CS412 Introduction to
Computer Networking &
Telecommunication
Local Area Networks
Chi-Cheng Lin, Winona State University
Topics

LANs - IEEE Project 802

Ethernet

Data Link Layer Switching
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Figure 12-1
LAN Compared with the OSI Model
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Figure 12-2
Project 802
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Ethernet

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Ethernet Cabling
Manchester Encoding
Ethernet MAC Sublayer Protocol
Binary Exponential Backoff Algorithm
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
Retrospective on Ethernet
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802.3 and Ethernet

802.3
1-Persistent CSMA/CD LAN, 1 - 10 Mbps


Ethernet
A specific product that almost implements
802.3
XBaseY
Cabling (baseband)
10Base5 (thick Ethernet)
10Base2 (thin Ethernet)
10Base-T (twisted pair)
10Base-F (fiber optics)
Channel
capacity
Cable type
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Ethernet Cabling

The most common kinds of Ethernet
cabling.
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Ethernet Cabling (2)

Three kinds of Ethernet cabling.
 (a) 10Base5, (b) 10Base2, (c) 10Base-T.
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Ethernet Cabling (3)

Cable topologies.
 (a) Linear (b) Spine (c) Tree (d) Segmented
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Manchester Encoding

Why Manchester encoding?
Differentiating 0 bit or idle
Synchronization

Encoding scheme:
Each bit period is divided into 2 equal
intervals
Each bit period has a transition in the
middle
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Manchester Encoding

Manchester encoding
Bit 1: high-low, bit 0: low-high

Differential Manchester encoding
Bit 1: no transition at the start of interval
Bit 0: transition at the start of interval
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Ethernet MAC Sublayer Protocol

Frame formats.
 (a) DIX Ethernet, (b) IEEE 802.3.


Preamble: 10101010 for synchronization
Start of frame: 10101011
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Ethernet MAC Sublayer Protocol

Addresses
Ethernet uses 6 bytes
Support
Unicast: address begins with 0
Multicasting: 1 + group number
Broadcasting: all 1’s
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Figure 12-6
Collision in CSMA/CD
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Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2 .
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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
As the network speed goes up
 minimum frame length must go up or
maximum cable length must come down
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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 = 1024, for i = 10,...,16
when i > 16, reset i = 0
Low delay for light load
 Reasonable delay for high load

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Switched Ethernet

A simple example of switched Ethernet
 If all ports on a card wired together, each
card becomes an on-card LAN and forms
one collision domain.
 If buffer used, one port is a collision
domain and no collision will occur.
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Figure 12-14
An Ethernet Network Using A Hub
One collision domain
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Figure 12-15
An Ethernet Network Using a Switch
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Fast Ethernet

The original fast Ethernet cabling.
100Base-T: hubs and switches
100Base-F: switches only, with one cable one collision domain
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Gigabit Ethernet

Configurations
 (a) A two-station Ethernet.
 (b) A multistation Ethernet.
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Gigabit Ethernet (2)

Gigabit Ethernet cabling.
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IEEE 802.2: Logical Link Control

LLC
 (a) Position of LLC. (b) Protocol formats.
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Logical Link Control
LLC forms the upper half of data link
layer (MAC is below LLC)
 Purposes

Provides error control and flow control
Hides differences between 802 networks
by providing a single format and interface
to network layer

Services
Unreliable datagram
Acknowledged datagram
Reliable connection-oriented service
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Logical Link Control

Sender
Network layer passes packet to LLC using LLC
access primitives
LLC sublayer adds LLC header
Source and destination access points
Control: sequence and acknowledgement numbers
802.x frame payload field =
(LLC header + packet)
Frame is transmitted

Receiver
Reversed process
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Restrospective 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
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Data Link Layer Switching





Bridges from 802.x to 802.y
Local Internetworking
Spanning Tree Bridges
Remote Bridges
Interconnection Devices
 Repeaters, hubs, bridges, switches, cutthrough switches, routers, gateways
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Data Link Layer Switching
Multiple LANs connected by a backbone
to handle a total load higher than the
capacity of a single LAN.
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Bridges from 802.x to 802.y

Operation of a LAN bridge from
802.11 to 802.3.
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Bridges from 802.x to 802.y (2)

General Problems
Different data formats
Different data rates
Different maximum frame length
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Local Internetworking

A configuration with four LANs and
two bridges.
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Transparent 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
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Transparent 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
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Figure 16.6
Learning bridge
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Spanning Tree Bridges

To increase reliability
Two or more bridges between 2 LANs

Problem: looping
F4
F3
with unknown destination
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Spanning Tree Bridges

Solution to looping: Spanning tree bridges
LAN  vertex
Bridge  edge(s)
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Remote Bridges
Connects LANs at remote sites
 Approach

Putting bridges on each LAN
Connecting bridges point-to-point
Point-to-point link considered as a “hostless” LAN
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Interconnection Devices


Repeaters, hubs, bridges, switches,
cut-through switches, routers, gateway
Issues: bandwidth and collision domain
(a) Which device is in which layer.
(b) Frames, packets, and headers.
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Interconnection Devices

(a) A hub. (b) A bridge. (c) a switch.
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Figure 16.2
Repeater
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Figure 16.3
Function of a repeater
A repeater is not an amplifier – an
amplifier does not regenerate signals.
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Figure 16.4
Hubs
A hub is a multiport repeater.
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Figure 16.5
Bridge
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Figure 14.16 A network with and without a bridge
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Figure 14.17
Collision domains in a nonbridged and bridged network
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Figure 21-16
Switch
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Figure 14.18
Switched Ethernet
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Cut-Through Switch
As soon as the destination header field
has been received, the frame can be
forwarded.
 Faster (shorter delay)
 No more store-and-forward?
 Bad frames propagation

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Figure 21-10
A Router in the OSI Model
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