Communication Networks

1
Communication Networks

• To interconnect a number of end-systems that want
  to communicate with each other
• Examples
• Telephone Network, Cellular phones, Satellite
  phone
• Radio, Broadcast TV, Cable TV, Satellite TV
• Internet, Ethernet, Token ring, FDDI
• Specialized mobile radio (taxi - dispatch)
• etc., etc., etc.
• First understand point-to-point communication
  (digital comm ECE 530)
• Components, Mechanisms, Performance

2
Telephone Network

• End-systems
• telephones
• fax
• modem
• How should we interconnect these?

3
Telephone Network

• Dedicated link between each pair of end-systems
• n(n-1)/2 links
• inefficient
• not scalable - each additional end system
  requires n additional links

4
Telephone Network - Switching

• 1 link per end-system
• switch to interconnect end-systems
• manual
• electro-mechanical
• electronic
• digital
• resource sharing

5
Telephone Network - Trunking/Multiplexing

• Add more switches
• Reduce switch to end-system link distance
• Interconnect switches with trunks
• bundles of links
• multiplexing

6
Telephone Networks - Multiplexing

• Multiplexing - mechanism for sharing a single
  physical channel amongst multiple users.
• Multiplexing Techniques
• Frequency division multiplexing (FDM) - partition
  channel bandwidth into disjoint frequency bands.
• Time division multiplexing (TDM) - partition
  channel into disjoint time slots. Assumes digital
  transmission.

7
Telephone Networks - Network Hierarchy

• The part of the network connecting the
  end-systems to the nearest switch (local
  exchange) is called the local access network.
• The part of the network interconnecting the local
  exchanges is called the inter-exchange network in
  a geographical area corresponding to a
  city/county/state.
• The part of the network interconnecting switches
  across an inter-state geographical area is called
  the long-distance network.
• The network at different spatial scales (access
  network) may be a star or a fully-connected mesh
  (core long distance network).
• RBOC, Long-distance carriers, ATT divestiture,
  Telecom deregulation

8
Telephone Networks - Wireless local loop/Cellular

• The local access may be wireless (base-station)
• Wireless local loop (WLL)
• Cellular - End-systems may be mobile the entire
  region is covered by a collection of
  base-stations region of coverage is a cell
• medium access control

9
Telephone Networks - Wireless/Cellular/Mobile

• Medium Access Control
• frequency division multiple access (FDMA)
• time division multiple access (TDMA)
• code division multiple access (CDMA)
• Mobility - As a mobile moves across cell
  boundaries, the base station it is communication
  with must be changed.
• location tracking - while the mobile is not in
  use
• handoff - during a call

10
Telephone Network Components

• End-Systems
• telephone
• fax
• modem
• Transmission medium
• twisted pair (copper)
• microwave link (long-distance trunks - FDM)
• fiber-optic links (long-distance, inter-exchange,
  part of local access - fiber to the curb/home)
• satelite
• Switch

11
Telephone Network Operation - Call Set Up

• Signaling
• off-hook, dial tone
• number acquisition
• subscriber authentication
• Routing, Call Admission Control
• Is there a route along which the call may be set
  up? The number of circuits in the trunk may be
  less than the maximum number of calls that could
  be in progress.
• What is the best available route?
• Even if a route is available should the call be
  accepted?
• Call set up
• ringing/busy tone
• switch settings

12
Telephone Network Operation - Call Duration

• Switching
• Multiplexing
• Medium Access Control (wireless)
• Handoff (mobile)

13
Telephone Network Operation - Call Tear Down

• Signaling
• on-hook
• billing
• Switch setting
• common channel signaling (SS7) - overlay network
  for call setup/teardown/routing/admission control

14
Telephone network - History

• Telephone
• Phillip Reis (German school teacher) - could
  transmit musical tones - 1861 - called it a
  telephone.
• Alexander Graham Bell (Boston) - 1876, Feb 14.
• Elisha Gray (Chicago) - filed the patent only a
  few hours later!
• Modem
• Bell 103 modem - 300 bps - early 1960s
• Switch
• Electromechanical - Almon Brown Strowger (Kansas
  city undertaker) - 1889. Apparently the
  switchboard operator was directing phone calls to
  his competitor!

15
Telephone network - Future challenges

• Integrated Services or Multimedia
• simultaneously transmit voice/data/video over the
  network
• different bandwidth requirements
• dedicated circuit for the duration of the call is
  wasteful
• Control/Signaling or Intelligent Network needs to
  move towards and open architecture
• Deregulation and Competition

16
Data Networks

• End-Systems
• computers
• printers
• servers
• other peripherals
• How should these be interconnected? Why not use
  modems on a telephone network?

