Transcript TDC 463-98-501/502, Summer II 2002 2-1

Unit 2: Underlying Technologies
• Transmission media (Section 3.1. Read on your own)
• Local Area Networks (LANs)
– Ethernet (CSMA/CD - Carrier Sense Multiple Access/Collision Detection ,
IEEE 802.3)
– Token Ring (IEEE 802.5)
• Switching
– Circuit switching
– Packet switching
• Datagram approach
• Virtual circuit approach
• Wide Area Networks (WANs)
–
–
–
–
PPP (Point-to-Point Protocol)
X.25
Frame Relay
ATM (Asynchronous Transfer Mode)- cell relay
• Interconnecting devices – repeaters, bridges, routers and gateways
• Shared media v.s. switched LAN architecture
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IEEE 802 LAN Layers
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Ethernet LANs
Features
1) Widest Industry Use and Acceptance
a)
b)
c)
d)
Product Availability
Many Vendors
Low Cost
High Knowledge Base
2) Standardized for Multiple Media Types
a)
b)
c)
d)
Twisted Pair (10Base-T)
Optical Fiber (10Base-F, FOIRL)
Coaxial Cable (10Base2, 10Base5)
Also high-speed Ethernets
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Ethernet Problems
1) Coaxial Cable Networks Hard to Troubleshoot
a)
b)
c)
d)
e)
Faulty connections and electrical failures hard to find
Improper grounding can cause stray voltages
Static electricity
Non-standard hardware
Problems are often intermittent
2) Ethernet Lacks Built-In Network Monitoring
3) Ethernet Lacks Any Priority Mechanism
4) Station Transmission Time May Grow Large under High Loads
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Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
The basic idea:
When a station has a frame to transmit:
1) Listen for Data Transmission on Cable (Carrier Sense)
2) When Medium is Quiet (no other station transmitting):
a) Transmit Frame, Listening for Collision
b) If collision is heard, stop transmitting, wait random time, and transmit again.
Frame format
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This portion must be at least 64 bytes
for the Ethernet to work correctly
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Figure 3-9
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Figure 3-11
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Figure 3-12
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Figure 3-13
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Token Ring Features
1) Predictable Performance
a)
Unlike Ethernet, there is a fixed limit on how long a station must wait to
transmit frame.
b) Eight data priority levels ensure that important data get sent first.
2) Ring-of -Stars Topology
a) Star layout is well understood.
b) Ring is easily expanded by adding additional Multistation Access Units (MAUs)
c) Only point-to-point data connections used.
3) Self-Monitoring and Reconfiguration Capabilities
a) Active Monitor station recovers from any token operation problems.
b) If any station goes down it will be detected and removed from the ring.
c) Any single cable can be cut or disconnected and network will reconfigure and
continue operation.
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Token Ring Features (continued.)
4)
IBM Support
a) IBM SNA data and LAN data can travel together on same token ring.
b) Token Ring is an integral part of IBM future networking.
Disadvantages of Token Ring
1)
2)
Higher price for NICs
Limited support for non-IBM products.
a) Fewer products available for Token Ring than Ethernet
b) Ethernet is still at the heart of some vendors’ future network plans.
Note: For this course, you do not need to know the details of Token Ring frame format
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Figure 3-14 Token Ring Operation
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Figure 3-19 Circuit Switching
• Dedicated physical connections
• Source and destination operate at the same speed
• Data arrive in sequence
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Packet Switching
• Store-and-forward
• Source and destination may operate at different rates
Figure 3-20 Packet Switching- Datagram approach
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Packet Switching- Datagram approach
•Connectionless
•No connection setup necessary before sending data
•Each packet sent independently
•Each packet may take different path to destination
•Each packet contains complete destination address
•Packets may arrive out-of-order (transport layer must do
reordering)
•Network load is completely unpredictable
•Protocol Examples: IP, Novell IPX, AppleTalk
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Figure 3-21 Packet switching – Virtual circuit approach
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Packet switching – Virtual circuit approach
•Connection-oriented
•Sender sends a Setup Request packet to establish a virtual circuit before sending
data
•Setup Request passes through all router/switches on path from source to
destination
•Path is assigned a Virtual Circuit Identifier (VCID)
•Each router/switch stores information about each VC
•Any router/switch or destination may deny the setup request (like a busy signal).
•When finished, Sender sends Clear Request to tear down VC.
•Each data packet contains VCID, not full source and destination
addresses
•All packets follow same path and arrive in order
•Network load can be controlled through admissions control (denying
setup requests if busy)
•Protocol Examples: X.25, Frame Relay, ATM
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WAN
PPP
• commonly used for dial-up access to the internet (connect through
a phone line to the access router)
• can also be used in a point-to-point link between two devices such
as routers
Figure 3-22 Point-to-point protocol (PPP) frame
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Figure 3-23 X.25 (Connection-oriented)
X.25 - An interface protocol to
access the network
Not defined by X.25
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Figure 3-25
Hop-by-hop error and flow
control in X.25 is not necessary
for newer more reliable
networks.
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Figure 3-28 ATM cells (Small fixed-size data units)
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Figure 3-29
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Internetworking Terms
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Transparent Bridge
DA: Destination Address
SA: Source Address
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Shared Media Hub
Shared-Media vs.
Switched LAN
Architecture
only one 10Mbps
connection at a
time
shared, single
10Mbps LAN
segment
10 Mbps
client workstations
serv ers
"10 Mbps f or ALL"
Switch-Based LAN Architecture
multiple dedicated
10Mbps LAN
segments
Switching Hub
switching matrix
Multiple,
simultaneous 10Mbps
connections
All connections at 10Mbps
shared media hub
shared media hub
Workgroup with sharedserv ers with dedicatedWorkgroup with shared
connection
connections
connection
Workstations with
dedicated connections
"10 Mbps f or EACH"
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