Network performance metrics include availability, reliability, robustness, and bit error rate. Obviously, these terms are subjective as single words. Including them in a contract for service is wise. Defining them in words that can be used to set performance expectations where money is to be paid in exchange is even wiser. Drawing a diagram and making a list of locations and facility or circuit identification numbers that the definitions cover is absolutely necessary to ensure services received meet expectations; otherwise, money may be paid for inferior or unsatisfactory services.

Here are some generalized statements in layman language to serve as a starting point for definitions in a service contract.

Availability is a measure of the time network resources being paid for that are actually available for use. The time period may be as specified as 24 hours a day, 7 days a week, 365 days a year.

Reliability is a measure of the time the network is performing in accordance with specifications when it is available.

Robustness is a measure of the network’s ability to recover from loss of service caused by a malfunction of one or more elements making up the network.

Bit error rate, also referred to as bit error ratio, is a measure of bit errors received, compared to the total number of bits received over a given time. Packet networks measure lost packets instead of measuring bit errors. The packet either makes it through the network, or it doesn’t. Obviously, if the network doesn’t successfully carry a packet, there will be errors in a file.
Transmission facilities in communications networks are built on terrestrial and satellite radio wave (wireless), wire, and light wave media.

In general, wire has significant distance limitations depending on the electrical bandwidth and level of signal it’s expected to carry. Radio waves are better than wire in both respects, but not nearly as capable as light waves. Light waves have many times the bandwidth capacity as radio waves.

For example, broadcast satellite service (BSS) facilities are designed around an architecture having 32 transponder channels, each with 32MHz occupied bandwidth. Each channel carries a single quadrature phase shift-keyed bitstream capable of about 30 Mbs payload. Direct satellite service (DSS) facilities occupying a single slot are typically configured with 36-, 54-, or 72-MHz transponders and are typically loaded with subcarriers. Each subcarrier is separated from its neighbors, depending on the amount of bandwidth in the payload it’s required to carry. 

Generally, a 36MHz transponder can accommodate a DS-3 (44.736 Mbs), depending on uplink antenna size, transmit power and receive antenna size. These variables all accumulate in a calculation involving reliability, robustness and bit error rate (BER). The lower the error requirement, the more expensive the facility becomes. Terrestrial radio operating in the same band, for example, 4 to 6 GHz or 11 to 12 GHz exhibit similar payload capacity and BER characteristics. Radio channel baseband BER ranges from 10 7 to 10 9.

Wireless LANs are proliferating as well, using the 1Ghz spectrum. Bandwidth ranges from 10 to approximately 50 Mbs, depending on equipment and physical free space path distance.
Single mode fibers running SONET/SDH optical carrier–based protocol has many times the payload capacity and significantly better BER. An OC-48/STM-16 runs at 2.5Gbs and has a capacity of 48 STS-1 (51.84 Mbs). Each STS-1 can carry 1 DS-3 and several more T1s. Baseband link performance is in the range of 10 9 to 10 11 BER.

When differentiating between satellite and terrestrial transmission, it’s important to consider several factors. These include the nature or topology of the network, reliability, robustness, and error rate performance. All things being equal, which of course they aren’t in every detail, but at a high level, satellite networks tend to be more cost-effective and reliable in point-to-multiple-point topology than terrestrial networks. This is not a fiber vs. radio technology consideration; it’s a physical consideration. Fundamentally, the satellite transponder is the common source for multiple receivers of the same information. The physical aspect is one transmitter (albeit an expensive one) to many inexpensive receivers. If the satellite could radiate light waves instead of, or in addition to, radio waves, the consideration wouldn’t change. What would change is the payload capacity for a given radiated source. It’s well known that light waves are capable of carrying far more information than radio waves. The difference is primarily the fact that free space propagation or attenuation is drastically different than physical conductor propagation or attenuation.

Network architecture is best described with a topology map or block diagram. Depending on the business requirements and nature of the traffic, the topology map and block diagram can be combined in a single figure and be used as a conceptual tool or have greater detail added and be used as a design detail document in test and acceptance work, network performance evaluation, or fault diagnosis. The topology map or block diagram is used to capture and convey locations, number of sites, type of facilities, number and type of connections, service demarcation points, bandwidth, phone numbers, network domains, IP address, service provider boundaries and interface points, type and class of service, etc.

No comments:

Telecom Made Simple

Related Posts with Thumbnails