17
Data Networks - Inadequacies of the TN

• Only one connection at a time to each end-system
• use multiplexing and a modem pool (e.g. ISP, DoIT
  dial up)
• Call set up and tear down overhead and delay
• many applications have a relatively short call
  duration for which the call setup time becomes
  significant).
• Bursty traffic
• Circuit switching and multiplexing inefficient
• Example N traffic streams each producing L bits
  every T secs. Tolerable delay is D
• Using TDM, we would need N logical channels, each
  with capacity L/D. Total capacity NL/D. Total
  traffic NL/T
  idle time!

18
Circuit versus packet based multiplexing

• Packet based multiplexing
• Combine all traffic streams on a single channel.
• May be able to exploit the idle time
  (inefficiency) of circuit based multiplexing.
  (example)
• Traffic streams interfere with each other - cause
  (additional) queueing.
• Impact of queueing on average delay, worst-case
  delay, and delay distribution.
• Multiplexing gain depends on the delay statistic
  of interest.
• Scheduling

19
Packet based multiplexing and switching

• Packetization and Framing
• Data from different users will have to be
  identified properly and beginning and end
  demarkated.
• Packet Switching or Store-and-Forward Switching
• Since different traffic streams no longer arrive
  on different logical channels, the switch will
  have to look at each packet individually to
  determine which traffic stream it corresponds to.
  
• Packet based medium access control
• Other alternatives besides packet based?
• Burst level circuits
• intermediate time scale between call and packets
• end-to-end or hop-by-hop

20
Computer Networks - Internet

• End-Systems
• computers
• printers
• servers
• other peripherals
• Transmission medium (point-to-point/shared)
• twisted pair
• coaxial cable
• satellite
• optical fibers
• leased telephone lines
• Switches/Routers

21
Internet - Operation - Call Set Up / Tear Down

• At the network layer there is no call set up or
  tear down. End-systems are always connected.
• At higher layer applications may set up or tear
  down calls (sessions) (e.g. telnet, ftp, http,
  email)
• The session set up involves signaling to between
  the end-systems using the permanent connectivity
  at the network layer (in-band signaling). The
  switches/routers do not participate in this
  session set up except for carrying the signaling
  message between the end-systems.

22
Internet - Operation - Call Duration

• Packetization/Framing
• Packet Multiplexing - Scheduling
• Packet Medium Access Control
• random access with collision resolution
• scheduled access
• Packet Switching - Datagram Routing
• Flow Control (end-to-end, hop-by-hop)
• congestion control
• fairness, sharing of reources
• Error detection and retransmission (end-to-end,
  hop-by-hop)

23
Data Networks - History

• Packet-switching - Baran (US) - 1961, Zimmerman
  (France)
• ARPAnet (funded by DARPA) - 1969
• motivation share expensive computing resources
• built by universities and research labs
• mechanisms decentralized routing, flow control
• Internet to interconnect a packet radio network
  (built at Stanford Research Institute) and
  ARPAnet (early 1970s)
• mechanisms common packet format, routing,
  addressing
• gateway
• ARPA-like networks - CSNET, NEARnet
• Other proprietory networks - SNA, DECnet

24
Data Networks - History

• ISO-OSI seven layer architecture
• IETF (Internet Engineering Task Force)
• Local Area Networks
• Ethernet (Xerox PARC) late 1970s
• Token Ring (IBM)
• NSFnet - late 1980s - early 1990s - NSF replaces
  DARPA funding
• Acceptable Use Policy - no commercial use -
  withdrawn 1992
• National backbone privatized - 1995
• World Wide Web (CERN particle physicists)

25
Integrated Services Networks

• Data, Voice, Video, Images (all bits)
• Different traffic characteristics - average bit
  rate, bursty/non-bursty
• Different requirements - real time interactive,
  loss tolerance.
• Unicast, broadcast, multicast, anycast
• Mobility
• Pricing, Billing

26
Intergated Services Networks - ATM

• Fixed size packets - no need to demarkate packet
  boundaries (framing)
• Small packet sizes - reduces cross-traffic
  intereference
• Virtual circuit routing
• same route for all packets belonging to a session
• route selection to be done at time of call setup
• packets for this session will be tagged with a
  session ID
• each switch to be traversed maintains a
  forwarding table that tells it which outgoing
  link, packets with a particular ID need to go out
  on.
• session IDs may be themselves be changed at each
  switch, so that they have only local significance
  (why?)

