New York Institute of Technology

Engineering and Computer Sciences

CSCI-370Computer Networks:

Shrinking the globe one click at a time

Lecture 6

Khurram Kazi

CSCI 370

NYITMajor sources of the slides for this lecture

Computer Networks: A Systems Approach, Larry Peterson

The Internet and Its Protocol, Adrian Farrel’s book. http://www.pcc.qub.ac.uk/tec/courses/network/

SDH-SONET/sdh-sonetV1.1a_1.html http://electrosofts.com/sonet/index.html SONET by Walter Goralski, McGraw-Hill Optical Networking Standards: A Comprehensive

Guide by K. Kazi, Springer.

CSCI 370 2

NYITReference Network: For discussion purposes

Router A

Router B

Router C

Router D

Router F

Router E

LAN 1

LAN 2

Wide Area Network or Metro Area Network

Edge Router

Edge Router

Source of IP Traffic

Destination of IP Traffic (server)

Shows traffic flow

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

CSCI 370 3

NYITVector/Distance vs. Link State Routing

Link State Keeps the volume of

information passed along to other routers to a minimum

Each router periodically checks on the status of neighboring routers, reporting which links are alive to all the other participating routers

With the this information each router can then create its own map of the internetwork

Network A

Network B

Network C

Network D

Router

Router

Router

Router

Network A connects to Network B and CNetwork B connects to Network A and DNetwork C connects to Network A and DNetwork D connects to Network B and C

CSCI 370 4

NYITAutonomous Systems Who owns the internet (one happy family)

Wide variety of organizations National governments Large Internet Service Providers (ISPs) Telephone companies with wide geographic footprint

In the real world, each organization wants the largest possible amount of control and secrecy Each organizational grouping of computers/servers defines itself as an

Autonomous System (AS) AS can operate in isolation from all other groupings Within an AS, routing information is generally widely distributed One router can clearly see the path through the AS network to another

router within the same AS Protocols that distribute routing information within as AS is referred as

Interior Gateway Protocols (IGPs). The word gateway is the old name for a router

CSCI 370 5

NYITAutonomous Systems Organizations and ASs require connectivity to make

the Internet work Connectivity operates in a largely hierarchical way

Home users and small companies paying smaller ISPs for private access (dial-up, wireless, leased lines, cable etc.)

Smaller ISPs and larger corporations buy access to the backbone network operated by larger ISPs

The larger ISPs create a peering agreement with each other to glue the whole thing together

CSCI 370 6

NYITAutonomous Systems Just the connectivity is not enough Must have the ability to route from a router in one AS to a

router in another AS Key to this is the routers that sit on the links between ASs These Autonomous Systems Border Routers (ASBRs) are

responsible for leaking routing information from one AS to another AS

These routers do not divulge too much information about their internal network infrastructure

They reveal just enough information such that IP packets can be routed to the hosts that AS supports

Such routing protocols are called Exterior Gateway Protocols (EGPs) EGPs distribute reachability information in terms of subnetted

and aggregated IP addresses and unique AS identifiers called AS numbers

CSCI 370 7

NYITAutonomous Systems within the Internet

Autonomous System

EGP Link

Connection to Other Autonomous System

IGP LinkASBR

Customer Network

CSCI 370 8

NYIT

Moving onto Physical layer:

Optical Transport Technologies

CSCI 370 9

NYIT

Reference Network: For discussion purposes

Router A

Router B

Router C

Router D

Router F

Router E

LAN 1

LAN 2

Wide Area Network or Metro Area Network

Edge Router

Edge Router

Source of IP Traffic

Destination of IP Traffic (server)

Shows traffic flow

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

ip addr

Physical Layer

CSCI 370 10

NYITPrior to SONET/SDH: The need for Synchronous Optical Networks

Previous technology - PDH - Plesiochronous Digital Heirarchy was limited: US and European systems had little in common -

expensive translators required for transatlantic traffic

"Standard" equipment from different vendors was incompatible

No self checking - expensive manual check and repair system

No standard for high bandwidth links - proprietary Not synchronous above US DS-1 bandwidth

