Data Communications Theory Lecture-8 Dr. Anwar Mousa University of Palestine Faculty of Information Technology.

Data Communications Theory

Lecture-8

Dr. Anwar Mousa

University of PalestineFaculty of Information Technology

SONET / SDH

3

What is SONET & SDH?SONET and SDH are a set of related standards for

synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical NETwork SDH is an acronym for Synchronous Digital Hierarchy. SONET is the United States version of the standard

published by the American National Standards Institutue (ANSI).

SDH is the international version of the standard published by

the International Telecommunications Union (ITU).

4

Optical Level Electrical Level Line Rate (Mbps) SDH Equivalent

OC-1 STS-1 51.840 –

OC-3 STS-3 155.520 STM-1

OC-12 STS-12 622.080 STM-4

OC-48 STS-48 2488.320 STM-16

OC-192 STS-192 9953.280 STM-64

OC-768 STS-768 39813.120 STM-256

OC-1536 STS-1536 79,626,120 STM-512

OC-3072 STS-3072 159,252,240 STM-1024

STS (electrical signaling levels called Synchronous Transport

Signals)

OC: Optical Carrier

STM: Synchronous Transport Modul

The following table lists the hierarchy of the most common SONET/SDH data rates:

5

The SONET/SDHThe "line rate" refers to the raw bit rate carried over the

optical fiber. A portion of the bits transferred over the line are designated

as "overhead ."The overhead carries information that provides OAM&P

(Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, and

performance monitoring .The "line rate" minus the "overhead rate" yields the

"payload rate" which is the bandwidth available for transferring user data such as packets or ATM cells .

6

The SONET/SDHThe SONET/SDH level designations sometimes include a

"c" suffix (such as "OC-48c") .The "c" suffix indicates a "concatenated" or "clear" channel .

This implies that the entire payload rate is available as a single channel of communications (i.e. the entire payload

rate may be used by a single flow of cells or packets) .The opposite of concatenated or clear channel is

"channelized." In a channelized link the payload rate is subdivided into

multiple fixed rate channels. For example, the payload of an OC-48 link may be

subdivided into four OC-12 channels .

7

The SONET/SDHIn practice, the terms STS-1 and OC-1 are sometimes used

interchangeably, though the OC-N format refers to the signal in its optical form .

It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

Note that the typical data rate progression starts at OC-3 and increases by multiples of 4 .

As such, while OC-24 and OC-1536, along with other rates such as OC-9, OC-18, OC-36, and OC-96 may be defined in some standards documents, they are not available on a wide-

range of equipment.As of 2007, OC-3072 is still a work in progress.

8

The SONET/SDHSynchronous optical networking, is a method for

communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber .

The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large

amounts of telephone and data traffic

and to allow for interoperability between equipment from different vendors.

9

The SONET/SDHSynchronous networking differs from PDH in that the exact

rates that are used to transport the data are tightly synchronized across the entire network ,

made possible by atomic clocks.SONET/SDH is a synchronous network using synchronous

TDM multiplexing .All clocks in the system are locked to a master clock .

This synchronization system allows entire inter-country networks to operate synchronously ,

greatly reducing the amount of buffering required between elements in the network.

10

The SONET/SDHBoth SONET and SDH can be used to encapsulate

earlier digital transmission standards, such as the PDH standard, or used directly to support

either ATM or so-called Packet over SONET/SDH (POS) networking .

As such, it is inaccurate to think of SDH or SONET as communications protocols ,

but rather as generic and all-purpose transport containers for moving both voice and data.

11

Structure of SONET/SDH signals SONET and SDH often use different terms to describe

identical features or functions .

The main difference between the two:SONET can use either of two different basic framing

units while SDH has one

12

The basic unit of transmissionThe basic unit of framing in SDH is an STM-1 (Synchronous

Transport Module level - 1), which operates at 155.52 Mbit/s.

SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal - 3, concatenated) ,

but it is otherwise identical in size, bit-rate, and high-level functionality.

SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal - 1), operating at 51.84

Mbit/s - exactly one third of an STM-1/STS-3c.

13

Concatenated signalsIn normal operation, an STS-n signal is made of n

multiplexed STS-1 signal.Sometimes we have a signal with a data rate higher than

what an STS-1 can carry.In this case, SONET allows us to create an STS-n signal

that is not considered as n STS-1 signal; it is one STS-n signal that can not be demultiplexed into n STS-1 signal

(STS-nc).STS-3c can not be demultiplexed into three STS-1 signals.An STS-3c can accommodate 44 ATM cells, each of 53

bytes.

14

SONET LAYERS The SONET standards includes four functional layer:

the photonic, the section, the line and the path layer.They corresponds to both the physical and data link

layer in OSI..1The path layer

The path layer is responsible for the movement of a signal from its optical source to its optical destination.

