Exploration Network Chapter 9 (Rev 1.0)

7/29/2019 Exploration Network Chapter 9 (Rev 1.0)

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2007 Cisco Systems, Inc. All rights reserved. Cisco Public

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Chapter 1 1

Ethernet

Network Fundamentals

Chapter 9


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Chapter 1 2 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Ethernet - Standards and Implementation


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Physical Layer Limitations


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LLC Connecting the Upper Layers

Can be considered the driver software for theNIC


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Media Access Control

Lower sublayer of the Data Link layer; implemented byhardware


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Physical Implementations of Ethernet


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Historic Ethernet

The first version of Ethernet is known as Carrier SenseMultiple Access with Collision Detection (CSMA/CD).


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Legacy Ethernet

Technology MediumPhysical

Topology

Max. Segment

Length

10 BASE 5Thick coax

(Thicknet) Bus 500 m

10 BASE 2Thin coax

(Thinnet)Bus 200 m (185 m)

10 BASE TTwisted

pair

Star 100 m


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Ethernet Collision Management

Legacy Ethernet Current Ethernet

central point of the network

segment was a hub

switches replaced hubs in

Ethernet-based networks

half-duplex communication full-duplex communications

As the number of devices and data

traffic increase, the rise in

collisions can have a significant

impact on the user's experience.

The switch reduces or minimizes

the possibility of collisions.


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Moving to Gigabit Ethernet

New networking services require higherbandwidth LANs

Upgrading to Gigabit Ethernet does not always

mean that the existing network infrastructurehas to be completely replaced

Ethernet can now be applied across a city inwhat is known as a Metropolitan Area Network

(MAN).


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The Frame Encapsulating the Packet

The differences between framing styles are minimal.


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Ethernet Frame Size

Original Ethernet frame: minimum - 64 bytes;maximum -1518 bytes

The IEEE 802.3ac standard (1998), extended

the maximum allowable frame size to 1522bytes. to accommodate a technology calledVirtual Local Area Network (VLAN).

If the size of a transmitted frame is less than

the minimum or greater than the maximum, thereceiving device drops the frame.


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Addressing in Ethernet


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Ethernet MAC Address Structure


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Viewing the MAC Address


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Addressing Layers L2 and L3


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Ethernet Unicast, Broadcast and Multicast

Unicast IP and MAC destination addresses areused by the source to forward a packet.

Broadcast - all hosts on that local network willreceive and process the packet; broadcast MAC is

FF-FF-FF-FF-FF-FF Multicast IP and MAC destination addresses

deliver packets/frames to a specific group of hosts;Multicast MAC address is a special value that

begins with 01-00-5E in hexadecimal; value endsby converting the lower 23 bits of the IP multicastaddress into the remaining 6 hexadecimalcharacters of the Ethernet address. The remainingbit in the MAC address is always a "0".


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Media Access Control in Ethernet In a shared media environment, all devices have guaranteed

access to the medium, but they have no prioritized claim on it.


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CSMA/CD The Process

All network devices that have messages to send mustlisten before transmitting.

If a device detects a signal, it will wait some of timebefore attempting to transmit.

When there is no traffic detected, a device will transmit

its message.

If two devices transmit signals at the same time, thesignals mix and the message becomes corrupted.

Once detected, every device transmitting will continue

to transmit to ensure that all devices detect the collisionand send out a jamming signal, invoke a backoffalgorithm.

After the delay has expired on a device, the devicegoes back into the "listening before transmit" mode.


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Hubs and Collision Domain

Hubs and repeaters therefore have the effect of increasing the sizeof the collision domain.


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Drill:

How many collision domains and broadcast domains are seen?


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Drill:



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Drill:



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Ethernet Delay (Latency)

As hubs or repeaters are added, the accumulated delay increasesthe likelihood that collisions will occur.


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Timing and Synchronization

10 Mbps Ethernet uses the first 64 bits of theframe preamble to synchronize the receiver. Ituses asynchronous communication.

Ethernet implementations with throughput of100 Mbps and higher are synchronous.Synchronous communication means that timinginformation is not required.


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Ethernet Bit and Slot Time

Determining slot time is a trade-off between the need to reduce theimpact of collision recovery (backoff and retransmission times) andthe need for network distances to be large enough toaccommodate reasonable network sizes.


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Interframe Spacing

- time measured from the last bit of the FCS field of one frame to thefirst bit of the Preamble of the next frame


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Jam Signal

As soon as a collision is detected, sendingdevices transmit a 32-bit "jam" signal that willenforce the collision. This ensures all devices inthe LAN to detect the collision.

The jam signal is not detected as a valid frame;otherwise the collision would not be identified.The most commonly observed data pattern for

a jam signal is simply a repeating 1, 0, 1, 0pattern, the same as the Preamble.


