Introduction to Telephony, Cable and Internet Technologies

1
Introduction to Telephony, Cable and Internet
Technologies

• http//www.pde.rpi.edu/
• Or
• http//www.ecse.rpi.edu/Homepages/shivkuma/
• Shivkumar Kalyanaraman
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu

• Based in part upon slides of S. Keshav (Ensim),
  J. Bellamys book, Prof. Raj Jain (OSU), L.
  Peterson (Princeton), J. Kurose (U Mass)

2
Overview

• Connectivity
• direct (pt-pt, N-users),
• indirect (switched, inter-networked)
• Telephony, Internet, Cable Networks Basic
  Concepts
• Concepts Topologies, Framing, Multiplexing,
  Flow/Error Control, Reliability, Multiple-access,
  Circuit/Packet-switching, Addressing/routing,
  Congestion control
• Data link/MAC layer SLIP, PPP, LAN technologies
  
• Interconnection Devices
• S. Keshav book (Chapter 2), Opt Nets (Sec 11.1,
  13.1, 13.2)

3
Connectivity...

• Building Blocks
• links coax cable, optical fiber...
• nodes general-purpose workstations...
• Direct connectivity
• point-to-point
• multiple access

4
Connectivity (Continued)

• Indirect Connectivity
• switched networks
• gt switches
• inter-networks
• gt routers

5
What is Connectivity ?

• Direct or indirect access to every other node in
  the network
• Connectivity is what you get instead of a direct
  physical link
• Key Tradeoff Performance characteristics worse!

6
Connectivity

• Internet
• Best-effort
• (no performance guarantees)
• Packet-by-packet
• A pt-pt link
• Always-connected
• Fixed bandwidth
• Fixed delay
• Zero-jitter

7
Telephony
8
Telephone Network What is It?

• Specialized to carry voice traffic
• Aggregates like T1, SONET OC-N can also carry
  data
• Also carries
• Telemetry, video, fax, modem calls
• Internally, uses digital samples
• Switches and switch controllers are special
  purpose computers
• Pieces
• 1. End systems
• 2. Transmission
• 3. Switching
• 4. Signaling

9
Telephone Network What is It?

• Single basic service two-way voice
• low end-to-end delay
• guarantee that an accepted call will run to
  completion

• Endpoints connected by a circuit, like an
  electrical circuit
• Signals flow both ways (full duplex)
• Associated with reserved bandwidth and buffer
  resources

10
Telephone Network Design

• Fully connected core
• simple routing
• telephone number is a hint about how to route a
  call
• But not for 800/888/700/900 numbers these are
  pointers to a directory that translates them into
  regular numbers
• hierarchically allocated telephone number space

11
Telephone Network Design
12
Telephone Pieces End Systems
13
Telephone Pieces End Systems

• Transducers key to carrying voice on wires
• Dialer
• Ringer
• Switch-hook

14
Last-Mile Transmission Environment

• Wire gauges19, 22, 24, 26 gauge(smaller better)
• Diameters 0.8, 0.6, 0.5, 0.4 mm (larger better)
• Various forms of noise (twisting reduces noise)
• Bridged-tap noise bit-energy diverted to
  extension phone sockets
• Crosstalk
• Ham radio
• AM broadcast
• Insertion loss -140 dBm noise floor
• 100 million times more sensitive than normal
  modems
• Bandwidth range 600 kHz
• Notch effects in insertion loss due to
  bridged-taps
• Transmission PSD -40dBm gt 90 dBm budget

15
2-wire vs 4-wire Sidetones and Echoes

• Both trans reception circuits need two wires
• 4 wires from every central office to home
• Alternative Use same pair of wires for both
  transmission and reception
• Signal from transmission flows to receiver
  sidetone

• Reverse Effect received signal at end-system
  bounces back to CO (esp if delay gt 20 ms) echo
• Solutions balance circuit (attenuate side-tone)
  echo-cancellation circuit (cancel echoes).

