CSCI-690 Computer Networks: Shrinking the globe one click at a time Lecture 6

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CSCI-690Computer NetworksShrinking the globe
one click at a timeLecture 6

• Khurram Kazi

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Major sources of the slides for this lecture

• Computer Networks A Systems Approach, Larry
  Peterson
• The Internet and Its Protocol, Adrian Farrels
  book.
• http//www.pcc.qub.ac.uk/tec/courses/network/SDH-S
  ONET/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.

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Reference Network For discussion purposes
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Vector/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

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RIP Routing Protocol RFC 1058

• RIP is one of a class of algorithms known as
  "distance vector algorithms".
• RIP is intended for use within the IP-based
  Internet. The Internet is organized into a number
  of networks connected by gateways. The networks
  may be either point-to-point links or more
  complex networks. Hosts and gateways are
  presented with IP datagrams addressed to some
  host.
• Limitations of the protocol
• This protocol does not solve every possible
  routing problem. Its is primary intended for use
  as an IGP, in reasonably homogeneous networks of
  moderate size.
• The protocol is limited to networks whose longest
  path involves 15 hops.
• It is inappropriate to use this for larger
  networks
• The protocol depends upon "counting to infinity"
  to resolve certain unusual situations
• Routing messages received from
• This protocol uses fixed "metrics" to compare
  alternative routes. It is not appropriate for
  situations where routes need to be chosen based
  on real-time parameters such a measured delay,
  reliability, or load. The obvious extensions to
  allow metrics of this type are likely to
  introduce instabilities of a sort that the
  protocol is not designed to handle.

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RIPv2 message format
RIPv2 message consists of a 4-byte header
followed by from 1 to 125 route entries
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RIP Details commands
1 Request A request for the responding system to send all or part of its routing table
2 Response A message containing all or part of the senders routing table. This message may be sent in response to a request or poll, or it may be an update message generated by the sender.
3 Traceon Obsolete (should be ignored)
4 Traceoff Obselete
5 -- reserved
Rest of the datagram contains a list of
destination, with information about each. Each
entry in this list contains a destination or
host, and the metric for it.
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RIP Details

• RIP can carry routing information for several
  different protocols. For IP the address family
  identifier is 2.
• The IP address is the usual Internet address,
  stored as 4 octets in network order.
• The metric field must contain a value between 1
  and 15 inclusive, specifying the current metric
  for the destination, or the value 16, which
  indicates that the destination is not reachable.
• Metric" measuring the total distance to the
  entity. Distance is a somewhat generalized
  concept, which may cover the time delay in
  getting messages to the entity, the dollar cost
  of sending messages to it, etc.

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Autonomous 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

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Autonomous 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

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Autonomous 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

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Autonomous Systems within the Internet
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Moving onto Physical layer Optical Transport
Technologies
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Reference Network For discussion purposes
Physical Layer
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Prior 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

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Prior 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

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Prior 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

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Time Division MultiplexingPDH (Plesichronous
Digital Hierarchy) Networks

• The T1 carrier (1.544 Mbps).

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Time Division MultiplexingPDH (Plesichronous
Digital Hierarchy) Networks

• Multiplexing T1 streams into higher carriers.

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Time 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)

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Time 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

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SONET/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

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Time Division Multiplexing (5)

• SONET and SDH multiplex rates.

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SONET/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

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SONET/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

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An example of adding/dropping of timeslots
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SONET 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.
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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

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SONET 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.

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SONET 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
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SONET 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.

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SONET 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.

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SONET Frame Overhead ExplainedLine 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.

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Pointer Function
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SONET Frame Overhead ExplainedLine 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.

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SONET Frame Overhead ExplainedLine 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

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SONET Frame Overhead ExplainedLine 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.

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SDH Frame Structure

• STM-N Frame Format
• STM - "Synchronous Transmission Module"
• STM-N general format
• Originally the basic frame STM-1 consists of
• 270x92430 octets
• 9x981 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)

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Scrambling in SONET/SDHas 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.

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Client Signals of SONET/SDH
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SONET Multiplexing Structure
AU Administrative Unit TUG Tributary Unit Group
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Virtual Concatenation Link sizes provided by VC
SDH SONET from to In steps of
VC-11 (1-64) VT1.5 (164) 1.6 Mbit/s 102.4 Mbit/s 1.6 Mbit/s
VC-12 (1-64) VT2 (164) 2.2 Mbit/s 139.3 Mbit/s 2.2 Mbit/s
VC-3 (1-256) STS-1 (1256) 49 Mbit/s 12.7 Gbit/s 49 Mbit/s
VC-4 (1-256) STS-3c (1256) 150 Mbit/s 38.3 Gbit/s 150 Mbit/s

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Virtual Concatenation Link sizes provided by VC
Virtual concatenation SONET 89 98 99 100 100 95 95 95 95
Virtual concatenation SONET VT-1.5-7v VT-1.5-16v VT-1.5-63v STS-1-2v STS-1-4v STS-1-21v STS-3-7v  
Virtual concatenation SDH 92 98 92 100 100 100 95  
Virtual concatenation SDH VC-12-5v VC-12-12v VC-2-4v VC-12-46v VC-3-2v VC-3-4v VC-4-7v  
Contiguous concatenation SONET 67 33 42 42
Contiguous concatenation SONET none none STS-3c STS-12c STS-48c  
Contiguous concatenation SDH 92 33 42  
Contiguous concatenation SDH none VC-2-4c none VC-4-4c VC-4-16c  
No concatenation SONET 20 50
No concatenation SONET STS-1 STS-1 none none none  
No concatenation SDH 20 50 67  
No concatenation SDH 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 Gigabit Ethernet / 1 Gbit/s