Path layer overhead is added at this layer.STS multiplexers provide path layer functions

15

SONET LAYERS .2The line layer

The line layer is responsible for the movement of a signal across physical line.

Line layer overhead is added to the frame at this layer.

STS multiplexers and ADM provide line layer functions

16

SONET LAYERS .3The Section layer

The line layer is responsible for the movement of a signal across physical section.

It handles framing, scrambling and error control.

Section layer overhead is added to the frame at this layer.

17

SONET LAYERS .4The Photonic layer

Corresponds to the physical layer of OSI Model.Includes physical specification of the optical fiber

channel,SONET uses NRZ encoding with the presence of

light representing one and the absence of light representing Zero .

18

Framing

In packet or frame oriented data transmission (such as Ethernet), a frame usually consists of a header and a payload,

with the header of the frame being transmitted first, followed by the payload (and possibly a trailer).

In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists,

but instead of the overhead being transmitted before the payload, it is interleaved,

with part of the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next part

of the payload, until the entire frame has been transmitted .

19

MultiplexingThree STS-1 signals may be multiplexed by time-division

multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s .

The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing

2,430 bytes and transmitted in 125 microseconds.Higher speed circuits are formed by successively aggregating

multiples of slower circuits, their speed always being immediately apparent from their designation.

For example, four STS-3 signals can be aggregated to form a 622.08 Mbit/s signal designated as OC-12 or STM-4.

OC-12 STS-12 622.080 STM-4OC-3 STS-3 155.520 STM-1

20

MultiplexingThe highest rate that is commonly deployed is the OC-192 or

STM-64 circuit, which operates at rate of just under 10 Gbit/s .Speeds beyond 10 Gbit/s are technically viable and are under

evaluation.[ Few vendors are offering STM-256 rates now, with speeds of nearly

40Gbit/s .]Where fiber exhaust is a concern, multiple SONET signals can

be transported over multiple wavelengths over a single fiber pair by means of Wavelength division multiplexing ,

including Dense Wave Division Multiplexing (DWDM) and Coarse Wave Division Multiplexing (CWDM). DWDM

circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.

21

SONET/SDH and relationship to 10 Gigabit/// Ethernet

Another fast growing circuit type amongst data networking equipment is 10 Gigabit Ethernet

(10GbE) .This is similar in rate to OC-192/STM-64, and, in its

wide area variant, encapsulates its data using a light-weight SDH/SONET frame.

OC-192 STS-192 9953.280 STM-64However, 10 Gigabit Ethernet does not explicitly

provide any interoperability at the bitstream level with other SDH/SONET systems .

22

SONET equipment

SONET regeneratorSONET Regens extend long haul routes in a way similar to

most regenerators,A regenerator is a repeater that takes a received optical signal

(OC-n) (that has already traveled a long distance ), demodulates it into the corresponding electrical signal (STS-

n), regenerates the electrical signal, and finally modulates the electrical signal into its corresponding OC-n signal .

Since the late 1990s, SONET regenerators have been largely

replaced by Optical Amplifiers .

23

SONET equipment STS (Synchronous Transport Signals) Multiplexer/

Demultiplexer

Mark the beginning points and endpoints of a SONET linkProvide interface between an electrical tributary network and

the optical networkAn STS multiplexer multiplexes signals from multiple

electrical sources and creates the corresponding OC signals. An STS demultiplexer demultiplexes an optical OC signal

into corresponding electricalsignals.

24

SONET equipment

SONET Add-Drop Multiplexer (ADM)SONET ADMs are the most common type of SONET

Equipment .SONET ADMs allows insertion and extraction of signals.SONET ADMs can add STSs coming from different sources

into a given path or can remove a desired signal from a path and redirect it without demultiplexing the entire signal.

Instead of relying on timing and bit positions, add/drop multiplexing use header info such as addresses and pointers

to identify individual streams.

25

SONET equipment

TerminalsA terminal is a device that uses the services of a

SONET network.For example, in the Internet, a terminal can be a

router that sends packets to another router at the other side of a SONET network.

SONET is used as a transport network to carry loads from other WANs.

26

SONET Network ArchitecturesCurrently, SONET (and SDH) have a limited number of

architectures defined .These architectures allow for efficient bandwidth usage

as well as protection (i.e. the ability to transmit traffic even when part of

the network has failed ,)and are key in understanding the almost worldwide usage

of SONET and SDH for moving digital traffic .The three main architectures are:

27

SONET Network Architectures.1Linear networks

Linear networks can be point-to-point or multipoint

Point-to-point NetworksA point-to-point network is normally made of an STS

multiplexer, an STS demultiplexer and zero or more regenerators with no add/drop multiplexers.

The signal flow can be uidirectional or bidirectional.