16
Dialing

• Pulse
• sends a pulse per digit
• collected by central office (CO)
• Interpreted by CO switching system to place call
  or activate special features (eg call
  forwarding, prepaid-calls etc)
• Tone
• key press (feep) sends a pair of tones digit
• also called Dual Tone Multifrequency (DTMF)
• CO supplies the power for ringing the bell.
• Standardized interface between CO and end-system
  gt digital handsets, cordless/cellular phones

17
Telephone Pieces Transmission Muxing

• Trunks between central offices carry hundreds of
  conversations
• Cant run thick bundles! Instead, send many calls
  on the same wire
• Multiplexing (a.ka. Sharing)
• Analog multiplexing
• Band-limit call to 3.4 KHz and frequency shift
  onto higher bandwidth trunk
• obsolete
• Digital multiplexing
• first convert voice to samples
• 1 sample 8 bits of voice
• 8000 samples/sec gt call 64 Kbps

18
Transmission Multiplexing (contd)

• How to choose a sample?
• 256 quantization levels, logarithmically spaced
  (why?)
• sample value amplitude of nearest quantization
  level
• Two choices of levels (? law and A law)
• Time division multiplexing
• Trunk carries bits at a faster bit rate than
  inputs
• n input streams, each with a 1-byte buffer
• Output interleaves samples
• Need to serve all inputs in the time it takes one
  sample to arrive
• gt output runs n times faster than input
• Overhead bits mark end of frame (why?)

19
Transmission Multiplexing

• Multiplexed trunks can be multiplexed further
• Need a standard! (why?)
• US/Japan standard is called Digital Signaling
  hierarchy (DS)

20
Telephone Pieces Switching
21
Telephone Pieces Switching

• Problem
• each user can potentially call any other user
• cant have (a billion) direct lines!
• Switches establish temporary circuits
• Switching systems come in two parts switch and
  switch controller

22
Switching System Components
23
Switch What does it do?

• Transfers data from an input to an output
• many ports (up to 200,000 simultaneous calls)
• need high speeds
• Some ways to switch
• 1. space division switching eg crossbar
• if inputs (or crosspoints) are multiplexed, need
  a schedule (why?)

24
Crossbar Switching Elements
25
Switching (Contd)

• Another way to switch
• time division (time slot interchange or TSI)
• also needs a service schedule (why?)

• To build larger switches we combine space and
  time division switching elements

26
Telephone pieces Signaling

• A switching system has a switch and a switch
  controller
• Switch controller is in the control plane
• does not touch voice samples
• Manages the network
• call routing (collect dialstring and forward
  call)
• alarms (ring bell at receiver)
• billing
• directory lookup (for 800/888 calls)

27
Signaling

• Switch controllers are special purpose computers
• Linked by their own internal computer network
• Common Channel Interoffice Signaling (CCIS)
  network
• Earlier design used in-band tones, but was hacked
• Also was very rigid (why?)
• Messages on CCIS conform to Signaling System 7
  (SS7)

28
Signaling (contd)

• One of the main jobs of switch controller keep
  track of state of every endpoint
• Key is state transition diagram

29
Telephony Routing of Signaled Calls

• Circuit-setup (I.e. the signaling call) is what
  is routed.
• Voice then follows route, and claims reserved
  resources.
• 3-level hierarchy, with a fully-connected core
• ATT 135 core switches with nearly 5 million
  circuits
• LECs may connect to multiple cores

30
Telephony Routing algorithm

• If endpoints are within same CO, directly connect
• If call is between COs in same LEC, use one-hop
  path between COs
• Otherwise send call to one of the cores
• Only major decision is at toll switch
• one-hop or two-hop path to the destination toll
  switch.
• Essence of telephony routing problem
• which two-hop path to use if one-hop path is
  full
• (almost a static routing problem )

31
Features of telephone routing

• Resource reservation aspects
• Resource reservation is coupled with path
  reservation
• Connections need resources (same 64kbps)
• Signaling to reserve resources and the path
• Stable load
• Network built for voice only.
• Can predict pairwise load throughout the day
• Can choose optimal routes in advance
• Technology and economic aspects
• Extremely reliable switches
• Why? End-systems (phones) dumb because
  computation was non-existent in early 1900s.
• Downtime is less than a few minutes per year gt
  topology does not change dynamically

32
Features of telephone routing

• Source can learn topology and compute route
• Can assume that a chosen route is available as
  the signaling proceeds through the network
• Component reliability drove system reliability
  and hence acceptance of service by customers
• Simplified topology
• Very highly connected network
• Hierarchy full mesh at each level simple
  routing
• High cost to achieve this degree of connectivity
• Organizational aspects
• Single organization controls entire core
• Afford the scale economics to build expensive
  network
• Collect global statistics and implement global
  changes
• gt Source-based, signaled, simple alternate-path
  routing

