their T-1 lines for voice and FR access. For example, 18 channels of the 24 T-1 channels may be connected to the telephone system for voice traffic. The other six channels carry FR traffic to the network service provider's FR port.

Frame Relay

A public network offering that enables customers to transmit data between LANs at multiple locations. It also is used to access the Internet. Frame relay (FR) was first promoted as a lower-priced substitute for private lines. By using FR, organizations do not have to plan, build, and maintain their own duplicate paths to each of their sites. Multiple users share the FR networks. It is offered by local and long distance (LD) telephone companies.

Frame relay's requirement of defining links between sites in advance, which can be cumbersome for large organizations that want to connect each site to every other site, is leading some companies to choose MPLS VPNs. Falling prices on MPLS service and frame relay's unsuitability for voice are other reasons organizations are starting to migrate to MPLS. However, FR is still a popular, cost-effective choice for organizations.

Source: http://upload.wikimedia.org/wikipedia/en/2/2e/Frame_relay.jpg