8/2/2019 Cisco Module 5
1/51
8/2/2019 Cisco Module 5
2/51
Development of Ethernet
Grew out of Alohanet developed at
University of Hawaii in 70s. Multiple access to shared radio frequency
Formed basis for Ethernet MAC method
CSMA/CD (Carrier Sense Multiple Access /Collision Detect
Original Ethernet was first LAN
First Standard published as consortium ofDigital Equipment Company, Intel, Xerox (DIX)in 1980
8/2/2019 Cisco Module 5
3/51
IEEE Standards
IEEE standardized original Ethernet
standards
IEEE official Ethernet standards are802.3
802.3 standard has been supplementedas improvements in physical media haveoccurred. Still remains 802.3
Reason Ethernet is scalable
8/2/2019 Cisco Module 5
4/51
IEEE Ethernet Naming Rules
Ethernet refers to family: Ethernet, Fast
Ethernet, Gigabit Ethernet (Gig-E), and10-Gb Ethernet
As Ethernet is expanded, IEEE issues new
supplement. New supplements are given 1 or 2 letter
designation and an abbreviateddescription (Identifier) 802.3u = Fast Ethernet
10BASE2 (IEEE 802.3a)
1000BASE-TX (IEEE 802.3X)
8/2/2019 Cisco Module 5
5/51
IEEE Ethernet Naming Rules (Contd)
Description consists of three parts
Number indicating bps Tx Word BASE indicating Baseband TX Numbers referring to Coax Cable segment
length
One or more letters indicating type of mediumused F = Fiber optic cable T = Copper unshielded twisted pair
Baseband signaling whole bandwidth usedfor signal. Used in Ethernet Broadband signaling Analog carrier signal is
modulated by data signal
8/2/2019 Cisco Module 5
6/51
IEEE
Standards making organization
Goals
Supply information to build devices to
comply with Ethernet standard Not stifle innovation
Equipment Mfgs not required to
meet standards
8/2/2019 Cisco Module 5
7/51
802.3/Ethernet and OSI Model
Standards specify
Physical media and connectors Communication via Data Link Layer
Encapsulation of protocol-specific traffic
Data link layer broken into 2-sublayers Media Access Control (MAC) [802.3]
How to Tx frames on physical medium,physical addressing, line discipline, network
topology Logical Link Control (LLC) [802.2]
Logically ID different protocols forencapsulation.
8/2/2019 Cisco Module 5
8/51
Layer 2 comparisons
Layer 1 Limitation Layer 2 Solution
Cannot communicate with theupper-level layers
Communicates with upper-level layers via the LLC sublayer
Can only describe stream ofbits
Uses framing to organize orgroup bits. (makes bits havemeaning
Cannot identify computers Identifies computer usingMAC Addressing
Cannot decide whichcomputer will Tx data from
group of computers trying toTX at same time
Uses MAC sublayer toaccomplish decisions
8/2/2019 Cisco Module 5
9/51
LLC and MAC
MAC is concerned with physical
media
LLC is independent of physical
media Allows multiple Layer 3 protocols (IP,
IPX, AppleTalk) to be supported along
with multiple frame types
8/2/2019 Cisco Module 5
10/51
MAC Addressing
All computers on a network must be
able to be uniquely identified inorder for frame delivery
Media Access Control (MAC) addressis located on NIC Hardware address
NIC address
Layer 2 address
Ethernet address
8/2/2019 Cisco Module 5
11/51
MAC addressing (Contd)
MAC address is 48 bits
Expressed at 12 hexadecimal digits
First 6 digits are assigned to Mfg by IEEE
Organizat ionally Unique I dent ifier (OUI )
Last 6 digits are assigned by vendor
OUI Vendor Assigned
00 60 2F 3A 07 BC Cisco Particular Device
8/2/2019 Cisco Module 5
12/51
MAC addressing (Contd)
Data link layer adds H e a d e r and
T r a i l e r to upper level data Header and Trailer contain control
information for corresponding Layer 2at destination
Upper-layer data is encapsulatedbetween Layer 2 Header and Trailer
8/2/2019 Cisco Module 5
13/51
Broadcast LANs
Ethernet and 802.3 are B r o a d c a s t
Networks All stations see all frames
Each station checks every frame to
determine if frame is destined for thatstation
Part of data contained in frame isdestination MAC address On finding matching address, destination
passes data in frame to upper-levelprotocols
8/2/2019 Cisco Module 5
14/51
Framing
Framing allows essential
information to encoded into the bitstream
Which computer is talking to whichcomputer
Where data starts, stops, what protocol
to use
8/2/2019 Cisco Module 5
15/51
Generic Frames
Different types of frames described
by different standards
Generic Frame Fields
Frame Start Address
Length / Type / Control
Data
Frame Check Sequence (FCS)
8/2/2019 Cisco Module 5
16/51
Frame Fields Frame Start
Tells NIC where frame starts
Address Source and Destination MAC Addresses
Length and Type If required describes length of the frame and
protocol type
Data Field Contains data being Tx
Frame Check Sequence Field Number based on data in frame. Sending computer
calculates the value and places it in FCS. Rxcomputer calculates number and checks it againstFCS value
8/2/2019 Cisco Module 5
17/51
Ethernet Frame Structure
Ethernet II Frame
Structure
Oct et s in Each Fr am e Field Fr am e Field
8 Preamble
6 Destinatin MAC Address
6 Source MAC Address
2 Type Field
46 to 1500 Data and Pad
4 Frame Check Sequence (CRC
Checksum)
8/2/2019 Cisco Module 5
18/51
Ethernet Frame Fields Preamble
Alternating 1s and 0s used in timing for 10 Mbps and slower
Start Frame Delimiter (SFD) Marks end of Start Frame
Destination Address Destination MAC Address
Source Address Source MAC Address
Length/Type If value 1536 indicates Type
Decoded per protocol indicated Data and Pad
Any length > 64 octets and < 1518 octets that does not exceedMaximum Transmission Unit (MTU)
Frame Check Sequence
8/2/2019 Cisco Module 5
19/51
Ethernet Operation
Carrier Sense Multiple Access /
Collision Domain (CSMA/CD) Provides media access control
strategies
Media Access Control protocolsthat determine which computer on a
shared domain (collision domain) isallowed to Tx.