33
Telecommunications Regulation History

• FCC regulations cover telephony, cable, broadcast
  TV, wireless etc
• Common Carrier provider offers conduit for a
  fee and does not control the content
• Customer controls content/destination of
  transmission assumes criminal/civil
  responsibility for content
• Local monopolies formed by ATTs acquisition of
  independent telephone companies in early 20th
  century
• Regulation forced because they were deemed
  natural monopolies (only one player possible in
  market due to enormous sunk cost)
• FCC regulates interstate calls and state
  commissions regulate intra-state and local calls
• Bells 1000 independents interconnected
  expanded
• FCC rulemaking process
• Intent to act, solicitation of public comment etc

34
Deregulation of telephony

• 1960s-70s gradual de-regulation of ATT due to
  technological advances
• Terminal equipment could be owned by customers
  (CPE) gt explosion in PBXs, fax machines,
  handsets
• Modified final judgement (MFJ) breakup of ATT
  into ILECs (incumbent local exchange carrier) and
  IXC (inter-exchange carrier) part
• Long-distance opened to competition, only the
  local part regulated
• Equal access for IXCs to the ILEC network
• 1 long-distance number introduced then
• 800-number portability switching IXCs gt retain
  800 number
• 1995 removed price controls on ATT

35
Telecom Act of 1996

• Required ILECs to open their markets through
  unbundling of network elements (UNE-P),
  facilities ownership of CLECs.
• Today UNE-P is one of the most profitable for
  ATT and other long-distance players in the local
  market due to apparently below-cost regulated
  prices
• ILECs could compete in long-distance after
  demonstrating opening of markets
• Only now some ILECs are aggressively entering
  long distance markets
• CLECs failed due to a variety of reasons
• But long-distance prices have dropped
  precipitously (ATTs customer unit revenue in
  2002 was 11.3 B compared to 1999 rev of 23B)
• ILECs still retain over 90 of local market
• Wireless substitution has caused ILECs to develop
  wireless business units

36
US Telephone Network Structure (after 1984)
37
Exchange Area Network
38
Cable TV Networks
39
Cable Technology

• Coaxial cable RF distribution networks.
• Attributes
• Broadcast, low-band reverse channels
• Mainly one-way video channels
• Reasonably secure network (private conduit to
  home)
• Free from free-space interferences
• Good signal capacity (over 1 GHz) and flexibility
• Multiple signaling channels
• Significant attenuation that increases
  proportional to frequency gt (active) RF
  amplification (every 1000 ft)
• Freq responses of deployed amps and filters limit
  practical usage of frequencies gt 1 GHz

40
Cable Building Blocks
41
Cable Spectrum Upto 750 Mhz
42
Cable Technology Architecture

• Head-end signal processing center
• Each carrier Baseband analog or digital
  modulation
• Carriers multiplexed w/ freq-selective diplex
  filters
• allows simultaneous info transfer in both
  directions
• Tree-and-branch architecture
• Well-suited for one-way broadcast video
  transmission (same signals to every customer)
• Accumulates noise distortions (amplifiers)
• Affects plant reliability and received signal
  quality
• Limits on the number of amplifiers cascaded
• Limits on bandwidth in operation (few 100s of
  MHz) below cable potential
• Makes delivery of switched services (separate
  stream for each customer) difficult

43
Tree-and-Branch Architecture
44
Fiber Optics For Cable Networks

• Key Leave the laser ON and intensity-modulate
  with the analog signal
• Such analog modulated lasers are very different
  from their digital counterparts
• Low internal noise and high linearity in the
  range
• Receiver simple photo-detector -gt back to RF
  spectrum
• Result Hybrid fiber-coax infrastructure, with
  fiber closer to headend
• Coax plant serves smaller range (segmentation),
  but overall HFC reach dramatically increased
• Also, it allows the economical support of remote,
  smaller clusters of homes
• Each part could also provide different services
  to area (micro-market segmentation)
• Assign different portions of HFC spectrum to diff
  uses many virtual networks sustained
  investments possible

45
Hybrid Fiber Coax (HFC) Networks
46
Multiple Services over HFC
47
Future Potential of HFC Broadband

• Due to smaller loops, the region from 900MHz 1
  GHz can be used for data.
• Reduced noise in this region gt increased bit
  rate (200 Mbps) per segment
• Future fiber moves closer, smaller
  coax-segments, reduced homes per coax run (60
  homes), use of frequencies above 1 Ghz using new
  electronics
• Latest DOCSIS 2.0 spec 256 QAM (gt 8 bits/Hz) or
  S-CDMA on cable for more robust transmissions

