Networks What is a Network?

1
Networks What is a Network?

• a set of applications and/or switches connected
  by communication links
• many topologies'' possible
• local area networks (LAN) versus wide-area
  networks (WAN)
• many different media fiber optic, coaxial cable,
  twisted pair, radio, satellite
• for applications topology and media unimportant

2
Networks What is a Network?

• a software/hardware infrastructure
• original justification allows shared access to
  computing resources (e.g., computers, files,
  data)
• a medium through which geographically dispersed
  users communicate (e.g., email, teleconferencing)
  
• a medium through distributed services/applications
  are implemented
• an electronic village
• an information highway, national information
  infrastructure
• cyberspace - "a consensual environment
  experienced daily by billions of operators, in
  every nation, ...." Hotlink Wiliam Gibson on
  Cyberspace

3
Networks Packet-Switching

• data entering network divided into chunks called
  "packets''
• packets traversing network share network
  resources (e.g., link bandwidth, buffers) with
  other packets
• on demand resource use statistical resource
  sharing
• resources demands may exceed resources available
  
• e.g., A and B packets arrive at R1, destined for
  C
• resource contention queueing (waiting), delay

4
Networks Circuit Switched Networks

• all resources (e.g. communication links) needed
  by call dedicated to that call for duration
• example telephone network
• resource demands may exceed resources available
• A and B want to call C
• resource contention blocking (busy signal)
• drawbacks ??
• advantages ??

5
Networks Why statistically share resources?

• More efficient
• example 1 Mbit/sec link each user requires 100
  Kbits/sec when transmitting each user has data
  to send only 10 of time.
• circuit-switching give each caller 100 Kbits/sec
  capacity. Can support 10 callers.
• packet-switching with 35 ongoing calls,
  probability that 10 or more callers
  simultaneously active lt 0.0004!
• Can support many more callers, with small
  probability of "contention.''
• if users are bursty'' (on/off), then
  packet-switching is advantageous

6
NetworksElements of a Network

• communication links
• point-to-point (e.g., A-to-B)
• broadcast (e.g., Ethernet LAN)
• host computer running applications which use
  network (e.g. H1)
• router computer (often w/o applications-level
  programs) routing packets from input line to
  output line. (e.g., C)
• gateway a router directly connected to two
  networks (e.g. A)
• network set of nodes (hosts/routers/gateways)
  within single administrative domain
• internet collection of interconnected networks

7
NetworksProtocols

• protocol rules by which active network elements
  (applications, hosts, routers) communicate with
  each other
• protocols define
• format/order of messages exchanged
• actions taken on receipt of message
• rules by which two or more people communicate to
  provide a service, or to get something done
• protocols in every day life

8
Networks Layered Architecture

• complex system architecture simplified by
  layering.
• layer N relies on services of layer N-1 to
  provide a service to layer N1
• service from lower layer independent of how that
  service implemented
• information/complexity hiding
• layer N change doesn't affect other layers
• interfaces define how services requested

9
Networks Layered network architecture

• the network consists of geographically
  distributed hardware/software components
• a distributed layered view

10
Networks Layering and protocols

• peer entities (e.g., processes) in layer N
  provide service by communicating (sending
  "packets") with each other, using communication
  service provided bylayer N-1.
• logical versus physical communication

11
Networks The Internet and ISO/OSI reference
models

• ISO International Standards Organisation
• OSI Open System Interconnection

12
Networks OSI reference model

• Physical Layer Concerned with transmitting of
  raw bits over a communication channel. Common
  issues are Voltage, bit duration, simplex,
  duplex, full duplex, connection establishment,
  cables and connectors
• Data Link Layer 1 and 0 organised into packets
  or frames and error detection and correction
  applied.
• Network Layer Data is organised into packets or
  frames and switching, queuing, routing and
  congestion control is applied.
• Transport Layer Multiplexing and demultiplexing
  of data from/to different sources. Flow control
  of the source.
• Session Layer Connection establishment,
  connection management, connection tear-down.
• Presentation Layer Data compression encoding
  and decoding, security encryption, format
  conversion
• Application layer commerce, betting,
  entertainment applications.

