Local Area Networks and Wide Area Networks

1
Local Area Networks and Wide Area Networks

• Computers in Business
• Foundation Year

2
Local Area Networks

• LANs
• generally privately-owned networks within a
  single building or campus of a few kilometres in
  size.
• Distinguished from other networks by 3
  characteristics
• their size
• transmission technology
• topology

3
LANs Characteristics

• Restricted in size
• worst case transmission time is bounded and known
  in advance.
• Transmission technology usually consists of a
  single cable
• Various topologies employed in designing LANs

4
LAN ApplicationsPC Networks

• Client/Server Communication
• Shared databases
• Shared hardware resources
• Shared Internet access
• Peer-to-Peer Communication
• Sharing work and information with colleagues
• Low cost is high priority
• Attachment costs in the hundreds of dollars

5
LAN Topologies Bus

• Multipoint medium
• Stations attach to linear medium (bus) using tap
• Full-duplex between station and tap
• Transmission from any stations travels entire
  medium (both directions)
• Termination required at ends of bus

6
Bus LAN Diagram
7
LAN Topologies Tree

• Generalization of bus topology
• Branching cable with no closed loops
• Cable(s) begin at headend, travel to branches
  which may have branches of their own
• Each transmission propagates through network, can
  be received by any station

8
Tree LAN Diagram
9
Bus/Tree Topology Problems

• How do you identify who the transmission is
  intended for?
• Data transmitted in frames
• Each frame has header with addressing info
• How do you regulate access?
• Stations take turns sending, by monitoring
  control information in frames

10
LAN Topologies Ring

• Repeaters are joined by unidirectional
  point-to-point links in a ring
• As a frame circulates past a receiver, the
  receiver checks its address, and copies those
  intended for it into a local buffer
• Frame circulates until it returns to source,
  which removes it from network

11
Ring LAN Diagram
12
LAN Topologies Star

• Each station connected directly to central node,
  usually with two undirectional links
• Central node can broadcast info, or can switch
  frames among stations

13
Star LAN Diagram
14
Choosing a Topology

• Factors to consider include reliability,
  flexibility/expandability, and performance
• Bus/tree is most flexible
• Tree topology easy to lay out
• Ring provides high throughput, but reliability
  problems
• Star can be high speed for short distances, but
  has limited expandability

15
Transmission Media Options

• Twisted pair--digital signaling
• Optical fiber--analog signaling
• Baseband coax--digital signaling
• Broadband coax--analog signaling
• Uses FDM to carry multiple channels
• Can be used over longer distances
• Inherently unidirectional, due to amplifier
  limitations

16
Selecting Transmission Media

• Capacity Can it support expected network
  traffic?
• Reliability Can it meet requirements for
  availability?
• Types of data supported Is it well-suited to the
  applications involved?
• Environmental scope Can it provide service in
  the environments required?

17
Ethernet(IEE 802.3)

• Bus-based broadcast network with decentralised
  control operating at 10 or 100 Mps.
• Computers on an Ethernet may transmit data
  whenever they want to.
• If two or more packets collide, each computer
  waits a random time and tries again.

18
Wide Area Networks (WANs)

• WAN span a large geographic area, often a country
  or a continent.
• Collection of host computers connected by a
  communication subnet.
• WANs consist of two distinct components
• transmission lines (aka circuits, channels or
  trunks)
• switching elements

19
Switching elements

• Specialised computers used to connect two or more
  transmission lines.
• When data arrives on an incoming transmission
  line, the switching element selects an outgoing
  line to forward them on.
• Routers, aka packet switching nodes or data
  switching exchanges

20
Relation between hosts and subnet
Router
Subnet
Host
LAN
21
WANS communication

• Most WANs consist of cables or telephone lines
  connecting a pair of routers.
• If two routers that do not share a connecting
  cable wish to communicate with each other they do
  so through other intermediate routers.
• Subnets using this principle called
  packet-switching subnet (or point-to-point).

22
WANS topology

• LANs have a symmetric topology, WANs typically
  have irregular topologies.

Star
Tree
23
LANs and WANs
24
Example networks

• Novell Netware
• Most popular network system in the PC world.
• Based on the Client/Server Model
• Netware used its own proprietary protocol stack
  based on Xerox Network System XNS.

25
The Novell NetWare reference model
Layer
Application
Transport
Network
Data link
Physical
26
The Internet (history)

• Predated by the ARPAnet
• mid 1960s US Department of Defense created a
  command and control network that could survive a
  nuclear war
• Advanced Research Projects Agency (ARPA) designed
  a packet-switching network consisting of a subnet
  and host computers.
• 1974 development of TCP/IP model facilitated the
  growth of the ARPAnet, Universities connect to
  the network (TCP/IP established as the official
  internet protocol in 1983)
• 1980s ARPAnet continued to grow, shut down in 1990

27
The Internet

• Collection of existing networks
• ARPAnet, NSFNET, SPAN (NASAs network), HEPNET,
  BITNET, EARN etc.
• all based on the TCP/IP protocol stack
• When is a computer on the Internet?
• Runs the TCP/IP protocol stack
• has an IP address
• has the ability to send packets to all the other
  machines on the Internet.

28
TCP/IP

• Transport layer protocols, basis of the Internet.
• Transmission Control Protocol/Internet Protocol
• Allows a byte stream originating on one machine
  to be delivered without error to another machine
  on the Internet.

29
Application Layer

• Layer on top of the transport layer, containing
  all the higher-level protocols
• Protocols include
• TELNET - virtual terminal
• FTP - file transfer protocol
• SMTP - simple mail transfer protocol
• DNS - Domain name server
• NNTP - network news transfer protocol
• HTTP - Hyper-text transfer protocol