48
Cable Regulation

• Very different from telephony not common-carrier
• Able to control content AND the conduit!
• Grew by providing an alternative (and extension)
  to broadcast TV and had initial growth troubles
• Did not have to offer service on a
  non-discriminatory basis (unlike common carriers)
• Asserted first-amendment rights to maintain
  control over content
• Not required to provide access to their
  distribution system to other providers (some
  portion of capacity required to be offered to
  unaffiliated players eg CNN)
• But they reserve rights to appropriately bundle
  these channels
• Limited regulation basic tier is rate-regulated
  by local authorities till 1999 based upon FCC
  rules

49
Cable regulation (contd)

• Cable networks limited in horizontal expansion,
  and from vertically integrating w/ CNN etc
• Note ILECs like Bell Atlantic in contrast merged
  with IXCs like GTE
• ATTs cable acquisitions were interesting (and
  will be explored later)
• Cable service is multi-faceted and varied from
  area to area gt regulation formulation more
  complicated
• Over-builders (satellite providers) got access to
  independent content providers otherwise
  regulation achieved little for cable
• Local authorities get revenue from cable
  regulation
• HFC dominates franchise regulation talks, but
  cable providers are not obligated to provide
  broadband access..

50
Data Networking and the Internet
51
Recall Indirect Connectivity

• Indirect Connectivity
• switched networks
• gt switches
• inter-networks
• gt routers

52
Inter-Networks Networks of Networks


Internet



The internet is just a big switch providing
indirect connectivity
53
Recall Connecting N users Directly

• Pt-pt connects only two users directly
• How to connect N users directly ?
• What are the costs of each option?
• Does this method of connectivity scale ?

A
B
. . .
Bus
Full mesh
54
Point-to-Point Connectivity Issues

• Physical layer coding, modulation etc
• Link layer needed if the link is shared betn
  apps is unreliable and is used sporadically
• No need for protocol concepts like addressing,
  names, routers, hubs, forwarding, filtering

A
B
55
Link Layer Serial IP (SLIP)

• Simple only framing Flags byte-stuffing
• Compressed headers (CSLIP) for efficiency on low
  speed links for interactive traffic.
• Problems
• Need other ends IP address a priori (cant
  dynamically assign IP addresses)
• No type field gt no multi-protocol
  encapsulation
• No checksum gt all errors detected/corrected by
  higher layer.
• RFCs 1055, 1144

56
Link Layer PPP

• Point-to-point protocol
• Frame format similar to HDLC
• Multi-protocol encapsulation, CRC, dynamic
  address allocation possible
• key fields flags, protocol, CRC
• Asynchronous and synchronous communications
  possible
• Link and Network Control Protocols (LCP, NCP) for
  flexible control peer-peer negotiation
• Can be mapped onto low speed (9.6Kbps) and high
  speed channels (SONET)

57
Connecting N users Directly ...

• Bus Low cost vs broadcast/collisions, MAC
  complexity
• Full mesh High cost vs simplicity
• New concept
• Address to identify nodes.
• Needed if we want the receiver alone to consume
  the packet!

. . .
Bus
Full mesh

• Problem Direct connectivity does not scale.

58
How to build Scalable Networks?

• Scaling system allows the increase of a key
  parameter. Eg let N increase
• Inefficiency limits scaling
• Direct connectivity is inefficient hence does
  not scale
• Mesh inefficient in terms of of links
• Bus architecture 1 expensive link, N cheap
  links. Inefficient in bandwidth use

59
Filtering, forwarding

• Filtering choose a subset of elements from a set
• Dont let information go where its not supposed
  to
• Filtering gt More efficient gt more scalable
• Filtering is the key to efficiency scaling
• Forwarding actually sending packets to a
  filtered subset of link/node(s)
• Packet sent to one link/node gt efficient
• Solution Build nodes which focus on
  filtering/forwarding and achieve indirect
  connectivity
• switches routers

60
Connecting N users Indirectly

• Star One-hop path to any node, reliability,
  forwarding function
• Switch S can filter and forward!
• Switch may forward multiple pkts in parallel for
  additional efficiency!

Star
S
61
Connecting N users Indirectly

• Ring Reliability to link failure, near-minimal
  links
• All nodes need forwarding and filtering
• Sophistication of forward/filter lesser than
  switch

Ring
62
Topologies Indirect Connectivity
S
Ring
Star
Tree
63
Protocol Issues in Data Networks

• Pt-Pt connectivity
• Framing
• Error control/Reliability
• Flow control Windowing protocols
• Multiplexing, Virtualization
• Circuit vs Packet Switching a muxing view
• MAC arbitration schemes
• Random access/CSMA, TDMA, CDMA
• Interconnection components repeater, hub,
  bridge, switch, router