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Networks Layers of a protocol architectureApplic
ation, socket and presentation layers

• application layer
• process-to-process communication
• examples WWW, email, teleconferencing, info.
  retrieval
• socket layer (Internet only)
• buffering and delivery of data at end systems
• presentation layer (OSI only)
• conversion of data to a common format (e.g.,
  little endian versus big-endian byte orders,
  integer and floating point numbers).
• Internet stack data conversion a user-level
  concern

14
Networks Layers of a protocol architectureSessio
n and Transport layers

• session layer (OSI only)
• session set up (e.g., authentication), recovery
  from failure (broken session)
• a "thin" layer
• transport layer
• transport service end-to-end delivery of data
• may multiplex several streams from higher layers
• sender/receiver speed matching
• Internet TCP and UDP

15
Networks Layers of a protocol architectureData
Link and Physical layers

• network layer
• at end hosts start packets on their way
• at routers control packet routing
• bottleneck avoidance, congestion control
• Internet IP packets, BGP, RIP

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Networks Layers of a protocol architectureData
Link and Physical layers

• data link layer
• point-to-point error free communication over a
  single link
• multiaccess LAN protocols
• speed matching between sender/receiver
• Ethernet, HDLC, PPP
• physical layer
• transmitting raw bits (0/1) over media

17
Networks Internetworks the Internet

• an internet interconnection of many networks
• a network of networks
• each network administered separately
• the Internet each network runs same software
  the Internet protocols

18
Networks Protocol packets

• packet unit of data exchanged between protocol
  entities in a given layer
• data at one layer encapsulated in packet at lower
  layer
• "envelope within envelope"

19
Networks Generic issues in a layer

• error control make "channel" more reliable
• flow control avoid flooding slower peer
• fragmentation dividing large data chunks into
  smaller pieces reassembly
• multiplexing several higher level session share
  single lower level connection
• connection setup handshaking with peer
• addressing/naming locating, managing identifiers
  associated with entities

20
Networks IP Networks version 4

• The Internet Protocol (IP) provides unreliable,
  connectionless packet delivery.
• IP is connectionless because it treats each
  packet of information independently.
• IP is unreliable because it does not guarantee
  delivery. That is, it does not require
  acknowledgments from the sending host, the
  receiving host, or intermediate hosts.
• IPv4 addresses consists of four 8-bit words
• Addresses are represented as four 8-bit
  hexadecimal words, each separated by a colon e.g.
  385FCA2E

21
Networks IP Networks version 4
22
Networks IP Networks version 4

• Version The IP version number, 4
• Length The length of the datagram header in
  32-bit words
• Type of service Contains five subfields that
  specify the precedence, delay, throughput,
  reliability, and cost desired for a packet.
• Total length The length of the datagram in bytes
  including the header, options, and the appended
  transport protocol segment or packet.
• Identification An integer that identifies the
  datagram.
• Flags Controls datagram fragmentation together
  with the identification field. The flags indicate
  whether the datagram may be fragmented, whether
  the datagram is fragmented, and whether the
  current fragment is the final one.
• Fragment offset The relative position of this
  fragment measured from the beginning of the
  original datagram in units of 8 bytes.
• Time to live How many routers a datagram can
  pass through. Each router decrements this value
  by 1 until it reaches 0 when the datagram is
  discarded. This keeps misrouted datagrams from
  remaining on the Internet forever.

23
Networks IP Networks version 4

• Protocol The high-level protocol type.
• Header checksum A number that is computed to
  ensure the integrity of the header values.
• Source address The 32-bit IPv4 address of the
  sending host.
• Destination address The 32-bit IPv4 address of
  the receiving host.
• Options A list of optional specifications for
  security restrictions, route recording, and
  source routing. Not every datagram specifies an
  options field.
• Padding Null bytes which are added to make the
  header length an integral multiple of 32 bytes as
  required by the header length field.

24
Networks IP Networks version 6

• IPv6 is the latest evolution of the Internet
  Protocol from IPv4.
• IPv4 is limited by two factors
• The Internet is running out of addresses to
  assign. In fact, the assigned address space is
  actually very sparsely populated but there is no
  satisfactory way of releasing the unused
  addresses without seriously complicating routing
  or disrupting existing networks.
• The 32-bit addresses used by IPv4 provides
  insufficient flexibility for global Internet
  routing. The deployment of Classless InterDomain
  Routing (CIDR) has extended the lifetime of IPv4
  routing by a number of years, but the effort
  required to manage routing continues to increase.
  
