Proxy Servers

1
Proxy Servers

• A server that sits between a client application,
  such as a web browser, and a real server. It
  intercepts all requests to the real server to see
  if it can fulfill the requests itself. If not, it
  forwards the request to the real server.
• Usually has software built in to make it act as a
  router.
• Improve Performance
• Recognizes multiple requests and caches popular
  data
• Filter Requests
• Allows control over data served, and generates
  statistics

2
Cable modems

• Speeds up to 30 Mbps claimed
• Actually, Ethernet connection restricts speed to
  10 Mbps
• Actual speed obtained depends on
• Speed of PC in handling IP traffic
• Speed of PC to modem interface
• Speed of cable modem system
• Congestion on cable network
• Size of pipe at head end to rest of Internet
• Speeds achieved have been up to 1.5 Mbps

3
Cable Internet

• Convergence between Communications and Computer
  Industries
• LANs could be deployed with baseband coax or
  broadband coax cables
• The Ethernet Cable
• The baseband cable was based on Ethernet
  development, using a 20-MHz, 50-ohm coax cable.
• Broadband coaxial cable, operating with a
  bandwidth of approximately 350 MHz on a 75-ohm
  cable.
• capable of higher speeds

4
Cable Internet

• The New Market
• The cable television companies are in the midst
  of a transition from their traditional core
  business of entertainment video programming to a
  position as a full-service provider of video,
  voice, and data telecommunications services
• The cable platform is steadily evolving into a
  hybrid digital and analog transmission system
  from its one-way, analog system

5
ADSL modems

• Asymmetric digital subscriber line modem
• The word asymmetric is used because these modems
  send data faster in one direction than they do in
  another
• 1mb/s Upstread, 8mb/s Downstream
• Uses the higher bandwidth above 3000 hz on the
  telephone companys copper wire phone line.
• The phone line's bandwidth between 24,000 hertz
  and 1,100,000 hertz is divided into 4,000-hertz
  bands
• A virtual modem is assigned to each band
• Each of these 249 virtual modems tests its band
  and does the best it can with the slice of
  bandwidth it is allocated
• Higher frequency creates much crosstalk to lower
  bandwiths. Frequency filters must be used.

6
HDSL modems

• High Bit-Rate ADSL Lines
• Developed to circumvent cable problems
• Does not require repeaters up to 12,000 ft.
• Bridge taps and splices do not bother it
• Sends 768 Kbps down two pairs of wires gives T1
  speed

7
Performance variables

• Actual speed obtained depends on
• Modem connection to PC
• Quality of twisted pair used in local phone
  wiring
• All networking connections into a PC are
  regulated by software flow control, ie-port speed
  settings.
• Frame size can be changed in Windows, both
  sending and receiving. This is governed by the
  TCP Window size. Default is not the fastest
  setting. Free software exist to optimize these
  Windows settings. These can be tweaked manually
  as well.
• http//cable-dsl.home.att.net/

8
They hype of these services

• The providers are trying to bump up their
  revenues, without major new investments
• The key ingredient for success is to minimize
  costs and satisfy the consumer
• Both services cost roughly the same
• Cable is slower if you have a lot of subscribers
  sharing your line
• Cable generally has higher upload speeds at any
  given time

9
Multiple Computers on One Connection

• Both ADSL and Cable providers sell IP addresses,
  weather they are from DHCP or static (more
  expensive)
• With 1 IP you can have only 1 computer on your
  line using that IP at any given time
• Why not create your own subnet like the ISPs do?
  All you need is a router, or a computer running a
  router emulator
• MS Internet Connection Sharing emulates a
  Gateway, a really intelligent router
• Need 2 network cards, one for input and one for
  output if emulating.

10
ISDN

• integrated services digital network
• an international communications standard for
  sending voice, video, and data over digital
  telephone lines or normal telephone wires,
  (Broadband or Baseband)
• Supports data transfer rates of 64Kbps
• Most services offer 2 pairs of wires, 1 for data
  and 1 for voice. You can use the voice line with
  ADSL to increase BPS rate
• You can use both lines for data and achieve
  128Kbps

11
ISDN overview

• ISDN vision
• Universal access
• User services
• End-to-end digital communication
• Intended to increase reliability and lower costs
• Part of increasing digitization of communication
• Any kind of data can be carried
• Evolution from telephone-based service
• Integrated digital network combines digital
  transmission, switching
• Analog transmission has to be decoded before
  digital switching
• Digital transmission does not need decoding,
  directly switched

12
How ISDN was expected to evolve

• ISDN transition time of one or more decades
• Gradual cutover
• Need to replace several infrastructures
• Use of existing networks
• Existing voice and data infrastructure needed to
  be utilized
• Eventually, connections at other than 64 Kbps
• Need to improve digitizing technology (e.g.
  ADPCM)
• Need higher rates for other data applications
• Purpose was to move beyond bandwidth limits of
  current analog telephony

13
ISDN principles

• Set of standard facilities to support range of
  inputs
• Video, data, text, sound, fax, etc.
• Support for switched, nonswitched connections
• Packet-switching, circuit-switching, leased lines
• Connections at 64Kbps
• Network intelligence for services, maintenance,
  network management
• Layered protocol model uses existing standards
• Configuration flexibility in different countries
• Allows physical configuration/architecture
  flexibility
• Different countries can implement different
  policies (competition, etc.)

