Basic Communications Model
Standards are needed at all Layers
User Layer
Application Layer
Computer (Transport) Layer
Transmission Layer
1
Subnet Layer
Station B
Station A
2
2
1
1
4
3
Transmission of Messages
1. Within a Single Subnet, or
2. in Point-to-Point Links Between Subnets
2
Internet Layer
Station B
Station A
2
1
4
3
Routing of Messages
Across multiple subnets in an internet
3
Internetting
Station B
Station A
2
1
4
3
internet: collection of subnets such that any station
on any subnet can communicate with any other
station on any other subnet simply by giving the
4
internet address of the other station.
Layering in Major Architectures
Basic Model
TCP/IP
IPX/SPX
OSI
Application (7)
Application
Application
Application
Presentation (6)
Computer
(Transport)
Transport:
TCP, etc.
Complex:
SPX, etc.
Internet
Internet
Internet
Subnet
(OSI)
Subnet
(OSI)
Subnet
(OSI)
Session (5)
Transport (4)
Network (3):
Internet &
Subnet
Data Link (2)
Physical (1)
5
OSI Architecture
 Reference Model for Open Systems
Interconnection (OSI)
 Standards Agencies
• ISO (computers)
• Don’t confuse OSI and ISO!
• ITU-T (telecommunications)
 De jure standards (official/open)
6
OSI Architecture
 Standards are sophisticated
 Sophistication brings complexity
• Slow standards development
• Products are late to market and expensive
• The result: low acceptance of OSI standards
in general
 But OSI dominates at the single network
(subnet) transmission layer
7
Physical Layer (OSI Layer 1)
 Physical transmission standards
 Physical
• Connector
• Transmission media
 Electrical
• Voltage levels
• Meanings of various voltage levels
8
The Data Link Layer (OSI Layer 2)
9
Data Link Layer (OSI Layer 2)

Organize data for transmission over a data link
• Data packages at various layers are called protocol
data units (PDUs)
• The Data Link Layer PDU is called a frame
• Organization allows addressing on shared data links
• Organization allows error checking and other meta
delivery processes

Media Access Control
• Controls when stations may transmit in shared media
LANs and other shared data links
10
TCP/IP Architecture
 Internet Engineering Task Force (IETF)
• Of the Internet Society
 De jure (official/open)
11
TCP/IP Architecture
 Standards are simple
• Often have “Simple” in the name
• Standards and products are fast to market
• Products are inexpensive and fast to market
 Widely Used
• On the Internet
• In many corporate networks
• Dominates at internet and transport
layers
12
IPX/SPX
 Novell Corporation
 De facto (proprietary/closed)
 Used only in Novell NetWare PC networks
• But NetWare dominates PC networking
• Second most widely used architecture at
transport layer
13
SNA
 Systems Network Architecture
 IBM Corporation
 De facto (proprietary/closed)
 Dominates in mainframe communication
• On IBM mainframe systems
• On mainframe systems of other vendors
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Transport-Layer Traffic
TCP/IP
 IPX/SPX (NetWare)
 SNA (Mainframes)
 Other (including OSI)
 Total

38%
34%
18%
10%
100%
Rising
Falling
Source: Molloy (1994), 1993 data
15
Transmission Speeds
 Bit:
single 1 or 0
 Transmission speed is measured in bits
per second (bps)
• Duration of a single bit
• Not velocity of propagation
101101000011101010100101111010101
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Notations for Transmission Speed
Prefix
Use
Metric Meaning
Example
Interpretation
kilo
kbps
1,000
64 kbps
64,000 bps
Mega
Mbps
1 million
1.544 Mbps
1,544,000 bps
Giga
Gbps
1billion
2.4 Gbps
2,400,000,000 bps
Tera
Tbps
1 trillion
2 Tbps
2,000,000,000,000
Note upper case T
17
LANs, MANs, & WANs
 LAN
•
•
•
•
Local area network
Single office, building, campus
10 Mbps to 100 Mbps to the desktop common
1 Gbps coming
 Will carry most traffic, because most traffic
is local
18
PC Networking on a small LAN
Each client and server needs a NIC
rather than a modem
Network Interface
Card (NIC)
In each PC
19
PC Networking on a small LAN
Wiring Hub
15 to 50 cm (6 to 19 inches)
Telephone wiring
and RJ-45 plugs
Usually, a wiring hub
connects the computers
together
20
LANs, MANs, & WANs
 WAN
•
•
•
•
Wide area network
Intercity, international
9,600 bps to 1 Mbps common to the desktop
Links with higher speed are usually shared
(multiplexed) by several desktops
 Emerged before LANs, due to high cost of
long-distance telephone charges
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