CSCI 370 11

NYITPrior to SONET/SDH: The need for Synchronous Optical Networks

Synchronous? What does synchronous mean to a telephone

engineer "bits from one telephone call are always in the

same location inside a digital transmission frame"

US telephone calls, DS-0, are multiplexed 24 per DS-1 channel DS-0 refers to 64 Kb/s digitized voice signal that is

carried over digital telephone networks

DS-1 lines are synchronous it is easy to remove or insert a call

CSCI 370 12

NYITPrior to SONET/SDH: The need for Synchronous Optical Networks

Plesiochronous? Plesiochronous means "almost synchronous because bits are stuffed into

the frames as padding and the calls location varies slightly - jitters - from frame to frame"

4 DS-1 lines are multiplexed for DS-2 7 DS-2s are multiplexed to DS-3 To isolate a particular call from DS-3 it must be

demultiplexed to DS-1 Very expensive equipment is needed at every

exchange to demultiplex and multiplex high speed lines

CSCI 370 13

NYITTime Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks

The T1 carrier (1.544 Mbps).

CSCI 370 14

NYITTime Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks

Multiplexing T1 streams into higher carriers.

CSCI 370 15

NYITTime Division MultiplexingPDH (Plesichronous Digital Hierarchy) Networks

Bellcore originally proposed SONET - Synchronous Optical NETwork

1985 ANSI T1X1 committee

1986 CCITT SDH standards published: G.707, G.708, G.709

1987 Bellcore submitted SONET to CCITT - much European opposition

G.709 was reassigned to “Interfaces for Optical Transport Network (OTN)”

CSCI 370 16

NYITTime Division MultiplexingPrecursor to SONET/SDH

Compromises Basic rate for SONET increased to 51.840

Mbs to permit more bandwidth for OAM (operation, administration and maintenance functions) - concession to Europeans - a good move

Europeans dropped demand for level 2 and 3 rates to be directly supported

SDH/SONET merged on DS-3 and CEPT-4 rates

CSCI 370 17

NYITSONET/SDH

SDH/SONET would: Improve on existing DS-3 multiplexing standard Provide a non-proprietary solution Establish a hierarchy of digital standards

compatible with European and US systems

CSCI 370 18

NYITTime Division Multiplexing (5)

SONET and SDH multiplex rates.

CSCI 370 19

NYITSONET/SDH Model

4 layers Photonic - physical

characteristics of the optical equipment

Section - frame format and electro-optic conversion

Line - synchronization and multiplexing onto SONET frames

Path - end to end transport

Physical realization: Section - single run of

fibre optic cable Line - one or more

sections Path - end to end circuit

Path

Line

Section

Photonic

Payload Mux

Line

Section

Photonic

STS-1Mux

Section

Photonic

Regenerator

Section

Photonic

Regenerator

Line

Section

Photonic

STS-1Mux

Path

Line

Section

Photonic

Payload Mux

Other CPE

SONET Customer Premises

Equipment

Other CPE

SONET Service Provider

Equipment

Regenerator(s)

SONET Service Provider

Equipment

Other CPE

SONET Customer Premises

Equipment

Other CPE

Section Section Section(s) Section Section

Line Line Line

Path

CSCI 370 20

NYITSONET/SDH Model SONET/SDH networks are

configured as linear networks, where SONET/SDH nodes knows as Add Drop Multiplexers (ADMs) are hooked together in a line as shown in the figure. There may be two or four fibers between the two consecutive ADMs with one set serving as “protection” or “back up”.