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Backoff TimingAfter a collision occurs and all devices allow thecable to become idle, the devices whosetransmissions collided must wait an additional,longer period of time before attempting toretransmit the collided frame.

Intentionally designed to be random so that twostations do not delay for the same amount oftime before retransmitting

If media congestion results in the MAC layerunable to send the frame after 16 attempts, itgives up and generates an error to the Networklayer.


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Overview of Ethernet Physical Layer

10 Mbps - 10Base-T Ethernet 100 Mbps Fast Ethernet1000 Mbps - Gigabit Ethernet 10 Gbps - 10 Gb Ethernet


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10 BASE-T RJ-45 Pinouts


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Implementations of Fast Ethernet

100BASE-TX using Cat5 or later UTP (electrical)

100BASE-FX using fiber-optic cable (optical)

Both uses the 4B/5B as encoding technique.


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Gigabit Ethernet 1000 BASE-T

Gigabit Ethernet uses two separate encoding steps. Encoding thedata enables synchronization, efficient usage of bandwidth, andimproved signal-to-noise ratio characteristics.

1000BASE-T Ethernet provides full-duplex transmission using allfour pairs in Category 5 or later UTP cable.

1000BASE-T allows the transmission and reception of data in bothdirections, creating permanent collisions on the wire pairs. Thehybrid circuits detecting the signals use sophisticated techniquessuch as echo cancellation, Layer 1 Forward Error Correction(FEC), and prudent selection of voltage levels.

1000BASE-T uses many voltage levels. In idle periods, ninevoltage levels are found on the cable. During data transmissionperiods, up to 17 voltage levels are found on the cable.


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Gigabit Ethernet 1000BASE-SX and1000BASE-LX Ethernet Using Fiber-Optics

The fiber versions of Gigabit Ethernet - 1000BASE-SX and1000BASE-LX - offer the following advantages over UTP: noiseimmunity, small physical size, and increased unrepeated distancesand bandwidth.

The principal differences among the 1000BASE-SX and

1000BASE-LX fiber versions are the link media, connectors, andwavelength of the optical signal.


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Legacy Ethernet Using Hubs


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Ethernet Using Switches

- Switches allow the segmentation of the LAN into separate collisiondomains.


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Features of Switched-based LANs


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Switches Selective Forwarding

Can be thought of as establishing a momentary point-to-pointconnection between the transmitting and receiving node, longenough to forward a frame.

Store and forward switching - switch receives the entire frame,checks the FCS for errors, and forwards the frame to the

appropriate port for the destination node.

The switch maintains a MAC table that matches a destination MACaddress with the port used to connect to a node. For eachincoming frame, the destination MAC address is compared to thelist of addresses in the MAC table. If a match is found, the port

number in the table that is paired with the MAC address is used asthe exit port for the frame.

The terms switching and bridgingmay both be used in networking.


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Basic Switch Operation


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Switching Operation Master this Activity...


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Address Resolution Protocol (ARP)

The ARP protocol provides two basic functions:

Resolving IPv4 addresses to MAC addresses

Maintaining a cache of mappings

Each entry of the ARP table has a pair of values: an IPAddress and a MAC address.

To begin the process, a transmitting node attempts tolocate in the ARP table the MAC address mapped to an

IPv4 destination. If this map is cached in the table, thenode uses the MAC address as the destination MAC inthe frame that encapsulates the IPv4 packet.


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Maintaining the ARP Table

The ARP table is maintained dynamically.

As a node receives frames from the media, it can record thesource IP and MAC address as a mapping in the ARP table.

Another way a device can get an address pair is to broadcastan ARP request.

The dynamic entries in the ARP table are timestamped, same waythat MAC table entries are timestamped in switches.

Static map entries can be entered in an ARP table.

If an entry is not found on the ARP table, the processes send outan ARP request packet to discover the MAC address of thedestination device on the local network. The recipient sends an

ARP reply.

If no device responds to the ARP request, the packet is dropped.


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ARP Process Destinations Outside the LAN

If the destination IPv4 host is on the local network, theframe will use the MAC address of this device as thedestination MAC address.

If the destination host is not on the local network, the

source node needs to deliver the frame to the routerinterface (gateway) to reach that destination.

In the event that the gateway entry is not in the table,the normal ARP process will send an ARP request to

retrieve the MAC address associated with the IPaddress of the router interface.

For each device, an ARP cache timer removes ARPentries that have not been used for a specified period of

time.


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Drill:

Host A needs to send data to Host B. What Layer 2 and Layer 3addresses will be used to send data from Host A to Host B?


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Exploration Network Chapter 9 (Rev 1.0)