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SDH TRANSMISSION BACKBONE

Table of Contents DIGITAL HIERARCHIES SONET NETWORK ELEMENTS SONET-SDH BASICS SUMMARY

Back to Backbone Networks

• DIGITAL HIERARCHIES
  

  The physical connections netting the telephone network are usually digital and organized into concentration stages agglomerating the traffic of suburbs, cities, regions, nations and between nations. It's done via TDM (Time Division Multiplexing) equipments spread all over the telephone network and CODECS that convert analog signals into digital and vice versa.

  This digital infrastructure is used to carry the voice traffic but also is the carrier of digital data networks (digital leased lines and ISDN)

  There are two TDM transmission systems with their hierarchy in use: "plesiochronous (nearly synchronous) digital hierarchy (PDH)" and "synchronous digital hierarchy (SDH)", the most recent one providing higher speed channels.
  

  •  PLESIOCHRONOUS HIERARCHY

    This hierarchy was developed nearly 40 years ago to carry digital voice channels. It structures the transmission infrastructure into several layers. Different hierarchical levels are used in North America, in Europe and in Japan.

    Multiplex Levels	Alias	Voice Channels	BIT RATE (Mbps)
    			North America	Europe	Japan
    0	DS0	1	0.064	0.064	0.064
    1	DS1 or T1	24	1.544		1.544
    	E1	30		2.048
    	DS1c	48	3.152		3.152
    2	DS2	96	6.312		6.312
    		120		8.448
    3	DS3 or T3	480		34.368	32.064
    	E3	672	44.376
    		1344	91.053
    		1440			97.728
    4		1920		139.264
    		4032	274.176
    		5760			397.2
    5		7680		565.148
    ACRONYM: PDH : Plesiochronous Digital Hierarchy DS : Digital Stream - T or E refers to transmission repeater systems

     PDH is an asynchronous multiplexing scheme in the sense that the different tributary channels don't have to be clock synchronized between them and with the aggregate channel. A centralized and very stable network clock is unnecessary avoiding the problems of clock stability, recovery and distribution.
    But the consequence is the implementation complexity of cross connecting the aggregated channels, with demultiplexing and remultiplexing at each cross connect node.
    

  •  SYNCHRONOUS DIGITAL HIERARCHY (SONET-SDH)

    There are two standards for the synchronous digital hierarchy: SONET (Synchronous Optical NETwork) for North America and SDH (Synchronous Digital Hierarchy): the ITU international standard.

  The two Key benefits of this new hierarchy versus PDH are:
  • Higher transmission and aggregate rates
  • Direct multiplexing and cross connecting without intermediate multiplexing stages, due to its synchronous nature and pointers in multiplex streams that delineate the aggregated sub streams.
    

   The SONET or SDU transmission system is structured in several levels, each of them characterized by a transport channel called the Synchronous Transport Signal (STS) in SONET, the Synchronous Transport Module (STM) in SDH.
  This channel is transmitted over an Optical Carrier (OC).

  The different possible SONET /SDH levels are presented in the following table:

  STM level	STS OC level	Bit Rate Mbps	DS0s (64 Kbps)	DS1s (1.544 Mbps)	DS3s (44.376 Mbps)
  	1	51.84	672	28	1
  1	3	155.52	2016	84	3
  2	6	311.04	4032	168	6
  3	9	466.56	6048	252	9
  4	12	622.08	8064	336	12
  6	18	933.12	12096	504	18
  8	24	1244.16	16128	672	24
  9	36	1866.24	24192	1008	36
  12	48	2488.32	32256	1344	48
  24	96	4976.64	64512	2688	96
  48	192	9953.28	129024	5376	192

  Note that the Bit rate of STS-N (Level N) = N* STS-1 = N* 51.84 Mbps and STM-N = 3*STS-N.

  The predominant used channels are the STM-1 (or STS-3) and the STM-4 (or STS-12).

Table of Contents

• SONET-SDH BASICS
  

  It is a superhighway using fiber optics that rings most major cities and provides terabits worth of bandwidth. It is the basic foundation, the underlying transmission network of very high-speed networks such as ATM or SMDS, but also of terrestrial Video networks.

   

  • TRANSPORT CHANNELS

    The STS or STM transport channel aggregates lower speed channels (e.g. 4 STM-1 into 1 STM 4) and also multiplexes T1 or E1, T3 or E3. The aggregated streams are called Virtual Tributaries (in SONET), Virtual Containers (in SDH).

    There are also different levels of tributary channels as listed in the next table.
    

    SONET VT	SDH VC	SONET rate (Mbps)	SDH rate (Mbps)
    VT1.5	VC-11	1.544
    VT2.0	VC-12		2.048
    VT3.0		3.152
    VT6.0	VC-2	6.312	6.312
    	VC-3	44.736	34.368
    	VC-4		139.264
    STS-1		51.84
    STS-3	STM-1	155.52	155.52
    STS-12	STM-4	622.08	622.08
    NOTES: VTxx: "xx" designates the VT transfer rate Within STS-N channels, VTs can also be combined in VT groups (3*VT1.5s, 4*VT2s, 2*VT3s or 1*VT6)

    The multiplexing scheme is organized into functionally independent blocks with "header" and "payload" sections and built around (9 rows * 90 columns) frames.