Add/drop multiplexers (ADMs) are places where traffic enters and leaves. The traffic can be at various levels in the SONET/ SDH hierarchy

Also SONET network elements can

receive signals from a variety of facilities such as DS1, DS3, ATM, Internet, and LAN/MAN/WAN. They can also receive signals from a variety of network topologies

•ADMs drop some timeslots from the receive path and add timeslots to the transmit path

•In an STS-192, there could be 192 STS-1 timeslots that can be added or dropped at an ADM

CSCI 370 21

NYITAn example of adding/dropping of timeslots

Input Timeslot 0

Input Timeslot 0

Input Timeslot 0

Input Timeslot 0

Input Timeslot 1

Input Timeslot 1

Input Timeslot 1

Input Timeslot 1

Input Timeslot 2

Input Timeslot 2

Input Timeslot 2

Input Timeslot 2

Input Timeslot n

Input Timeslot n

Input Timeslot n

Input Timeslot n

Input Timeslot 1

Input Timeslot 0

Input Timeslot n

Input Timeslot 0

Input Timeslot 0

Input Timeslot 2

Input Timeslot 2

Input Timeslot 0

Input Timeslot 1

Input Timeslot 0

Input Timeslot 1

Input Timeslot 1

Input Timeslot 2

Input Timeslot 1

Input Timeslot n

Input Timeslot 2

Input Timeslot n

Input Timeslot 2

Input Timeslot n

Input Timeslot 1

Input Timeslot 2

Adding Timeslots

Dropping TimeslotsTDM Switch

Input Timeslot #s1 2 3 n 1 2 3 n

Output Timeslot #s

CSCI 370 22

NYITSONET Frame Structure

STS-1 Frame Format SONET is based on the STS-1

frame STS-1 consists of 810 octets

9 rows of 90 octects 27 overhead octets formed

from the first 3 octets of each row

9 used for section overhead 18 used for line overhead

87x9 = 783 octets of payload one column of the payload is

path overhead - positioned by a pointer in the line overhead

Transmitted top to bottom, row by row from left to right

STS-1 frame transmitted every 125 us: thus a transmission rate of 51.84Mbps

A1 and A2 are framing bytes and consist of F6 28 (hex). MSB is transport out first.

CSCI 370 23

SONET Frame Structure STS-3 Frame Format STS-3 is based on byte interleaving of 3 STS-1

frames STS-s frame transmitted every 125 us: thus a

transmission rate of 155 Mbps

CSCI 370 24

NYITSONET Frame Overhead Explained: Section Overhead

Framing Bytes (A1 and A2): These bytes are used to indicate the start of SONET/SDH frame. A1 byte is 1111 0110 and A2 byte is 0010 1000. These values remain the same in all STS-1s in an STS-N. SDH uses the same values for framing

Section Trace (J0)/Section Growth: This byte is used to trace the origin of an STS-1 frame as it travels across the SONET networks.  It allows two connected sections to verify the connections between them by transmitting a sixteen-byte message. This message is transmitted in sixteen consecutive frames with first byte carried in first frame, second byte in second frame and so on. If no such section trace message is defined or being transmitted, then in STS-48 or lower bit rate the, J0 and each Z0 shall be set corresponding to its order of appearance in the STS-N frame (i.e. J0 shall be set to 000000001, first Z0 to 0000010, second Z0 to 00000011 etc.) Where as in STS-192 frame each Z0 byte is set to the fixed pattern ‘11001100’.

CSCI 370 25

NYITSONET Frame Overhead Explained

Section BIP-8 (B1): B1 byte indicates bit error rate to the receiving terminal. This byte is known as Bit Interleaved Parity (BIP-8). The first bit in all the bytes in the previous frame are taken and then B1 is set so that the parity is even. Similarly all the other bits in B1 are set. The parity is calculated after scrambling and placed before scrambling. Scrambling is explained in later sections. The parity represented by this octet is the parity of the previous frame. It is used to estimate the bit error rate (BER) on the line. Note that the B1 parity is computed over all the bytes in the frame, no matter how large the frame. Because of this, the B1 byte does not provide a good BER estimation for large frames (perhaps STS-48 and larger) under adverse error conditions. SDH uses this byte for the same purpose.

0001 1000

1000 1000

1110 1101

0110 1010

0101 0101

0111 1000

1111 1111

1100 0101 BIP-8

CSCI 370 26

NYITSONET Frame Overhead Explained

Orderwire (E1): The E1 byte is located in the first STS-1 of an STS-N. It is called Local Orderwire (LOW). The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is used for a voice channel between two technicians as they installed and tested an optical link. It has a bit rate of 64kb/s. SDH uses this octet for the same purpose.

