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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 Acceptancea)   Product Availability

b)   Many Vendors

c)   Low Cost

d)   High Knowledge Base

2)   Standardized for Multiple Media Typesa)   Twisted Pair (10Base-T)

b)   Optical Fiber (10Base-F, FOIRL)

c)   Coaxial Cable (10Base2, 10Base5)

d) Also high-speed Ethernets

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Ethernet Problems

1)   Coaxial Cable Networks Hard to Troubleshoota)   Faulty connections and electrical failures hard to find

b)   Improper grounding can cause stray voltages

c)   Static electricity

d)   Non-standard hardware

e)   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

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 Performancea)    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 Topologya)    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 Ring1)    Higher price for NICs

2)    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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Figure 3-20 Packet Switching- Datagram approach

Packet Switching• Store-and-forward

• Source and destination may operate at different rates

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•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

Packet Switching- Datagram approach

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Figure 3-21 Packet switching – Virtual circuit approach

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•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

Packet switching – Virtual circuit approach

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Figure 3-22 Point-to-point protocol (PPP) frame

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

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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

TDC 463-98-501/502, Summer II 2002 2-27GOLDMAN: LAN FIG. 08-02

serversclient workstations

Shared Media Hub

shared, single 10Mbps LAN

segment

only one 10Mbps connection at a

time

Shared Media LAN Architecture

10 Mbps

"10 Mbps for ALL"

Switching Hubmultiple dedicated

10Mbps LAN segments

Workgroup with shared connection

Workgroup with shared connection

servers with dedicated connections

shared media hubshared media hub

Workstations with dedicated connections

Multiple, simultaneous 10Mbps

connections

All connections at 10Mbps

switching matrix

Switch-Based LAN Architecture

"10 Mbps for EACH"

Shared-Media vs. Switched LAN Architecture