15-441 Computer Networks
Ethernet I
Professor Hui Zhang
hzhang@cs.cmu.edu
1
Aloha Network
2
Hui Zhang
Original Ethernet
3
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High Level View
4

Goal: share a communication medium among
multiple hosts connected to it

Problem: arbitrate between connected hosts

Solution goals:

High resource utilization

Avoid starvation

Simplicity (non-decentralized algorithms)
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Medium Access Protocols



Channel partitioning

Divide channel into smaller “pieces” (e.g., time slots,
frequency)

Allocate a piece to node for exclusive use
Random access

Allow collisions

“recover” from collisions
Taking-turns

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Tightly coordinate shared access to avoid collisions
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Random Access Protocols


Transmit at full channel data rate R.

No a priori coordination among nodes

Two or more transmitting nodes -> “collision”,

Random access MAC protocol specifies:

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When node has packet to send

How to detect collisions

How to recover from collisions
Examples of random access MAC protocols:

Aloha

Slotted ALOHA

CSMA and CSMA/CD
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Aloha

Nodes sends the message when
it has data to send.

If it receives an ack, it considers
the transmission completed,
otherwise it retransmits after a
random delay.

Simple, distributed protocol, but
not very efficient


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Central
Computer
18% maximum utilization
Slotted Aloha: more efficient.

Reduces chances of collision

37% maximum utilization
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Slotted Aloha

Time is divided into equal size slots (= packet transmission
time)

Node with new arriving pkt: transmit at beginning of next slot

If collision: retransmit pkt in future slots with probability p, until
successful.
Success (S), Collision (C), Empty (E) slots
9
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CSMA/CD
Broadcast technology
host
host
host
host
host
host
host
host
hub

10
Carrier-sense multiple access with collision
detection (CSMA/CD).

MA = multiple access

CS = carrier sense

CD = collision detection
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CSMA/CD Algorithm

Sense for carrier.

If carrier present, wait until carrier ends.

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Sending would force a collision and waste time

Send packet and sense for collision.

If no collision detected, consider packet delivered.

Otherwise, abort immediately, perform “exponential back
off” and send packet again.

Start to send at a random time picked from an interval

Length of the interval increases with every retransmission
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Collision Detection
B
C
Time
A
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Collision Detection:
Implications

All nodes must be able to detect
the collision.


B
C
Any node can be sender
The implication is that either we
must have a short wires, or long
packets.


A
Or a combination of both
Can calculate length/distance
based on transmission rate and
propagation speed.

Messy: propagation speed is mediadependent, low-level protocol details,
..

Minimum packet size is 64 bytes
– Cable length ~256 bit times

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Example: maximum coax cable length
is 2.5 km
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CSMA/CD: Some Details


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When a sender detects a collision, it sends a
“jam signal”.

Make sure that all nodes are aware of the collision

Length of the jam signal is 32 bit times
Exponential backoff operates in multiples of 512
bit times.

Longer than a roundtrip time

Guarantees that nodes that back off longer will notice the
earlier retransmission before starting to send
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Ethernet Frame Format
8
6
6
2
Preamble
Dest
Source
Type



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Data
Pad
CRC
Preamble marks the beginning of the frame.


4
Also provides clock synchronization
Source and destination are 48 bit IEEE MAC addresses.

Flat address space

Hardwired into the network interface
Type field is a demultiplexing field.

What network layer (layer 3) should receive this packet?

Is actually a length field in the 802.3 standard
CRC for error checking.
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Minimum Packet Size
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
Why put a minimum packet size?

Give a host enough time to detect collisions

In Ethernet, minimum packet size = 64 bytes (two
6-byte addresses, 2-byte type, 4-byte CRC, and 46
bytes of data)

If host has less than 46 bytes to send, the adaptor
pads (adds) bytes to make it 46 bytes

What is the relationship between minimum packet
size and the length of the LAN?
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Minimum Packet Size (more)
Host 1
a) Time = t; Host 1
starts to send frame
Host 2
propagation delay (d)
Host 1
b) Time = t + d; Host 2
starts to send a frame
just before it hears from
host 1’s frame
c) Time = t + 2*d; Host 1
hears Host 2’s frame 
detects collision
Host 2
propagation delay (d)
Host 1
Host 2
propagation delay (d)
LAN length = (min_frame_size)*(light_speed)/(2*bandwidth) =
= (8*64b)*(2*108mps)/(2*107 bps) = 5.12 km
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Ethernet Physical Layer


10Base2 standard based on
thin coax.

Thick coax no longer used

Nodes are connected using
thin coax cables and “T”
connectors in a bus topology
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host
host
host
Host
10-BaseT uses twisted pair
and hubs.


host
Hub acts as a concentrator
The two designs have the
same protocol properties.

Key: electrical connectivity
between all nodes

Deployment is different
host
host
host
host
Hub
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Ethernet Technologies: 10Base2
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
10: 10Mbps; 2: under 200 meters max cable length

Thin coaxial cable in a bus topology

Repeaters used to connect up to multiple segments

Repeater repeats bits it hears on one interface to its other
interfaces: physical layer device only!
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10BaseT and 100BaseT
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
10/100 Mbps rate; later called “fast ethernet”

T stands for Twisted Pair

Hub to which nodes are connected by twisted pair, thus
“star topology”
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802.3u Fast Ethernet

Apply original CSMA/CD medium access protocol at
100Mbps

Must change either minimum frame or maximum
diameter: change diameter

Requires


2 UTP5 pairs (4B5B) or

4 UTP3 pairs (8B6T) or

1 fiber pair
No more “shared wire” connectivity.


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Hubs and switches only
4B/5B encoding
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Gbit Ethernet
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
Use standard Ethernet frame format

Allows for point-to-point links and shared broadcast
channels

In shared mode, CSMA/CD is used; short distances
between nodes to be efficient

Uses hubs, called here “Buffered Distributors”

Full-Duplex at 1 Gbps for point-to-point links
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Traditional IEEE 802 Networks:
MAC in the LAN and MAN

Ethernet defined as IEEE 802.3.


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Not quite identical
The IEEE 802.* set of standards defines a common
framing and addressing format for LAN protocols.

Simplifies interoperability

Addresses are 48 bit strings, with no structure

802.3 (Ethernet)

802.5 (Token ring)

802.X (Token bus)

802.6 (Distributed queue dual bus)

802.11 (Wireless)
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LAN Properties

Exploit physical proximity.

Typically there is a limitation on the physical distance between the
nodes, for example,
– to collect collisions in a contention based network
– to limit the overhead introduced by token passing or slot
reservations

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Relies on single administrative control and some level of
trust.

Broadcasting packets to everybody and hoping everybody (other
than the receiver) will ignore the packet

Token passing protocols assume everybody plays by the rules
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Why Ethernet?


Easy to manage.

You plug in the host and it basically works

No configuration at the datalink layer
Broadcast-based.

In part explains the easy management

Some of the LAN protocols (e.g. ARP) rely on broadcast
– Networking would be harder without ARP

Not having natural broadcast capabilities adds complexity to a LAN
– Example: ATM

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Drawbacks.

Broadcast-based: limits bandwidth since each packets consumes the
bandwidth of the entire network

Distance
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802.3z Gigabit Ethernet

Same frame format and size as Ethernet.


Full duplex point-to-point links in the
backbone are likely the most common use.

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This is what makes it Ethernet
Added flow control to deal with congestion

Choice of a range of fiber and copper
transmission media.

Defining “jumbo frames” for higher efficiency.
Hui Zhang