CSCI 370 27

NYITSONET Frame Overhead Explained

Section User Channel (F1): The F1 byte is located in the first STS-1 of an STS-N, and is used by the network provider. The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is passed from Section to Section within a Line and can be read, written, or both at each Section Terminating Equipment (STE) in that line. The use of this function is optional. SDH also uses this byte for the same purpose.

Section Data Communication Channel (D1, D2 and D3): These are the bytes, which form communication channel. These bytes are defined only for first STS-1 of an STS-N frame. These three bytes are considered as one 192-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and other communication needs between STE. This channel is used for internally generated, externally generated and supplier-specific messages. SDH uses this channel for the same purpose.

CSCI 370 28

NYIT

SONET Frame Overhead Explained:Line Overhead

Pointers (H1 and H2): The processing of H1 and H2 bytes in SONET and SDH is a beautiful concept. The Synchronous Payload Envelop (SPE) can be floating in a SONET frame. It can start in one frame and end in the next frame. Now these two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS-1s within an STS-N to align the STS-1 Transport Overheads in the STS-N, and to perform frequency justification. SDH handles these pointer bytes in the same way.

Pointer Action Byte (H3): The pointer action byte is allocated to compensate for the SPE timing variations. The value carried by H3 is not defined when there is no negative frequency justification. SDH handles this byte in the same way.

CSCI 370 29

NYITPointer Function

CSCI 370 30

NYIT

SONET Frame Overhead Explained:Line Overhead

Line BIP-8 (B2): The operation of this B2 byte is same as that of B1 byte in the SOH except that B2 is calculated over Line Overhead and Synchronous Payload Envelope of the previous frame before scrambling and placed in the current STS-1 frame before scrambling. SDH uses this byte for the same purpose.

Automatic Protection Switching (APS) Channel (K1, K2): Set of fibers is used for protection. These K1 and K2 are the bytes, which are transmitted over these protection channels for Automatic Protection Switching (APS) signaling between line level entities. These bytes are defined only for first STS-1 of an STS-N. In the remaining STS-1s it is undefined. These bytes are used to indicate a number of defects, alarms etc. detected at the receiving terminal back to the corresponding transmitting terminal through protection channels. SDH  uses these bytes for the same purpose. There is lot more explanation to be done on this concept of APS.

CSCI 370 31

NYIT

SONET Frame Overhead Explained:Line Overhead

Line Data Communication Channel (D4-D12): These bytes form a communication channel to send administrative messages just as D1 to D3. These nine bytes are considered as one 576-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and other communication needs. This channel is available for internally generated, externally generated and supplier-specific messages. These bytes are defined only for STS-1 number 1 of an STS-N signal. SDH uses these bytes for the same purpose but with additional codes.

Synchronization Status (S1): This byte is allocated for transporting synchronization status messages. S1 is defined only for first STS-1 of an STS-N signal. Currently only bits 5-8 of S1 are used to transport synchronization status messages. Bits 1-4 are undefined.  These messages contain clock quality labels that allow a SONET NE to select the most suitable synchronization reference from the set of available references. The purpose of these messages is to allow SONET NEs to reconfigure their synchronization references autonomously while avoiding the creation of timing loops. As an example for bits 5-8 in S1. Bits 5-8 are 0001 for stratum 1 traceable, 0111 for stratum 2 traceable, 0000 Synchronized traceability unknown etc. SDH uses this byte for the same purpose

CSCI 370 32

NYIT

SONET Frame Overhead Explained:Line Overhead

Growth (Z1): Z1 byte is located in second through Nth STS-1s of an STS-N. This byte is undefined.

STS-1 REI (M0): The M0 byte is defined only for the STS-1 in an OC-1 or STS-1 electrical signal. Bits 5 through 8 of the M0 byte are allocated for a Line Remote Error Indication function (REI-L), which conveys the error count detected by LTE (using the B2 code) back to its peer LTE. Bits 1 through 4 of the M0 byte are currently undefined. The error count shall be a binary number from zero (i.e., ‘0000’) to 8 (i.e., ‘1000’). The remaining seven values represented by the four REI-L bits (i.e., ‘1001’ through ‘1111’) shall not be transmitted, and shall be interpreted by receiving LTE as zero errors. Since there is no rate in SDH equivalent to STS-1, SDH does not define an M0 value for this byte.

