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.It was intended to provide interoperability among switching systems madeby different manufacturers, as well as to buffer the disparities in capacity and transmission rates of thosemyriad systems.To fulfill this role, physical interfaces, framing conventions, and two families of signaling standards weredeveloped.These two families range from 51.84 Mbps to 2.48 Gbps.Note SONET is an ANSI standard that has been embraced, but not wholly adopted, by the ITU.TheITU's version varies subtly, but in significant ways, and is known as the Synchronous Digital Hierarchy(SDH).The basic rate of transmission in SDH is 155.52 Mbps, unlike the 51.84 Mbps basic rate ofSONET.Another fairly minor difference is that SDH's copper-based transmission standards are known assynchronous transport modules (STMs) rather than synchronous transport signals (STSs).SONET supports two transmission systems, both discussed in the following sections:The OC systemThe Synchronous Transport Signal (STS) systemThe OC Systemhttp://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2906.htm (17 of 20) [02/02/2001 11.37.37] Transmission TechnologiesOne last series of standards that warrants examination is the Optical Carrier (OC) system.Table 6-3presents the OC standards, their bandwidth, and the number of DS-0 and DS-1 channels that eachsupports.Table 6-3: Optical Carrier BandwidthsOptical Carrier Line Bandwidth DS0 Channels DS1 ChannelsOC-1 51.84 Mbps 672 28OC-3 155.52 Mbps 2,016 84OC-9 466.56 Mbps 6,048 252OC-12 622.08 Mbps 8,064 336OC-18 933.12 Mbps 12,096 504OC-24 1.244 Gbps 16,128 672OC-36 1.866 Gbps 24,192 1,008OC-48 2.488 Gbps 32,256 1,344In theory, this table can be expanded almost indefinitely by continuing to multiply the basic OC-1.Service is available at OC-192 rates, for example, even though OC-192 has not yet been standardized.Ofthese specified standards, only OC-1, OC-3, OC-12, and OC-48 are commonly used.The other standardshave fallen into disuse.They are defined, but not used.As optical signaling technology continues to improve, it is reasonable to expect greater multiples of theOC-1 rate to become standardized.In fact, the technology exists to transmit at an OC-192 rate! It is alsolikely that these larger multiples will be retrofitted into the SONET and/or SDH standards.The STS SystemSONET's OC rates can also be implemented over an electrical signaling system using copper wiring.These electrical rates are indicated with an STS designation rather than the OC designation.Aside fromthis physical difference, there is a one-to-one correlation between the two standards.In other words,STS-1 is directly equivalent to OC-1; STS-3 equals the OC-3 rate of 155.52 Mbps; and so forth.It is important to note that the STS-n standards span up to STS-48, which yields 2.488 Gbps ofbandwidth.Providing this bandwidth to electrical signals over copper presents some significant technicalchallenges.For that matter, driving a copper wire at rates in excess of 155.52 Mbps becomes problematicover distances longer than 100 meters.Consequently, only STS-1 and STS-3 are actually defined andusable.The remainder are little more than theoretical constructs.The T-Carrier SystemThe digital signaling standards listed in Table 6-1 are implemented through a physical carrier system.The most prevalent carrier system in North America is the T-carrier system.The leased lines deliveredby this carrier system are prefaced with the letter T.A leased line delivered by T-carrier that conforms tothe DS-1 standard is known as a T-1, for example.This important distinction is frequently lost on eventhe most technical people! Consequently, the terms DS-n and T-n (where n identifies a specific number)http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2906.htm (18 of 20) [02/02/2001 11.37.37] Transmission Technologiesare often incorrectly used interchangeably.They are not interchangeable.DS-n identifies a standard,whereas T-n identifies a standards-compliant circuit.The T-carrier system was originally designed to provide multiplexed voice communications over a singletransmission facility.These facilities were used to transport calls between the various switching centers.A single T-1 circuit could carry 24 calls.Because the calls were in digital form, they could be amplifiedand regenerated en route to their destination.Therefore, digitized voice signals were inherently clearerthan analog voice communications, which couldn't be regenerated.Digitization of voice signals, however, presented some technical challenges.The T-carrier systemimplemented several improvements over the earlier carrier systems.These improvements werespecifically designed to improve the quality of digitized voice transmission.In effect, theseimprovements made the T-carrier system a better transport mechanism for digitized transmissions,without bias toward voice or data.Specific improvements were made in line encoding techniques andframing formats.T-Carrier ServicesAs described earlier in the section titled "ANSI's Digital Signal Hierarchy" only two of the DS standardsare available as commercial services: T-3 and T-1.A third service is also available: fractional T-1.However, fractional T-1 is a T-1 with a fraction of the 24 available channels actually in use.The T-3 transmission facility provides 44.736 Mbps of bandwidth.This bandwidth can be channelizedinto 672 separate channels of 64 kbps each or 28 T-1-equivalent channels.T-3s tend to be veryexpensive, especially as geographic distances increase.For any networking application that requiresmore bandwidth than a couple of T-1s can provide, a T-3 may actually prove to be the more economicalsolution.The T-1 transmission facility is the foundation of the T-carrier system.This basic service providescustomers with 1.544 Mbps of gross bandwidth.This can be channelized into as many as 24 channels of64 kbps each, or left whole for high-bandwidth networking applications.Networks that either can't afford the cost of a full T-1 or don't need its bandwidth may find the fractionalT-1 service more attractive.Fractional T-1 is actually constructed from a T-1 circuit.Bandwidth on thatcircuit is fractionalized using a device known as a channel bank.The subcircuit is usually a 56 kbpscircuit, but can be as small as 9.6 kbps.These subrate channels can then be delivered to differentcustomers [ Pobierz całość w formacie PDF ]