14
ISDN today

• Considerable pilot testing, availability in N.
  America
• Telus has ISDN switches in Calgary
• ISDN services available to public
• Commercial availability globally
• Considerable progress by European telecoms
• Some standards already out of date
• Enormity of undertaking results in slow
  implementation
• Vision of global digital network may not
  materialize
• Digital services may be provided by smaller
  enterprises
• ISDN standards work will still be valuable

15
Channel configurations I

• Multiple configuration possibilities
• Specific configurations have been proposed
• 2BD, 23BD, etc.
• Utilizes existing digital link capacities
• Basic access (basic rate interface)
• 2BD configuration
• Provides two FDX 64 Kbps channels, one FDX 16
  Kbps channel
• Including overhead information, channel needs 192
  Kbps data rate
• Assumed to be suitable for residential subscribers

16
Broadband ISDN

• Based on two technologies
• Asynchronous Transfer Mode (ATM)
• Synchronous Optical NETwork (SONET), SDH
• Limited availablity at present
• Work on the standard began in late 1980s
• Vision
• Massive information sharing across media, from
  multiple sources, in a shared electronic
  environment
• Large-scale distribution of material on demand
• Greater capacity
• User channel rates of 155 and 622 Mbps

17
Synchronous Optical NETwork (SONET)

• Based on optical carrier (OC) hierarchy
• Conceptually similar to DS hierarchy
• Refers to 8-bit segments as octets, to indicate
  transport of a variety of communication
• OC hierarchy

OC-18 (933.12 Mbps) OC-24 (1.244 Gbps) OC-36
(1.866 Gbps) OC-48 (2.488 Gbps)
OC-1 (51.84 Mbps) OC-3 (155.52 Mbps) OC-9
(466.56 Mbps) OC-12 (622.08 Mbps)
18
B-ISDN services

• Interactive services
• Conversational services
• Real-time dialogues (user-user, user-machine,
  machine-machine)
• Messaging services
• Electronic, voice, video mail
• Retrieval services
• Film and data libraries
• Distribution services
• Distribution services without user presentation
  control
• Broadcast services
• Distribution services with user presentation
  control
• User control over start times (e.g., pay-per-view
  TV)

19
Frame relay overview

• Also known as frame mode bearer service
• Developed in conjunction with ISDN
• Involves transmission of variable-length frames
• Can achieve speeds up to 1.544 Mbps
• Improvement over X.25
• Call control and data on separate connections
• Results in less network overhead as user data
  connection is simpler for network nodes to
  maintain
• Implements only physical and data link layers
• Point-to-point (hop-by-hop) flow, error control
  eliminated
• Streamlined acknowledgement not required at each
  node

20
Frame relay advantages and disadvantages

• Advantage dynamic bandwidth allocation
• If more data sent, network transmits more
  frames/second
• Unlike other networks where bandwidth is fixed
• Good for bursty inter-LAN traffic
• Advantage multi-connections per access line
• Unlike leased lines, where each access line
  connected to a single leased line
• Advantage protocol independence
• Creates frames with any kind of data
• Disadvantage lack of predictability of arrival
  times of variable length frames
• Especially a problem for voice and video

21
Applications suited to frame relay

• Block-interactive data applications
• Requires high throughput, low delays
• e.g., high-resolution graphics
• File transfer
• High throughput required for reasonable transfer
  speed
• Multiplexed low bit rate
• Large group of applications multiplexed on a
  frame relay connection
• e.g., character-interactive traffic, like
  text-editing

22
ATM overview

• Asynchronous transfer mode (a form of cell relay
  transmission)
• Also developed in conjunction with work on ISDN
• Designed to achieve data rates considerably
  higher than frame relay
• Involves transmission of fixed-length frames
• Like frame relay, avoids point-to-point error,
  flow control

23
ATM basics

• Virtual channels
• Basic connection established from end to end
• Each channel used for a specific data source
• Virtual paths
• Several virtual channels with common source,
  destination
• Useful unit for operations on multiple channels
• Each channel may have different data (e.g.,
  voice, video, data, image)
• Control signaling
• Several methods available to dynamically set up
  control connection
• Meta-signaling channel available to set up
  signaling channel

24
ATM advantages and disadvantages

• Same advantages as frame relay
• Advantage predictable delay of fixed length
  cells make it more useful for video, voice
• Advantage scalability
• Can be used as wide area ATM network, as an ATM
  LAN, or as future transport for services like
  SMDS
• Disadvantage will take time before standards are
  well-defined and agreed upon