Growth (Z2): These bytes are allocated for future growth, and their use is currently undefined. Note that STS-1 signal does not contain a Z2 byte.

Orderwire (E2): This byte has the same purpose for line entities as the E1 byte has for section entities. It is called Express Orderwire (EOW) channel. The corresponding bytes in the second through the Nth STS-1s of an STS-N frame are currently undefined. SDH uses this byte for the same purpose.

CSCI 370 33

NYITSDH Frame Structure

STM-N Frame Format STM - "Synchronous

Transmission Module" STM-N general format Originally the basic frame

STM-1 consists of 270x9=2430 octets 9x9=81 octets section

overhead 2349 octets payload

Higher rate frames are derived from multiples of STM-1 according to value of N

Later STM-0 was standardized by ITU (which corresponds to STS-1 rate)

CSCI 370 34

Scrambling in SONET/SDH:as an Aid to Clock Recovery on the Rx Side Scrambling of outgoing data ensures enough 1 to 0 and 0 to 1

transitions Helps in clock recovery on the receiver

The framing bytes A1 and A2, Section Trace byte J0 and Section Growth byte Z0 are not scrambled to avoid possibility that bytes in the frame might duplicate A1/A2 and cause an error in framing. The receiver searches for A1/A2 bits pattern in multiple consecutive frames, allowing the receiver to gain bit and byte synchronization. Once bit synchronization is gained, everything is done, from there on, on byte boundaries – SONET/SDH is byte synchronous, not bit synchronous.

CSCI 370 35

NYITClient Signals of SONET/SDH

SONET/SDHSONET/SDH

ATMATM

PDH

SONET/SDHSONET/SDH

ATMATM

CBR IP

10 GbE GFPGFPGFPGFP

GFPGFPGFPFibre ChannelPDHPDHPDHDVBMPLS 1 GbE

CSCI 370 36

NYITSONET Multiplexing Structure

AU : Administrative Unit

TUG: Tributary Unit Group

CSCI 370 37

Virtual Concatenation: Link sizes provided by VC

SDH SONET from to In steps of

VC-11 (1-64) VT1.5 (1 64) 1.6 Mbit/s 102.4 Mbit/s 1.6 Mbit/s

VC-12 (1-64) VT2 (1 64) 2.2 Mbit/s 139.3 Mbit/s 2.2 Mbit/s

VC-3 (1-256) STS-1 (1 256) 49 Mbit/s 12.7 Gbit/s 49 Mbit/s

VC-4 (1-256) STS-3c (1 256) 150 Mbit/s 38.3 Gbit/s 150 Mbit/s

CSCI 370 38

Virtual Concatenation: Link sizes provided by VC

Virtual concatenati

on

SONET

89% 98% 99%100%

100% 95%95%

VT-1.5-7v VT-1.5-16v

VT-1.5-63v

STS-1-2v

STS-1-4v STS-1-21vSTS-3-7v  

SDH

92% 98%92%

100%100%

100% 95% 

VC-12-5v VC-12-12vVC-2-4v

VC-12-46vVC-3-2v

VC-3-4v VC-4-7v 

Contiguous concatenati

on

SONET67% 33% 42%

none none STS-3c STS-12c STS-48c  

SDH92% 33% 42%  

none VC-2-4c none VC-4-4c VC-4-16c  

No concatenati

on

SONET20% 50%

STS-1 STS-1 none none none  

SDH20% 50% 67%  

VC-3 VC-3 VC-4 none none  

Service / bitrate

Ethernet /10 Mbit/s

ATM /25 Mbit/s

Fast Ethernet /100 Mbit/s

ESCON /200 Mbit/s

Gigabit Ethernet / 1 Gbit/s

CSCI 370 39