Instrumentation and Timing EG30109

1
Instrumentation and TimingEG30109
Data Communication Part 1
Chris Carey
2
OSI 7 Layer Model
This graphic is taken from The Abdus Salam
International Centre for Theoretical Physics.
3
Layer 7 Application

• This layer supports end-user processes
• Communicating hosts are identified
• Quality of service is identified
• Authentication takes place here
• Syntax constraints are identified here
• Examples are e-mail, Telnet FTP

4
Layer 6 Presentation

• This layer provides independence from differences
  in data representation
• Formats and encrypts data to be sent across a
  network
• Provides freedom from compatibility problems
• Sometimes referred to as the syntax layer

5
Layer 5 Session

• This layer establishes, manages and terminates
  connections
• Deals with session and connection coordination.
• Sets up and coordinates information exchange
  between applications.

6
Layer 4 Transport

• This layer provides transparent transfer of data
  between end systems
• Responsible for end to end error recovery
• Implements flow control
• Ensures complete data transfer
• TCP operates here

7
Layer 3 Network

• This layer provides for switching and routing
• Creates logical paths known as virtual circuits
  for transmitting data between nodes
• Responsible for error handling, packet sequencing
  and congestion control
• IP operates here

8
Layer 2 Data Link

• At this layer data packets are encoded and
  decoded into bits
• Provides transmission protocol knowledge and
  management
• Handles errors, flow control and frame
  synchronisation in the physical layer
• Divided into two sub-layers

9
Layer 2 Data Link (contd)

• The Media Access Control (MAC) sub-layer controls
  how a host on the network gains access to the
  data and permission to transmit it.
• The Logical Link Control (LLC) sub-layer controls
  frame synchronisation, flow control and error
  checking

10
Layer 1 Physical

• This layer conveys the bit stream as electrical
  impulses, light or radio signal across the
  network
• Provides the hardware means of sending and
  receiving data over a carrier medium.
• Ethernet is an example of a physical layer
  protocol.

11
Connection Based Protocols

• Require a channel to remain open for the duration
  of the data exchange
• Incorporate flow control
• Packets are guaranteed to arrive in the correct
  order
• Examples include telephone networks and TCP.

12
Connectionless Protocols

• Do not require a channel to be opened
• Do not use flow control
• Have fewer overheads
• Delivery is not guaranteed
• Examples include the postal service, answer phone
  messages, UDP and IP

13
Error Detection and Correction

• Parity Checking
• Checksums
• Cyclic Redundancy Checks (CRCs)
• Hamming Code

14
Local Area Networks - LANs

• Usually contained within one organisation or
  small geographical area (E.g. a building or site)
• Fast data transmission rates
• Access is usually restricted to trusted members
  of the owner organisation
• Enables the sharing of IT resources between many
  users
• A user at a remote location can operate as part
  of a LAN using tunnelling or VPN technologies

15
Ethernet

• Wireless (hence Ether net) LAN architecture
  developed by Xerox, DEC Intel in mid 1970s
• Basis of the IEEE 802.3 standard
• Uses bus or star topology
• Uses CSMA/CD access method
• Data rate of up to 1Gbit/sec

16
Token Ring

• LAN architecture developed by IBM and has since
  been standardised as IEEE 802.5
• Uses a ring topology
• Uses CSMA/CA access method
• Used in applications where guaranteed delivery
  times are crucial, e.g. real-time control systems
• Data rate up to 16Mbit/sec

17
Bluetooth

• Short-range wireless LAN technology developed by
  leading IT and mobile communications companies
• Designed to connect electronic devices together
  automatically (PDAs, printers, cell phones,
  digital cameras, etc.)
• Effective range up to 10 metres
• Data rate up to 2Mbit/s using 2.45GHz band

18
Metropolitan Area Networks MANs

• No longer a commonly used term as many MANs are
  referred to as LANs or WANs
• Usually span a moderate geographical area (e.g. a
  town or city)
• Data transmission rates are generally slower than
  those of a LAN
• Examples include cable television networks, ADSL
  and the University of Bath

19
Wide Area Networks WANs

• Span large geographical areas
• Used to connect multiple LANs together
• Usually make use of third-party infrastructure
  (E.g. leased lines)
• Data transmission rates generally slower than
  LANs
• Examples include large corporations and of course
  THE internet.

20
Frames and Packets

• Data is packaged into discrete blocks known as
  packets
• A packet consists of a header and a body
• The packet header contains protocol specific
  information such as source and destination
  addresses, time to live and sequence number
• The data stream is often split and grouped into
  frames prior to transmission across a physical
  media

21
Switching and Routing

• Most modern digital communication networks use
  packet switching technology
• Packets are examined and then forwarded to their
  destination based on knowledge stored in the
  router in the form of routing tables
• Routing tables can be constructed automatically
  using dedicated protocols which search out other
  routers and hosts