27
Intergated Services Networks - ATM

• VC routing
• forwarding table entries need to be set and
  cleared at time of call set up and tear down,
  much like for circuit switching.
• Admission control
• Setting of scheduling parameters - resource
  reservation
• Many of ATMs ideas are being incorporated in the
  standards for the future Internet.

28
Data Networks - ATM - History

• Fixed-size packet multiplexing/switching/VC
• Kasahara, Tezuka, Nakanishi, Hasegawa (Osaka U.)
  - 1961
• Chu (Bell Labs) - 1968 - ATDM
• Fraser (Bell Labs) - 1969-1972 - virtual circuit,
  Spider, Datakit
• Coudreuse (France) - Prelude network
• ISDN - mid 1980s - 2 x 64 kbps 16 kbps
• B-ISDN using ATDM - ATM standard - CCITT - mid
  1980s
• ATM Forum (consensus body of service providers
  and equipment manufacturers)

29
Networking Mechanisms

• Packetization, Framing - fixed/variable size,
  small size?
• Multiplexing - circuit (FDM, TDM), packet -
  scheduling
• Medium Access Control - circuit (FDMA, TDMA, CDMA
  (some amount of multiplexing gain)), packet
  (random access, scheduled access)
• Scheduling, Regulation
• Switching (Forwarding)
• Flow Control
• Routing, Forwarding Table Setting
• Admission Control, Resource Reservation
• Pricing, Billing

30
Networking Mechanisms

• Need to implement these mechanisms in the
  end-systems and routers/switches.
• Some mechanisms require coordination amongst
  multiple components (end-systems and switches).
  E.g. flow control, routing, admission control,
  resource reservation). Distributed protocol
• Layered protocol architecture
• Software versus hardware

31
Network Performance

• Performance Measures
• call level
• blocking probabilities - revenue
• packet level
• delay - average, variance, maximum, jitter
• loss - probabilities, fraction (m out of any
  consecutive n)
• throughput
• network cost (transmission medium,
  switches/routers, network hardware and sofware in
  end-systems)

32
Network Performance

• Traffic characteristics
• call level
• arrivals
• holding times
• packet level
• arrivals
• packet lengths
• Both performance and characteristics could be
• probabilistic or worst-case
• long-term averages or over finite horizons

33
Course Focus Performance Analysis of Network
Traffic Control Mechanisms

• Tools
• Stochastic Models
• Markovian models
• Non-Markovian models
• Analysis or Simulation
• Deterministic or Worst-case models
• relatively new

34
Layered Architecture

• How should the various network mechanisms be
  realized?
• Architecture is a specific way of organizing
  these mechanisms
• Layered architecture uses a hierarchy of building
  blocks
• Layered architecture for data networks
• building blocks at various layers are distributed
• bottommost layer (0) is the physical
  communication link
• layer n communication system comprises
• layer n-1 communication system(s)
• layer n peer modules
• layer n peer modules interface with the layer n-1
  communication system via the peer modules of
  layer n-1

35
Layered Architecture for Data Networks

• The interface between a lower layer communication
  system and the higher layer modules must be
  standardized
• The functionality offered by the lower layer must
  be standardized
• Using the standard interface and lower layer
  functionality, the peer modules at layer n can
  communicate with each other, to provide a certain
  standardized functionality as part of the layer n
  communication system.
• The rules and format used by a set of peer
  modules to communicate with each other is called
  a protocol
• Thus, the each layers communication system
  comprises a distributed algorithm with possibly
  noisy communication over the lower layer
  communication system
• Advantages of layered architecture
• simplifies design, verification
• standardization, interoperability

36
Implementation of Layers

• Modules of adjacent layers in a host or switch
  interface with each other.
• exchange data
• exchange control information
• The data from the module at layer n1, once
  obtained by the module at layer n, is processed
  and sent over the layer n-1 communication system.
• Processing may involve fragmentation,
  encapsulation, etc.
• Processing at various layers may be done in
  special purpose hardware to reduce processing
  delay.

