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Networks What is a Network?

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Title: 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.

13
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

16
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
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