8/2/2019 Cisco Module 5
20/51
Media Access Control
MAC and LLC comprise Layer 2
2 categories of MAC
Deterministic
Token Ring, FDDI Nondeterministic (probalistic)
Ethernet / 802.3
8/2/2019 Cisco Module 5
21/51
Deterministic MAC Protocols
Token Ring
Hosts in ring
Token circulates around ring
Grabs Token
Tx data for limited time
Releases Token
8/2/2019 Cisco Module 5
22/51
Nondeterministic MAC Protocols
First come / First served (FCFS)
CSMA/CD
Listen for quiet
Begin Tx
More than 1 Tx at same time collision
Frame is lost
All other hosts hear collision
Wait random time (backoff) Retransmit
8/2/2019 Cisco Module 5
23/51
Topologies
Ethernet
Logical bus, physical star, extendedstar
Token Ring Logical ring, physical star
FDDI
Logical ring, physical dual-ring
8/2/2019 Cisco Module 5
24/51
Ethernet Broadcast Broadcast every host on net receives
every packet Only device that has matching MAC address
will pay attention to packet.
Remaining devices drop the packet
Ethernet not concerned with Layer 3 Checks packet for errors
Detects error packet is dropped
Destination does notnotify Source of droppedpacket
Connectionless architecture
Best-effort delivery system
8/2/2019 Cisco Module 5
25/51
Simplex, Half-, Full-Duplex
Simplex Unidirectional: data only
travels in one direction Half-duplex Data travels in both
directions but only one direction at
a time. Full-duplex Data travels both
directions at the same time With switching, no collisions occur
Can achieve full bandwidth
8/2/2019 Cisco Module 5
26/51
Ethernet Timing
8/2/2019 Cisco Module 5
27/51
Types of collisions
Single collisions
A collision detected while trying totransmit a frame
Multiple collisions
Same frame collided repeatedly beforebeing successfully Tx
Deferred Tx No collision frame was delayed in
being Tx because of busy medium
8/2/2019 Cisco Module 5
28/51
Result of Collisions
As a result of collisions, corrupted
or partial frames can occur
8/2/2019 Cisco Module 5
29/51
Local, Remote, Late collisions
Local collision
On coax, signals colliding in eachdirection overlap and cancel/emphasizewaveform causing overvoltage
(collision) On UTP, detects signal on RX at same
time sending on Tx (Half-duplex only)
Excessive crosstalk and be perceived ascollision
8/2/2019 Cisco Module 5
30/51
Remote collision
Typically a frame that is a runt and
FCS is invalid Results from collisions on far side of a
repeated connection
8/2/2019 Cisco Module 5
31/51
Late Collision Occurs after the first 64 octets of Tx have
been sent. Theoretical limits of network propagation have
been exceeded by that point
802.3 allows automatic retransmission of latecollided frames, but not required
Explicitly denies automatic retransmission inGigabit Ethernet
Occurs after slot t imehas elapsed and on farside of a repeater
8/2/2019 Cisco Module 5
32/51
Ethernet Errors Late collisions are considered to be errors
Frequency of errors will indicate urgency of problem
Simultaneous Tx before slot time Collision or runt
Simultaneous Tx after slot time Late collision
Excessively or Illegally Tx Jabber, long frame, range errors
Illegally short Tx Short Frame, Collision fragment, runt
Corrupted Tx FCS error
Insufficient or Excessive Number of Bits Tx Alignment error
Mismatch of actual or reported number of Octets in frame Range error
Unusually long preamble or jam event Ghost or jabber
8/2/2019 Cisco Module 5
33/51
Jabber
Tx of at least 20,000 to 50,000 bit-
times in duration More properly called Long frame
May or may not have valid FCS
8/2/2019 Cisco Module 5
34/51
Long Frame
Greater than maximum legal size
Takes into account if frame is tagged Does not count as an error if frame is
802.1Q tagged
8/2/2019 Cisco Module 5
35/51
Short Frame
< minimum 64 octets and good FCS
Sometimes called runts
8/2/2019 Cisco Module 5
36/51
FCS Errors
FCS Error occurs when Checksum
values differ by 1 or more bits fromwhat was Tx.