28

SONET Network ArchitecturesMultipoint networks

A multipoint network uses ADMs to allow the communications between several terminals .

An ADM removes (drops) the signal belonging to the termial connected to it and adds the signal

transmitted from another terminal.Each terminal can send data to one or more

terminals.The signal flow can be uidirectional or bidirectional.

29

SONET Network ArchitecturesLinear APS (Automatic Protection

Switching), also known as 1+1 :To create protection against failre in linear networks,

SONET defines Automatic Protection Switching (APS).

APS means protection between two ADMs or a pair of STSMux/Demux_

Three schemes are common:

30

SONET Network Architectures1 .ONE-PLUS-ONE APS

There are normally two lines: one working line and one protection line .

Both lines are active all the time.The sending multiplexer sends the same data on both lines;

the receiver multiplexer monitors the line and chooses the one with the better quality .

If one of the lines fails, the receiver selects the other line.The sheme is inefficient because two times the bandwidth is

required .

31

SONET Network Architectures2 .ONE-to-ONE APS

There are normally two lines: one working line and one protection line .

The data are sent on the working line until it fails.At this time the receiver , using the reverse channel, informs

the sender to use the protection line instead.The failer recovery is slower than that of the one-plus

scheme but this scheme is more efficient because the protection line is used only when the working line if failed .

32

SONET Network Architectures3 .ONE-to-Many APS

Similat to the one-to-one scheme except that there is only one protection line for many working lines.

When a filer occurs in one of the working lines, the protection line takes control until the failed

line is repaired. It is not as secure as the one-to-one scheme!

33

SONET Network Architectures.2UPSR (Unidirectional Path Switched

Ring) :A type of ring network

A UPSR is a unidirectional network with two rings: one ring used as the working ring and the opther as the

ptoterction ring.The idea is similar to the one-plus-one scheme in a linear

network.The same signal flows through both rings, one clockwise

and the other counterclockwise.

34

SONET Network ArchitecturesI t is called UPSR because monitoring is done at the path

layer.A node receives two copies of the electrical signal at the

path layer, compares them and chooses the one with the best quality.

If a part of a ring fails, the other ring still can guarantee the continuation of data flow.

UPSR, like the one-plus-one scheme, has fast failure recovery, but it is not efficient because we need to have

two rings that do the job of one !

35

SONET Network Architectures.3BLSR (Bidirectional Line Switched Ring) :A type of ring network

Communications is bidirectional, which means that we need two rings for working lines.

We also need two rings for protection lines.The operation is similar to on-to-one APS scheme.

If a working ring in one direction between nodes fails, the receiving node can use the reverse ring to inform the upstream

node in the failed direction to use the protection ring.

36

SONET Network ArchitecturesCombination of rings

SONET uses a combination of interconnected rings to create services in a wide area.

A SONET network may have a regional ring, several local rings and many site rings to give service to a wide area .

These rings can be UPSR. BLSR, or a combination of both .

Mesh Networks

37

Next-generation SONET/SDH SONET/SDH development was originally driven by the need to transport

multiple PDH signals like DS1(1.544 Mbps), E1 (2.048 Mbps), DS3 (44.736 Mbps) and E3 along with other groups of multiplexed 64 kbit/s

pulse-code modulated voice traffic. The ability to transport ATM traffic was another early application.

In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller SONET multiplexing

containers (eg, STS-1) are multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes .

SONET/SDH is therefore able to transport both voice and data simultaneously.

38

Next-generation SONET/SDH

One problem with traditional concatenation, however, is inflexibility.

Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left

over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s Fast Ethernet connection

inside a 155 Mbit/s STS-3c container leads to considerable waste.

Virtual Concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly

arbitrary size (e.g. 100 Mbit/s).

39

Next-generation SONET/SDH

Virtual Concatenation increasingly leverages X.86 or Generic Framing Procedure (GFP) protocols in order to

map payloads of arbitrary bandwidth into the virtually concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamic virtual

concatenation, multiplexing containers based on the short-term bandwidth needs in the network.

The set of next generation SONET/SDH protocols to enable Ethernet transport is referred to as Ethernet over

SONET/SDH (EoS).

Frame Relay

41

Frame Relay

Three types of switching exist: circuit switching, packet switching message switching.

Packet switching can use two approaches: virtual-circuit switching datagram.

Frame Relay is a virtual-circuit WAN with the following properties:

42

Frame Relay

Frame Relay is available in the following speeds: 56 kbit/s, 64 kbit/s, 128 kbit/s, 256 kbit/s, 512 kbit/s, 1.544

Mbps and 44.376 Mbps

Operates in just the physical and data link layersIt can be used as a backbone network to provide

services to protocols that already have a network layer protocol, such as the Internet.

43

Frame Relay

Allows a frame size of 9000 bytes, which can accommodate all LAN frame sizes.