• Even if IPv4 routing could be scaled up, the
  Internet will eventually run out of network
  numbers.
• IPv6 extends the maximum number of Internet
  addresses by using 128-bit addressing.
• As both IPv4 and IPv6 protocols may coexist on
  the same network, providing an orderly migration
  from IPv4 to IPv6.
• IPv6 has a simplified packet header and improved
  options.

25
Networks IP Networks version 6
26
Networks IP Networks version 6

• IPv6 addresses consists of eight 16-bit words
• Addresses are represented as eight 16-bit
  hexadecimal words, each separated by a colon e.g.
  38295FABCA272EB2AB23923CFAB45469
• IPv4-mapped IPv6 address'' has the following
  format
• 00000000000000000000FFFFx1.a2x.x3.x4
• IPv6 has three types of addresses
• A unicast address'' uniquely identifies an
  interface and a system.
• A multicast address'' uniquely identifies a
  number of interfaces and systems that belong to a
  multicast group.
• An anycast address'' is an address that has a
  single sender, multiple listeners, and only one
  responder (normally the nearest'' one, depending
  on the routing protocols' measure of distance).
  For example, several web servers may listen on an
  anycast address. When a request is sent to this
  address, only one responds.

27
Networks IP Networks v6 Global Unicast Address
format

• TLA ID Top-level aggregation identifier will be
  used to divide the address space into
  geographical regions and major subdivisions of
  these such as countries, states, and broad
  organizational types. Routers at the top level
  will have a routing table entry for every active
  TLA ID as well as additional lower-level entries
  for their TLA.
• NLA ID Next-level aggregation identifier
  assigned by the RIRs (Regional Internet
  Registries) to service providers and large
  organizations. The NLA will be used to divide the
  address space selected by a TLA ID between
  Internet service providers (ISPs) and individual
  large organizations such as governments and
  multinational companies.
• SLA ID Site-level aggregation identifier
  assigned within an organization. The SLA allows
  each site to allocate up to 65,536 subnets per
  NLA ID. Organizations that require additional
  subnets can achieve this by aggregating ranges of
  NLA IDs.
• Interface ID Identifies an individual interface
  on a system.

28
Networks IP Networks v6 Extension Header

• Specifically, IPv6 omits the following fields
  from IPv4
• header length (the length is constant)
• identification
• flags
• fragment offset
• header checksum
• IPv6 options improve over IPv4 by being placed in
  separate extension headers that are located
  between the IPv6 header and the transport-layer
  header in a packet.
• Newly defined extensions can be integrated more
  easily into IPv6 extension headers
• hop-by-hop options that apply to each hop
  (router) along the path
• routing header for loose/strict source routing
  (used infrequently)
• define the packet as a fragment and contains
  information about the fragmentation (IPv6 routers
  do not fragment)
• IP Security authentication
• IP Security encryption
• destination options for the destination node
  (ignored by routers)

29
Networks IP Networks v6 Extension Header

• IPv6 uses the priority field in the IP header to
  provide an explicit priority definition. A node
  can set this value to indicate the relative
  priority of a particular packet or set of
  packets. The node, routers, or the destination
  host can use the value to decide what to do with
  the packet, such as letting it pass or dropping
  it.
• Congestion-controlled traffic is defined as
  traffic that responds to congestion through a
  back-off'' or other limiting algorithm.
  Priorities for congestion-controlled traffic are
  
• 0 uncharacterized traffic
• 1 filler'' traffic such as netnews
• 2 unattended data transfer such as electronic
  mail
• 3 reserved
• 4 attended bulk transfer such as FTP
• 5 reserved
• 6 interactive traffic such as telnet
• 7 control traffic such as routing protocols

30
Networks IPv6 over IPv4 using Tunneling

• Tunneling allows the existing IPv4 routing
  infrastructure to carry IPv6 traffic.
• Dual-stack hosts and routers (that support both
  IPv4 and IPv6) can tunnel IPv6 datagrams over
  regions of IPv4 routing topology by encapsulating
  the IPv6 datagrams within IPv4 packets.