64
Reliability Types of errors effects

• Forward channel bit-errors (garbled packets)
• Forward channel packet-errors (lost packets)
• Reverse channel bit-errors (garbled status
  reports)
• Reverse channel bit-errors (lost status reports)
• Protocol-induced effects
• Duplicate packets
• Duplicate status reports
• Out-of-order packets
• Out-of-order status reports
• Out-of-range packets/status reports (in
  window-based transmissions)

65
Temporal Redundancy Model
Packets

• Sequence Numbers
• CRC or Checksum

Timeout

• ACKs
• NAKs,
• SACKs
• Bitmaps

Status Reports
Retransmissions

• Packets
• FEC information

66
Reliability Mechanisms

• Mechanisms
• Checksum detects corruption in pkts acks
• ACK packet correctly received
• Duplicate ACK packet incorrectly received
• Sequence number identifies packet or ack
• 1-bit sequence number used both in forward
  reverse channel
• Timeout only at sender
• Reliability capabilities achieved
• An error-free channel
• A forward reverse channel with bit-errors
• Detects duplicates of packets/acks
• NAKs eliminated
• A forward reverse channel with packet-errors
  (loss)

67
Stop and Wait Flow Control
Light in vacuum 300 m/?s Light in fiber
200 m/?s Electricity 250 m/?s
68
Sliding Window Protocols
Ntframe
U
2tproptframe
tframe
Data
N
tprop
2?1

1 if Ngt2?1
Ack
69
Multiplexing The Method of Sharing Costly
Resources

• Multiplexing sharing
• Allows system to achieve economies of scale
• Cost waiting time (delay), buffer space loss
• Gain Money () gt Overall system costs less

Full Mesh
Bus
70
Virtualization

• The multiplexed shared resource with a level of
  indirection will seem like a unshared virtual
  resource!
• I.e. Multiplexing indirection virtualization
• We can refer to the virtual resource as if it
  were the physical resource.
• Eg virtual memory, virtual circuits
• Connectivity a virtualization created by the
  Internet!
• Indirection requires binding and unbinding
• Eg use of packets, slots, tokens etc

A
B
. . .

A
B
Physical Bus
Virtual Pt-Pt Link
71
Statistical Multiplexing

• Reduce resource requirements (eg bus capacity)
  by exploiting statistical knowledge of the
  system.
• Eg average rate lt service rate lt peak rate
• If service rate lt average rate, then system
  becomes unstable!!
• First design to ensure system stability!!
• Then, for a stable multiplexed system
• Gain peak rate/service rate.
• Cost buffering, queuing delays, losses.
• Useful only if peak rate differs significantly
  from average rate.
• Eg if traffic is smooth, fixed rate, no need to
  play games with capacity sizing

72
Stability of a Multiplexed System
Average Input Rate gt Average Output Rate gt
system is unstable!

• How to ensure stability ?
• Reserve enough capacity so that demand is less
  than reserved capacity
• Dynamically detect overload and adapt either the
  demand or capacity to resolve overload

73
Whats a performance tradeoff ?

• A situation where you cannot get something
• for nothing!
• Also known as a zero-sum game.

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

Traffic intensity La/R
74
Whats a performance tradeoff ?

• La/R 0 average queuing delay small
• La/R -gt 1 delays become large
• La/R gt 1 average delay infinite (service
  degrades unboundedly gt instability)!

Summary Multiplexing using bus topologies has
both direct resource costs and intangible costs
like potential instability, buffer/queuing delay.
75
How to design large inter-networks?
Circuit-Switching

• Divide link bandwidth into pieces
• Reserve pieces on successive links and tie them
  together to form a circuit
• Map traffic into the reserved circuits
• Resources wasted if unused expensive.

• Mapping can be done without headers.
• Everything inferred from timing.

76
How to design large inter-networks?
Packet-Switching

• Chop up data (not links!) into packets
• Packets data meta-data (header)
• Switch packets at intermediate nodes
• Store-and-forward if bandwidth is not
  immediately available.

77
Packet Switching
10 Mbs Ethernet
statistical multiplexing
C
A
1.5 Mbs
B
queue of packets waiting for output link
45 Mbs
D
E

• Cost self-descriptive header per-packet,
  buffering and delays due to statistical
  multiplexing at switches.
• Need to either reserve resources or dynamically
  detect and adapt to overload for stability

78
Spatial vs Temporal Multiplexing

• Spatial multiplexing Chop up resource into
  chunks. Eg bandwidth, cake, circuits
• Temporal multiplexing resource is shared over
  time, I.e. queue up jobs and provide access to
  resource over time. Eg FIFO queueing, packet
  switching
• Packet switching is designed to exploit both
  spatial temporal multiplexing gains, provided
  performance tradeoffs are acceptable to
  applications.
• Packet switching is potentially more efficient gt
  potentially more scalable than circuit switching !

