CS 313 Introduction to
Computer Networking &
Telecommunication
Local Area Networks
Chi-Cheng Lin, Winona State University
Topics

Ethernet

Data Link Layer Switching
2
802.3 and Ethernet

802.3
1-Persistent CSMA/CD LAN, 1 - 10 Mbps

Ethernet
A specific product that almost implements
802.3
3
Classic Ethernet Physical Layer
Architecture of classic Ethernet
Classic MAC Sublayer Protocol

Frame formats.
 (a) DIX Ethernet, (b) IEEE 802.3


Preamble: 10101010 for synchronization
Start of frame: 10101011
Ethernet MAC Sublayer Protocol

Addresses
Ethernet uses 6 bytes
Support
Unicast: address begins with 0
Multicasting: 1 + group number
Broadcasting: all 1’s
6
Classic MAC Sublayer Protocol
Collision detection can take as long as 2.
Ethernet MAC Sublayer Protocol

Minimum frame size: 64 bytes
Why?
frame_size bits/channel_capacity bps > 2 s
In 10-Mbps Ethernet, 2 = 50 s, therefore
frame_size > 50 s x 10 Mbps = 500 bits,
rounded up to 512 bits = 64 bytes
8
Binary Exponential Backoff Algorithm

Wait time t time slots after a collision
t = a random number between 0 and 2i - 1
after i collisions
t = 1023, for i = 10,...,16
when i > 16, reset i = 0
Low delay for light load
 Reasonable delay for high load

9
Switched Ethernet
(a) Hub. (b) Switch.
Collision Domain?
Switched Ethernet
Switch
Hub
Switch ports
Twisted pair
An Ethernet switch.
Fast Ethernet
The original fast Ethernet cabling.
Xbase-Y
Channel
capacity
Cable type
Gigabit Ethernet
A two-station Ethernet
Gigabit Ethernet
A multistation Ethernet
Gigabit Ethernet
Gigabit Ethernet cabling.
10 Gigabit Ethernet
10-Gigabit Ethernet cabling
Retrospective on Ethernet
Has been 20+ years
 Simple and flexible

Reliable
Cheap
Easy to maintain

Works easily with TCP/IP
Both IP and Ethernet are connectionless

Evolution – no software change required
Speed: higher and higher
Hubs, switches
17
Data Link Layer Switching
•
•
•
•
Uses of bridges
Learning bridges
Spanning tree bridges
Repeaters, hubs, bridges, switches,
routers, and gateways
Learning Bridges
Bridge connecting two multidrop LANs
Learning Bridges
Bridges (and a hub) connecting seven point-topoint stations.
Learning Bridges

Transparency
Plug and play

Operates in Promiscuous Mode
Accepting every frame transmitted on all
LANs to which it is attached

Decides
Discard or forward
If forward, to which LAN?
Look up a huge destination address hash table
21
Learning Bridges

Hash Table
Initially empty
Flooding algorithm
Backward learning algorithm
Arrival time noted for dynamic topology
Scanned periodically to remove old entries

Routing procedure for an incoming frame
If dest LAN = src LAN then discard
If dest LAN != src LAN then forward
If dest LAN unknown then use flooding
22
Learning Bridges
Protocol processing at a bridge.
Spanning Tree Bridges

To increase reliability
Two or more bridges between 2 LANs

Problem: looping
Bridges with two parallel links
Spanning Tree Bridges (2)
A spanning tree connecting five bridges. The dotted lines
are links that are not part of the spanning tree.
Repeaters, Hubs, Bridges, Switches,
Routers, and Gateways
(a) Which device is in which layer. (b) Frames, packets, and headers.
Discussions:
• Collision domain?
• Plug-and-play?