37
Implementation of Layers

• How does the exchange of data take place?
• Since there may be a temporary mismatch in the
  processing speed at which the two modules are
  operating, there is a need to buffer this data
• the data waits in a queue (e.g. a linked list)
• if the processing does not require any changes to
  the data and only further encapsulation with
  headers is required the actual data need be
  physically moved from one memory location to
  another
• however, in typical implementation the data may
  be moved once from users memory space to the
  systems memory space to the network interface
  cards memory
• movements of data add delays and hence should be
  minimized

38
ISO OSI reference model

• A set of protocols is open if
• protocol details are publicly available
• changes are managed by an organization whose
  membership and transactions are open to the
  public
• A system that implements open protocols is called
  an open system
• International Organization for Standards (ISO)
  prescribes a standard to connect open systems
• open system interconnect (OSI)
• Has greatly influenced thinking on protocol stacks

39
ISO OSI

• Reference model
• formally defines what is meant by a layer, a
  service etc.
• Service architecture
• describes the services provided by each layer and
  the service access point
• Protocol architecture
• set of protocols that implement the service
  architecture
• compliant service architectures may still use
  non-compliant protocol architectures

40
The seven layers
41
Physical layer

• Implements an unreliable bit link
• Moves bits between physically connected
  end-systems
• Consists of
• transmitter
• receiver
• communication medium
• Standard prescribes
• coding scheme to represent a bit
• shapes and sizes of connectors
• bit-level synchronization

42
Datalink layer

• Reliable packet link
• Framing
• Medium access control on a broadcast link (such
  as Ethernet)
• end-system must receive only bits meant for it -
  need datalink-layer address
• also need to decide who gets to speak next
• these functions are provided by MAC sublayer
• Error detection and retransmission
• part of logical link control sublayer
• layered over MAC sublayer

43
Datalink layer (contd.)

• Datalink layer protocols are the first layer of
  software
• Very dependent on underlying physical link
  propeties
• Usually bundle both physical and datalink layer
  on network interface card (NIC)
• example Ethernet
• Internet
• a variety of datalink layer protocols
• most common is Ethernet
• others are Token Ring, FDDI, SONET, HDLC

44
Network layer

• End-to-end packet link by concatenating a set of
  point-to-point packet links
• Routing (on routers only)
• participates in routing protocol to create
  routing tables
• responsible for switching packets
• Scheduling - the transmission order of packets
• Buffer management - choosing which packets to
  drop
• Packetization - segmentation and reassembly (on
  host systems only)

45
Two types of network layers

• In datagram networks
• provides both routing and data switching
• In connection-oriented network
• we distinguish between data plane and control
  plane
• data plane only switches and schedules data
  (touches every byte)
• control plane responsible for routing,
  call-establishment, call-teardown (doesnt touch
  data bytes)

46
Network layer

• Internet
• network layer is provided by Internet Protocol
• segmentation and reassembly
• packet-switching, routing (datagram routing)
• no call set up or tear down
• can be layered over anything, but only
  best-effort service - no scheduling

47
Transport layer

• Error control
• message will reach destination despite packet
  loss, corruption and duplication
• retransmit lost packets detect, discard, and
  retransmit corrupted packets detect and discard
  duplicated packets
• Flow control
• match transmission rate to rate currently
  sustainable on the path to destination, and at
  the destination itself

48
Transport layer (contd.)

• Multiplexes multiple applications to the same
  end-to-end connection
• adds an application-specific identifier (port
  number) so that receiving end-system can hand in
  incoming packet to the correct application
• Some transport layers provide fewer services
• e.g. simple error detection, no flow control, and
  no retransmission
• lightweight transport layer
• Internet
• two popular protocols are TCP and UDP
• TCP provides error control, flow control,
  multiplexing
• UDP provides only multiplexing

49
Session layer

• Not common
• Provides full-duplex service, expedited data
  delivery, and session synchronization
• Duplex
• if transport layer is simplex, concatenates two
  transport endpoints togeter
• Expedited data delivery
• allows some messages to skip ahead in end-system
  queues, by using a separate low-delay transport
  layer endpoint
• Synchronization
• allows users to place marks in data stream and to
  roll back to a prespecified mark
• Internet
• doesnt have a standard session layer

50
Presentation layer

• Unlike other layers which deal with headers
  presentation layer touches the application data
• Hides data representation differences between
  applications
• e.g. endian-ness
• Can also encrypt and/or compress data
• Usually ad hoc
• Internet
• no standard presentation layer
• only defines network byte order for 2- and 4-byte
  integers

51
Application layer

• The set of applications that use the network
• file transfer, telnet
• Doesnt provide services to any other layer
• User applications run on top of this layer

52
Layering

• We have broken a complex problem into smaller,
  simpler pieces
• Provides the application with sophisticated
  services
• Each layer provides a clean abstraction to the
  layer above