79
Protocol Issues in Data Networks (Contd)

• Pt-Pt connectivity
• Framing
• Error control/Reliability
• Flow control Windowing protocols
• Multiplexing, Virtualization
• Circuit vs Packet Switching a muxing view
• MAC arbitration schemes
• Random access/CSMA, TDMA, CDMA
• Interconnection components repeater, hub,
  bridge, switch, router

80
Multi-Access LANs

• Hybrid topologies
• Uses directly connected topologies (eg bus), or
• Indirectly connected with simple filtering
  components (switches, hubs).
• Limited scalability due to limited filtering
• Medium Access Protocols
• ALOHA, CSMA/CD (Ethernet), Token Ring
• Key Use a single protocol in network
• Concepts address, forwarding (and forwarding
  table), bridge, switch, hub, token, medium access
  control (MAC) protocols

81
MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency)
• allocate piece to node for exclusive use
• Taking turns Token-based
• tightly coordinate shared access to avoid
  collisions
• Random Access
• allow collisions
• recover from collisions

Goal efficient, fair, simple, decentralized
82
Channel PartitioningMAC protocols. Eg TDMA

• TDMA time division multiple access
• Access to channel in "rounds"
• Each station gets fixed length slot (length pkt
  trans time) in each round
• Unused slots go idle
• Example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

83
Taking Turns MAC protocols - 1

• Channel partitioning MAC protocols
• share channel efficiently at high load
• inefficient at low load delay in channel access,
  1/N bandwidth allocated even if only 1 active
  node!
• Random access MAC protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• Taking turns protocols
• look for best of both worlds!

84
Taking Turns MAC protocols - 2

• Polling
• Master node invites slave nodes to transmit in
  turn
• Request to Send, Clear to Send messages
• Concerns
• polling overhead
• latency
• single point of failure (master)

• Token passing
• Control token passed from one node to next
  sequentially.
• Token message
• Concerns
• token overhead
• latency
• single point of failure
• (token)

85
Taking Turns Protocols 3

• Reservation-based a.k.a Distributed Polling
• Time divided into slots
• Begins with N short reservation slots
• reservation slot time equal to channel end-end
  propagation delay
• station with message to send posts reservation
• reservation seen by all stations
• After reservation slots, message transmissions
  ordered by known priority

86
Random Access Protocols

• Aloha at University of Hawaii Transmit
  whenever you likeWorst case utilization 1/(2e)
  18
• CSMA Carrier Sense Multiple Access Listen
  before you transmit
• CSMA/CD CSMA with Collision DetectionListen
  while transmitting. Stop if you hear someone
  else.
• Ethernet uses CSMA/CD.Standardized by IEEE 802.3
  committee.

87
10Base5 Ethernet Cabling Rules

• Thick coax
• Length of the cable is limited to 2.5 km, no more
  than 4 repeaters between stations
• No more than 500 m per segment ? 10Base5

Terminator
Repeater
2.5m
Transceiver
500 m
88
10Base5 Cabling Rules (Continued)

• No more than 2.5 m between stations
• Transceiver cable limited to 50 m

Terminator
Repeater
2.5m
Transceiver
500 m
89
Inter-connection Devices

• Repeater Layer 1 (PHY) device that restores data
  and collision signals a digital amplifier
• Hub Multi-port repeater fault detection
• Note broadcast at layer 1
• Bridge Layer 2 (Data link) device connecting two
  or more collision domains.
• Key a bridge attempts to filter packets and
  forward them from one collision domain to the
  other.
• It snoops on passing packets and learns the
  interface where different hosts are situated, and
  builds a L2 forwarding table
• MAC multicasts propagated throughout extended
  LAN.
• Note Limited filtering intelligence and
  forwarding capabilities at layer 2

90
Interconnection Devices (Continued)

• Router Network layer device. IP, IPX, AppleTalk.
  Interconnects broadcast domains.
• Does not propagate MAC multicasts.
• Switch
• Key has a switch fabric that allows parallel
  forwarding paths
• Layer 2 switch Multi-port bridge w/ fabric
• Layer 3 switch Router w/ fabric and per-port
  ASICs
• These are functions. Packaging varies.