31
Networks Digital Video Broadcast - Terrestrial,
Satellite, Cable

• Satellite Modulation Quadrature Phase Shift
  Keying (QPSK)
• Terrestrial Modulation Orthogonal Frequency
  Division Multiplexing (OFDM)
• Cable Modulation Quadratrure Amplitude
  Modulation (QAM)

32
Networks Digital Video Broadcast - Terrestrial,
Satellite, Cable

• Compression The audio-visual source material is
  compressed to get a low enough bit rate to make
  economic use of available transmission bandwidth.
• Packetisation and synchronisation Each
  Elementary Stream (ES) is split into access units
  (AU), (audio frames or pictures). AUs are
  packetised into a Packetised (PES) packet, by
  adding a header with information about the
  content of the packet. PES structure uses time
  stamps.
• Multiplexing The MPEG-2 multiplexes PESs in a
  synchronous way into one transport stream which
  contains all data required by a receiver to
  recognise services (PSI/SI), decode and present
  synchronously audio-visual material etc.
• Error Protection DVB has made several
  specifications in order to adapt the stream to
  different networks, e.g. satellite, terrestrial,
  and cable
• Modulation and transmission DVB specifies how
  the signal is adapted to different networks.

33
Networks Digital Video Broadcast - Packetised
Elementary Streams

• The PES packet consists of a header and a payload
  and may be of variable length up to 64 kBytes.
  However, PES packets containing a video
  elementary stream may have unbounded or
  unspecified PES packet length.

34
Networks Digital Video Broadcast Presentation
and Decoding Time Stamps

• When MPEG-2 bi-directional coding is used, a
  picture may have to be decoded some time before
  it is presented, so that it can be used as a
  source of data for a B-picture.
• The decoder needs to know when to decode a frame
  and when to display it. Consequently, two types
  of time stamps exists
• Presentation Time Stamp (PTS) indicates the
  time when a picture must be presented
• Decoding Time Stamp (DTS) indicates the time

35
Networks Transport Stream

• Transport layer converts PES packets and sections
  into small 188 bytes packets of constant size.
• Structure 188 bytes, min 4 bytes header,
  adaptation field (up to 183 bytes)

36
Networks Transport Stream Program Specific
Information PSI tables

• Program Association Table (PID0x0000) List of
  all available programs (i.e. services) in a TS.
  Provides the link between the program number and
  the PMT PIDs. Program number 0 always carry the
  NIT.
• Program Map Table (PID Assigned in PAT) List of
  elementary streams belonging to a program. Also
  contains info (descriptors) about each program
  and individual ESs.

37
Networks Transport Stream Service Information
SI tables

• Service Description Table (PID 0x0011) Contains
  data describing the services in the transport
  stream, e.g. service name and provider.

38
Networks Transport Stream Other Service
Information SI tables

• Network Information Table (PID 0x0010) Contains
  information about the physical network carrying
  the transport stream. Also included are details
  of other transport streams.
• Service Description Table (PID 0x0011) Contains
  data describing the services in the transport
  stream, e.g. service name and provider.
• Bouquet Association Table (PID 0x0011) Provides
  information about a collection of services
  marketed as a single product. Services may be
  located in different transport streams.
• Event Information Table (PID 0x0012) Contains
  information about program names, start time,
  duration etc. both on the actual TS and other
  transport
• Running Status Table (PID 0x0013) Contains
  information about the status of an event
• Time and Date Table (PID 0x0014) Carries the
  UTC-time and date.
• Time Offset table (PID 0x0014) Carries the
  UTC-time and date information and the local time
  offset.
• .. and other tables

39
Networks DVB - Data Broadcasting Profiles

• Data piping simple, asynchronous, end-to-end
  delivery of data through DVB
• Data streaming streaming-oriented, end-to-end
  delivery of data either asynchronously,
  synchronously or synchronised with other data
  streams (e.g. audio and video) through DVB
• Multiprotocol encapsulation (MPE) for services
  that require transmission of datagrams of
  communication protocols via DVB
• Data carousels for data services that require
  periodic, cyclical transmission of data modules
  through DVB
• Object carousels for data services that require
  periodic, cyclical broadcasting of Digital
  Storage Media Command and Control (DSM-CC)
  User-User objects through DVB