Fundamentals of Telecommunications Week 5: WANs and LANs

1
Fundamentals of TelecommunicationsWeek 5 WANs
and LANs

• Elizabeth Lane Lawley, Instructor

2
Review of Quiz 1

• On a take-home test, my expectations are higher
  for both content and presentation
• Reminder It is your responsibility to understand
  the policy on academic dishonesty and clarify
  anything you dont understand

3
Network Models

• Helpful to have a model to understand parts of a
  network
• Most commonly used model is OSI 7-layer model

4
OSI

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

• Open Systems Interconnection
• Developed by ISO
• Contains seven layers(see page 358)

5
OSI Lower Layers

• Physical
• Data Link
• Network

6
OSI Physical Layer

• Responsible for transmission of bits
• Always implemented through hardware
• Encompasses mechanical, electrical, and
  functional interfaces
• e.g. RS-232

7
OSI Data Link Layer

• Responsible for error-free, reliable transmission
  of data
• Flow control, error correction
• e.g. HDLC

8
OSI Network Layer

• Responsible for routing of messages through
  network
• Concerned with type of switching used
• Handles routing between networks, as well as
  through packet-switching networks

9
OSI Upper Layers

• Transport
• Session
• Presentation
• Application

10
OSI Transport Layer

• Isolates messages from lower and upper layers
• Breaks down message size
• Monitors quality of communications channel
• Selects most efficient communication service
  necessary for a given transmission

11
OSI Session Layer

• Establishes connections between systems
• Manages log-ons, password exchange, log-offs
• Tracks physical location of files on both sides
  of a transfer

12
OSI Presentation Layer

• Provides format and code conversion services
• Examples
• File conversion from ASCII to EBDIC
• Invoking character sequences to generate bold,
  italics, etc on a printer

13
OSI Application Layer

• Provides access to network for end-user
• Users capabilities are determined by what items
  are available on this layer

14
TCP/IP v. OSI
15
TCP/IP Advantages

• Simpler model (5 layers rather than 7)
• More widely implemented

16
OSI Lower Layer Devices

• Provide transmission facilities over physical
  media
• Generally sit between computers (DTEs), and
  provide data transfer functions only

17
Null Modems

• Created from cables
• Crossover wires within cable or connector
• Allows direct point-to-point communication

18
Short Haul Modems

• Inexpensive (as little as 40)
• Can transmit at up to 19.200bps for several miles
• Also called modem eliminators

19
CSU/DSU (Digital Modems)

• Channel Service Units/Data Service Units
• Used when connecting to digital leased lines,
  like 56 or T-1s
• Ensures that digital data is properly formed into
  digital signal
• Includes digital conversion and high-speed serial
  interface
• Provides line equalization, filtering, error
  control, testing, and signal regeneration

20
Data Networks

• Connect data processing devices for the purpose
  of data communication
• WAN/Wide Area Network packet-switched networks
  that provide communication over long distances
  and among multiple sites
• LAN/Local Area Network privately owned network
  that provides reliable, high-speed, switched
  connections between devices in a single building
  or complex

21
Packet Switching Networks

• Data is broken up into packets that may take
  diverse routes
• Use virtual circuits rather than dedicated
  circuits--only in place while data is being
  transferred

22
X.25 Standard

• Defines method for connecting a node to a packet
  switching network
• Similar to lower layers of OSI model
• If DTE is not X.25 compatible, a PAD (packet
  assembler/disassembler) is necessary

23
Frame Relay Networks

• Takes advantage of digital nature of IXC
  connections
• Uses less error detection/correction because
  digital transmission is more reliable
• Uses only Layers 1 2 of OSI (routing
  information in frame is used to relay
  information, rather than readdressing)

24
LANs

• Provide flexible, cross-platform data
  communication and resource sharing
• Typically covers an area of 1-10 miles (building,
  campus, industrial park)
• Can use centralized control or distributed control

25
LAN Topologies

• Mesh
• Star
• Bus
• Star-wired bus
• Tree
• Ring
• Star-wired ring

26
LAN Access Methods

• CSMA Carrier Sense Multiple Access
• CSMA/CD CSMA with Collision Detection
• Token Passing

27
CSMA/CD

• Checks for clear path prior to transmission
  attempt
• When channel is clear of a carrier signal, it
  transmits
• If two stations transmit simultaneously,
  monitoring picks up collision, and stations
  retransmit at randomly selected backoff times

28
LAN Standards

• 802.x series
• 802.3 - Ethernet (CSMA/CD)
• 802.5 - Token Ring
• Divided data link into two functions
• MAC (medium access control) handles access method
• LLC (logical link control) handles error
  checking/reliability

29
Ethernet (802.3)

• Bus network using CSMA/CD
• Different transmission media can be used
• Baseband coax allows 10mbps
• 10Base-T
• 10mbps
• Baseband
• Twisted pair wires
• Uses hubs in star-wired bus configuration

30
IBM Token Ring (802.5)

• Star-wired ring
• 4 or 16mbps using twisted pair

31
Fiber Distributed Data Interface (FDDI)

• ANSI standard
• Uses fiber optics for speeds of up to 100mpbs
• Token ring topology similar to IEEE 802.5
• Two rings one for traffic, one for backup

32
Internetworking

• Connecting two or more networks together
• Why?
• Distance limitations reached
• Overcrowding of LANs
• Need for remote sites to connect

33
Repeaters

• Layer 1 devices
• Simply receives and retransmits all bits
• Some repeaters provide basic error-checking
  (Layer 2 function)
• Inexpensive, used to overcome distance limitations

34
Bridges

• Connects two or more networks using the same
  addressing method
• Can provide some addressing information (Layer
  2-3 functions)
• Often used when LANs reach their capacity of nodes

35
Types of Bridges

• Adaptive/learning bridges configure themselves by
  watching traffic, determining what addresses are
  on each side of the bridge
• Remote bridges connect to each other via a
  digital phone circuit such as a T-1

36
Routers

• Layer 3 devices
• Maintain a map of the network, choose the best
  route for data
• Translates local addresses (eg. spot) into
  network addresses (e.g. 129.21.22.12)
• Can translate messages with different addressing
  methods
• Can act as firewalls (many ISPs block traffic at
  the router level)

37
Gateways

• Perform functions as high as layer 7
• Can connect very different networks, peform
  complex conversions
• Could be used to connect a packet-switching
  network to a LAN, convert e-mail from one format
  to another