91
Interconnection Devices
Extended LAN Broadcast domain
LAN CollisionDomain
B
H
H
Router
Application
Application
Transport
Transport
Network
Network
Datalink
Datalink
Physical
Physical
92
Ethernet (IEEE 802) Address Format
(Organizationally Unique ID)
OUI
10111101
G/I bit (Group/Individual)
G/L bit (Global/Local)

• 48-bit flat address gt no hierarchy to help
  forwarding
• Hierarchy only for administrative/allocation
  purposes
• Assumes that all destinations are (logically)
  directly connected.
• Address structure does not explicitly acknowledge
  indirect connectivity
• gt Sophisticated filtering cannot be done!

93
Ethernet (IEEE 802) Address Format
(Organizationally Unique ID)

• G/L bit administrative
• Global unique worldwide assigned by IEEE
• Local Software assigned
• G/I bit multicast
• I unicast address
• G multicast address. Eg To all bridges on this
  LAN

OUI
10111101
G/I bit (Group/Individual)
G/L bit (Global/Local)
94
Ethernet 802.3 Frame Format
IP
IPX
AppleTalk

• Ethernet

Size in bytes
Dest.Address
SourceAddress
Type
Info
CRC
4
6
6
2
IP
IPX
AppleTalk

• IEEE 802.3

Dest.Address
SourceAddress
Length
LLC
CRC
Pad
Info
6
6
2
4
Length

• Maximum Transmission Unit (MTU) 1518 bytes
• Minimum 64 bytes (due to CSMA/CD issues)

95
Network/Transport Layer Issues

• Inter-networking heterogeneity, scale
• Routing
• Congestion control
• Quality of Service (QoS)

96
Inter-Networks Networks of Networks

• What is it ?
• Connect many disparate physical networks and
  make them function as a coordinated unit -
  Douglas Comer
• Many gt scale
• Disparate gt heterogeneity
• Result Universal connectivity!
• The inter-network looks like one large switch,
• User interface is sub-network independent

97
Inter-Networks Networks of Networks

• Internetworking involves two fundamental
  problems heterogeneity and scale
• Concepts
• Translation, overlays, address name resolution,
  fragmentation to handle heterogeneity
• Hierarchical addressing, routing, naming, address
  allocation, congestion control to handle scaling
• Two broad approaches circuit-switched and
  packet-switched

98
Scalable Forwarding, Structured Addresses

• Address has structure which aids the forwarding
  process.
• Address assignment is done such that nodes which
  can be reached without resorting to L3 forwarding
  have the same prefix (network ID)
• A simple comparison of network ID of destination
  and current network (broadcast domain) identifies
  whether the destination is directly connected
• I.e. Reachable through L2 forwarding only
• Within L3 forwarding, further structure can aid
  hierarchical organization of routing domains
  (because routing algorithms have other
  scalability issues)

Network ID Host ID
Demarcator
99
Flat vs Structured Addresses

• Flat addresses no structure in them to
  facilitate scalable routing
• Eg IEEE 802 LAN addresses
• Hierarchical addresses
• Network part (prefix) and host part
• Helps identify direct or indirectly connected
  nodes

100
Internet Routing Drivers

• Technology and economic aspects
• Internet built out of cheap, unreliable
  components as an overlay on top of leased
  telephone infrastructure for WAN transport.
• Cheaper components gt fail more often gt topology
  changes often gt needs dynamic routing
• Components (including end-systems) had
  computation capabilities.
• Distributed algorithms can be implemented
• Cheap overlaid inter-networks gt several entities
  could afford to leverage their existing
  (heterogeneous) LANs and leased lines to build
  inter-networks.
• Led to multiple administrative clouds which
  needed to inter-connect for global communication.

101
Internet Routing Model

• 2 key features
• Dynamic routing
• Intra- and Inter-AS routing, AS locus of admin
  control
• Internet organized as autonomous systems (AS).
• AS is internally connected
• Interior Gateway Protocols (IGPs) within AS.
• Eg RIP, OSPF, HELLO
• Exterior Gateway Protocols (EGPs) for AS to AS
  routing.
• Eg EGP, BGP-4

102
Intra-AS and Inter-AS routing

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routers with other routers in
  their AS

b
a
a
C
B
d
A
103
Intra-AS and Inter-AS routing Example
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A
104
Requirements for Intra-AS Routing

• Should scale for the size of an AS.
• Low end 10s of routers (small enterprise)
• High end 1000s of routers (large ISP)
• Different requirements on routing convergence
  after topology changes
• Low end can tolerate some connectivity
  disruptions
• High end fast convergence essential to business
  (making money on transport)
• Operational/Admin/Management (OAM) Complexity
• Low end simple, self-configuring
• High end Self-configuring, but operator hooks
  for control
• Traffic engineering capabilities high end only

105
Requirements for Inter-AS Routing

• Should scale for the size of the global Internet.
  