Less expensive than other traditional WANs

Has error detection at the data link layer only.There is no flow control or error control .There is no transmission policy if a frame is damaged;

it is dropped

44

Frame relay

Unlike X.25, whose designers expected analog signals, frame relay offers a fast packet technology, which means that the

protocol does not attempt to correct errors .When a frame relay network detects an error in a frame, it

simply drops that frame. The end points have the responsibility for detecting and

retransmitting dropped frames .(However, digital networks offer an incidence of error

extraordinarily small relative to that of analog networks).

45

Frame Relay

Provide fast transmission capability for more reliable media and for those protocol that have flow and error

control at the higher layer.

Frame Relay consists of an efficient data transmission technique used to send digital information quickly and

cheaply in a relay of frames to one or many destinations from one or many end-points.

46

Frame Relay

Network providers commonly implement frame relay for voice and data as an encapsulation technique, used between local area networks (LANs) over a wide area network

(WAN).

Each end-user gets a private line (or leased line) to a frame-relay node.

The frame-relay network handles the transmission over a frequently-changing path transparent to all end-users.

47

Frame Relay

As of 2006 native IP-based networks have gradually begun to displace frame relay .

With the advent of dedicated broadband services such as cable modem and DSL, the end may loom for the frame relay

protocol and encapsulation .However many rural areas remain lacking DSL and cable

modem services .In such cases the least expensive type of "always-on"

connection remains a 128-kilobit frame-relay line .Thus a retail chain, for instance, may use frame relay for

connecting rural stores into their corporate WAN.

48

Frame relay description

Frame relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points.

This speeds up overall data transmission.

For most services, the network provides a permanent virtual circuit (PVC), Permanent virtual circuit (PVC), means that the customer sees a

continuous, dedicated connection without having to pay for a full-time leased line, while the service-provider figures out the route each frame

travels to its destination and can charge based on usage.

49

Virtual circuits

As a WAN protocol, frame relay is most commonly implemented at Layer 2 (data link layer) of the Open Systems Interconnection (OSI) seven layer model.

Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical end-to-

end links mapped over a physical network, switched virtual circuits (SVCs), analogous to the circuit-switching

concepts of the public-switched telephone network (or PSTN), the global phone network we are most familiar with today.

While SVCs exist and are part of the frame relay specification, they are rarely applied to real-world scenarios. SVCs are most often considered harder to configure and maintain and are generally avoided without appropriate justification.

50

FRAME RELAY LAYERS

Frame relay has only physical and data link layersPhysical layer

No specific protocol is defined It is left to the implementer to use whatever is

availableData link layer

Use a simple protocol that does not support flow or error control

51

Frame relay usage

Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 kbit/s, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbit/s to 622.080 Mbit/s.

Frame Relay originated as an extension of Integrated Services Digital Network (ISDN). Its designers aimed to enable a packet-switched network to transport the circuit-switched technology.

Frame relay has its technical base in the older X.25 packet-switching technology, designed for transmitting analog data such as voice conversations.

52

Frame relay usage Frame relay often serves to connect local area networks

(LANs) with major backbones as well as on public wide-area networks (WANs)

It requires a dedicated connection during the transmission period .

Frame relay does not provide an ideal path for voice or video transmission, both of which require a steady flow of

transmissions .However, under certain circumstances, voice and video

transmission do use frame relay.(VOFR: Voice Over Frame Relay )

53

VOFR: Voice Over Frame Relay

VOFR: Voice Over Frame Relay Voice is digitized using PCM and then compressed.The result is sent as data frame over the network.This feature allows inexpensive sending of voice

over long distances.However, the quality of voice is not as good as voice

over a circuit switched network as PSTN.Also, the varying delay sometimes corrupts real-

time voice .

54

Frame relay Assembler/Disassembler

(FRAD)

To handle frames arriving from other protocols (such as Ethernet,ATM and X.25)

Assembles and disassembles frames comming from other protocols to allow them to be carried by Frame Relay frames.

A FRAD can be implemented as a separate device or as a part of a switch

55

Frame Relay versus X.25

The design of X.25 aimed to provide error-free delivery over links with high error-rates.

Frame relay takes advantage of the new links with lower error-rates, enabling it to eliminate many of the services provided by X.25.

The elimination of functions and fields, combined with digital links, enables frame relay to operate at speeds 20 times greater than X.25.

X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at layers 1 and 2 only. This means that frame relay has significantly less processing to do at

each node, which improves throughput.

56

Frame Relay versus X.25

X.25 prepares and sends packets, while frame relay prepares and sends frames.

X.25 packets contain several fields used for error and flow control, none of which frame relay needs.

The frames in frame relay contain an expanded address field that enables frame relay nodes to direct frames to their destinations with minimal processing .

X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load dictates.

Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.