High number from single stationindicates faulty NIC, bad drivers, badcabling at station
Errors associated with many hostsusually indicates: bad cabling, baddrivers, faulty hub port, induced noise
8/2/2019 Cisco Module 5
37/51
Alignment Error
Data does not end at octet
boundary has extra bits left over Extra bits truncated and if FCS fails
then error reported
Usually caused by:
Bad drivers or collision
Read/Write error in software
Can overload router CPU leading to crash
8/2/2019 Cisco Module 5
38/51
Range Error
Legal value in field length but
doesnt match actual number ofoctets in data field
8/2/2019 Cisco Module 5
39/51
Ghost
Noise on the cable that appears to
be a frame, but isnt >= 72 octets long
Ghosting network is slow for no apparent
reason
8/2/2019 Cisco Module 5
40/51
Autonegotiation Allows interfaces to match Tx and Rx
speeds with other interfaces 10BASE-T required a link pulse every 16 ms.
Turned into Normal Link Pulse (NLP)
Series of NLP Tx called Fast Link Pulse (FLP)
FLP consists of 33 NLP bursts = 16 bit datacode
Pages can be added representing additionalmore sophisticated negotiation and link
parameters After decoding an FLP an acknowledgement is
sent. Communicating partners can keep movingup pages to establish best link parameters
8/2/2019 Cisco Module 5
41/51
Full, Half-duplex link establishment
Autonegotiation optional for most
implementations Required for Gigabit networks
Two ways of establishing Full duplex Completed autonegotiation cycle
Administratively set
If setting one, m u s t set the other Gigabit does not support half-duplex
8/2/2019 Cisco Module 5
42/51
Collision and Broadcast Domains
3 types of media environments
Shared-media Multiple hosts have access to same media
Extended shared-media
Networking devices have extendednetworking to longer cable lengths ormore hosts
Point-to-Point One device only connected to one device
8/2/2019 Cisco Module 5
43/51
Indirectly connected networks Circuit-switched
Point-to-Point communication on an electricalcircuit that exists for duration of theconnection.
Not shared environment no collisions Telephone system
Packet-switched Packets sent with sufficient address info to
reach destination. Packets may travel throughdifferent circuits
Logical point-to-point connection Not shared environment no collisions
Cell phones and Internet
8/2/2019 Cisco Module 5
44/51
Collisions and Collision Domains
Collision occurs when 2 hosts Tx at
same time on shared medium Collision domain connected
physical segments where collisionscan occur
Layer 1 devices do not break up
collision domains Layer 2 & 3 break up collision domains
Segmentation
8/2/2019 Cisco Module 5
45/51
Layer 1 Devices and collision domains
Layer 1 devices simply extend
Ethernet cable segments More potential users
All traffic is passed on through layer 1
devices Greater number of potential collisions
Four repeater rule in place so timing
conditions assure all hosts hear collisions Repeater, NIC, Propagation delay increase
latency
8/2/2019 Cisco Module 5
46/51
Segmentation
Layer 2 and 3 devices segment
large collision domains into more,smaller collision domains
Decisions based on MAC addresses
Signals only propagated to correctsegment
Less traffic on segment
Increased bandwidth for hosts on segment
8/2/2019 Cisco Module 5
47/51
Layer 2 Broadcasts When node needs to communicate with
all hosts on network, sends broadcastwith MAC destination address0xFFFFFFFFFFFF. Every NIC must
recognize this address. Layer 2 devices must forward all
broadcast traffic to all segments
Accumulation of broadcast and multicasttraffic called broadcast radiation.
8/2/2019 Cisco Module 5
48/51
Layer 2 Broadcasts 3 sources of broadcast traffic
Workstations broadcast ARP requests in orderto locate MAC address
Routers broadcast routing protocol updatesperiodically to keep routing tables corrects
RIP broadcasts updates every 30 seconds
IP multicasts send streaming multimedia data.
Unless Layer 3 is used, is sent to all segments
8/2/2019 Cisco Module 5
49/51
Broadcast domains Grouping of collision domains
connected by Layer 2 devices. Must be controlled at Layer 3
Layer 3 devices do not forward broadcasts
to other segments Layer 3 forwarding based on IP address,
not MAC address
8/2/2019 Cisco Module 5
50/51
Data Flow Layer 1 devices always forward a
frame Layer 2 devices forward frame
unless prevented
Layer 3 devices block frames unlesstold to forward
8/2/2019 Cisco Module 5
51/51
Network Segment Segment can have different meanings
when referring to networking Layer 4 context means breaking up data
stream from Layer 5
Network context means a section of a networkbounded by bridges, switches, and routers
LAN topology means continuous circuit oftenconnected to other segments by repeaters
Top Related