• Focus on reachability, not optimality
• Use address aggregation techniques to minimize
  core routing table sizes and associated control
  traffic
• At the same time, it should allow flexibility in
  topological structure (eg dont restrict to
  trees etc)
• Allow policy-based routing between autonomous
  systems
• Policy refers to arbitrary preference among a
  menu of available options (based upon options
  attributes)
• In the case of routing, options include
  advertised AS-level routes to address prefixes
• Fully distributed routing (as opposed to a
  signaled approach) is the only possibility.
• Extensible to meet the demands for newer policies.

106
The Congestion Problem
?i
?i
?
?

• Problem demand outstrips available capacity

?1
Capacity
Demand
?n

• If information about ?i , ? and ? is known in a
  central location where control of ?i or ? can be
  effected with zero time delays,
• the congestion problem is solved!
• Unfortunately, we have incomplete info, require a
  distributed solution with time-varying time-delays

107
Congestion A Close-up View
packet loss
knee
cliff

• knee point after which
• throughput increases very slowly
• delay increases fast
• cliff point after which
• throughput starts to decrease very fast to zero
  (congestion collapse)
• delay approaches infinity
• Note (in an M/M/1 queue)
• delay 1/(1 utilization)

Throughput
congestion collapse
Load
Delay
Load
108
Congestion Control vs. Congestion Avoidance

• Congestion control goal
• stay left of cliff
• Congestion avoidance goal
• stay left of knee
• Right of cliff
• Congestion collapse

knee
cliff
Throughput
congestion collapse
Load
109
Goals of Congestion Control

• To guarantee stable operation of packet networks
• Sub-goal avoid congestion collapse
• To keep networks working in an efficient status
• Eg high throughput, low loss, low delay, and
  high utilization
• To provide fair allocations of network bandwidth
  among competing flows in steady state
• For some value of fair ?

109
110
CC Techniques Self-clocking

• Implications of ack-clocking
• More batching of acks gt bursty traffic
• Less batching leads to a large fraction of
  Internet traffic being just acks (overhead)

111
CC Techniques Additive Increase/Multiplicative
Decrease (AIMD) Policy

• Assumption decrease policy must (at minimum)
  reverse the load increase over-and-above
  efficiency line
• Implication decrease factor should be
  conservatively set to account for any congestion
  detection lags etc

112
Quality of Service What is it?
Multimedia applications network audio and video
113
Fundamental QoS Problems

• In a FIFO service discipline, the performance
  assigned to one flow is convoluted with the
  arrivals of packets from all other flows!
• Cant get QoS with a free-for-all
• Need to use new scheduling disciplines which
  provide isolation of performance from arrival
  rates of background traffic

114
Fundamental QoS Problems

• Conservation Law (Kleinrock) ??(i)Wq(i) K
• Irrespective of scheduling discipline chosen
• Average backlog (delay) is constant
• Average bandwidth is constant
• Zero-sum game gt need to set-aside resources
  for premium services

115
QoS Big Picture Control/Data Planes
116
Internet Regulation

• FCC has largely had a hands-off policy
• Early development of internet in part was
  influenced by high cost of telecom links
• Packet switching developed as better multiplexing
  technology
• Common-carriage regulation has affected Inet
• Eg modems were like fax machine for the common
  carrier
• Use of basic service (eg telephony) to provide
  enhanced service (eg internet access) gt not
  subject to FCC or state jurisdiction
• Led to community bulletin-boards, ISPs,
  value-added networks (frame-relay?)
• Home-to-ISP treated as local call (even if
  crossed state-boundaries)
• ILECs prohibited from offering inter-LATA
  services
• DSL viewed as basic service gt must unbundle DSL
  to allow 3rd parties to offer internet access
  over ILEC DSL

117
Summary List of Internet Problems

• Basics Direct/indirect connectivity, topologies
• Link layer issues
• Framing, Error control, Flow control
• Multiple access Ethernet
• Cabling, Pkt format, Switching, bridging vs
  routing
• Internetworking problems Naming, addressing,
  Resolution, fragmentation, congestion control,
  traffic management, Reliability, Network
  Management

118
Additional Reading

• Internet Design Philosophy
• Saltzer, Reed, Clark "End-to-End arguments in
  System Design"
• Clark "The Design Philosophy of the DARPA
  Internet Protocols"
• RFC 2775 Internet Transparency In HTML