    Section OH 3 rows	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    Line OH  6 rows	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*
    	P	*	*	*	.....	*	*	*

    3*N columns	N columns	86*N columns
    NOTES: 1. P: Path Overhead - *: Payload data bytes N: level of STS channel - OH: OverHead 2.See "STS overheads" section, for the definition of "Section", "Line", "Path"

     The total frame size is N*9*90 bytes = N* 6480 bit. The frame rate is 8 Kbps.
    The frame bit rate is N* 51.84 Mbps.

    • The payload of an STS-1 frame multiplexing T1s is illustrated below

    1	1	1	…..	1	S	10	10	10	…..	10	S	19	19	19	…..	19
    2	2	2	…..	2	S	11	11	11	…..	11	S	20	20	20	…..	20
    3	3	3	…..	3	S	12	12	12	…..	12	S	21	21	21	…..	21
    4	4	4	…..	4	S	13	13	13	…..	13	S	22	22	22	…..	22
    5	5	5	…..	5	S	14	14	14	…..	14	S	23	23	23	…..	23
    6	6	6	…..	6	S	15	15	15	…..	15	S	24	24	24	…..	24
    7	7	7	…..	7	S	16	16	16	…..	16	S	25	25	25	…..	25
    8	8	8	…..	8	S	17	17	17	…..	17	S	26	26	26	…..	26
    9	9	9	…..	9	S	18	18	18	…..	18	S	27	27	27	…..	27

    1	2	3	......	28	Stuff	1	2	3	......	28	Stuff	1	2	3	.....	28
    T1s

    The T1 n°1 has its 27 bytes of data located in the 9*rows-columns n°1, 30 and 59 of the 86*columns payload, T28 in columns 28, 57, 86.
    A given T1 has an allocated frame space of 27*8 =216 bit. The first two bytes don' convey user 's data: the first is a VT pointer to let the VT floating within its frame space, the second one is a VT level path "housekeeping" Overhead. The byte 25 is used to carry the T1 framing bit. User's data is thus made of 24 bytes=192 bit as expected.

    The overall overhead of T1s transport over STS-1 is 17% (3*28+6*9=138 overhead bytes out of the 810 STS-1 frame bytes).

     

  • STS-N OAM CHANNELS

    In fact those so-called overheads, in fact Operation-Administration and Maintenance Channels, create the flexibility, the transmission quality and the supervision of the STS-N carrier channels.

    An STS-N channel object is characterized by three elements:

    • The Path: the user end to end connection delineated by so called Path-Terminating Equipments (PTE)
    • The Line: the connection between the PTE and the SONET internal transport network, delineated by the so called Line-Terminating Equipments (LTE), but also the network end to end connection as a tandem of sections
    • The Section: the connection between internal network regenerator (switching and non-switching) equipments so called Section-Terminating Equipments (STE).
      

    Each of those elements has its own data carried within the STS-N frame.

    • The Section Overhead provides for multiplex framing information, performance monitoring, management information
    • The Line Overhead for error monitoring, automatic line protection switching, payload pointers management information
    • The Path Overhead for end to end performance monitoring, user data channel, management information

    Payload pointers allow the payload to float anywhere within the STS-N frame. The payload will often overlap multiple frames. To cope with possible transmission jitter, network equipments will update the pointers.

    The pointers (within the Line OH and VT OH) allow to drop-insert and cross connect STS-N channels and their Virtual Tributaries, without any demultiplexing and remultiplexing

Table of Contents

• SONET/SDH NETWORK ELEMENTS
  

  To provide the transmission, the multiplexing and cross connect services, the network uses the following network elements:

  • Terminating multiplexers (PTE category) are the network access equipments on user's premises. They can multiplex multiple T1s/E1s, T3/E3s onto an optical carrier. They are usually connected in redundant fiber dual ring architecture.
  • Add/Drop multiplexers (ADM) with features such as Drop-and-insert, Drop-and-continue, Broadcast. They are also used in a ring structure.

    

    They provide T1/E1 and T3/E3 access to the SONET/SDH infrastructure. They are much more efficient in implementation than their PDH counterparts as underlined below.

    

  • Digital Cross Connect (DXC): cross connectsT1/E1s and T3/E3s, as well as STS-N channels
  • Digital Loop Carrier Systems: concentrate multiple 64 Kbps channels into an OC-3
  • Concentrators, concentrating OC-3, OC-12 and OC-48 into higher Optical Carriers.

• SUMMARY
  

  The SONET-SDH transmission and multiplexing system enables nation-wide and worldwide (in a near future) broadband communications. It is also an integrated multimedia network conveying digital voice, data and video.

  It provides a large variety of transport services with a large range of transfer rates from 64 Kbps to 622 Mbps.
  It is almost an error free transmission medium with built in redundancy to protect against line outages. It is quite flexible with on demand bandwidth capability and also on demand drop-insert features.

  All these reasons explain the success of SONET-SDH networks that are spreading in the public networks but also in Metropolitan networks (MAN) and even in Campus Networks (CAN). The new high speed ATM or SMDS services rely on this digital high-speed error free transmission infrastructure.

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