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15-441 Computer Networks

Ethernet I

Professor Hui Zhang

hzhang@cs.cmu.edu

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Aloha Network

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Original Ethernet

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High Level View

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 Tightly coordinate shared access to avoid collisions

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R. No a priori coordination among nodes

Two or more transmitting nodes -> “collision”, Random access MAC protocol specifies:

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 18% maximum utilization

Slotted Aloha: more efficient. Reduces chances of collision

37% maximum utilization

CentralComputer

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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

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CSMA/CD

Broadcast technology

Carrier-sense multiple access with collision detection (CSMA/CD). MA = multiple access

CS = carrier sense

CD = collision detection

host host host host

host host host host

hub

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CSMA/CD Algorithm

Sense for carrier. If carrier present, wait until carrier ends.

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 DetectionTime

A B C

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Collision Detection: Implications

All nodes must be able to detect the collision.

Any node can be sender

The implication is that either we must have a short wires, or long packets.

Or a combination of both

Can calculate length/distance based on transmission rate and propagation speed.

Messy: propagation speed is media-dependent, low-level protocol details, ..

Minimum packet size is 64 bytes

– Cable length ~256 bit times Example: maximum coax cable length is

2.5 km

A B C

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CSMA/CD: Some Details

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

Preamble marks the beginning of the frame. 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.

Preamble Type PadDest Source Data CRC

8 6 6 2 4

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Minimum Packet Size

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)

propagation delay (d)a) Time = t; Host 1 starts to send frame

Host 1 Host 2

propagation delay (d)

Host 1 Host 2b) Time = t + d; Host 2 starts to send a frame just before it hears fromhost 1’s frame

propagation delay (d)

Host 1 Host 2c) Time = t + 2*d; Host 1 hears Host 2’s frame detects collision

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

10-BaseT uses twisted pair and hubs.

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

host host host host

Hub

Host

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Ethernet Technologies: 10Base2

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

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. Hubs and switches only

4B/5B encoding

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Gbit Ethernet

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. 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

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

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. This is what makes it Ethernet

Full duplex point-to-point links in the backbone are likely the most common use.

Added flow control to deal with congestion

Choice of a range of fiber and copper transmission media.

Defining “jumbo frames” for higher efficiency.