Lec # 12
Computer communucation and Networks
Ethernet
History
↗ Ethernet is the most widespread
LAN technology
↗ Ethernet was
developed at Xerox (1973) inspired by ALOHAnet
which
Robert Metcalfe had studied as part of his PhD dissertation
↗ Standardized by Xerox/DEC/Intel in 1978
↗ Ethernet was officially
accepted as IEEE standard 802.3 in 1985
Ethernet & IEEE 802.3
What the IEEE standard covers- Physical layer and interface to the link layer.
IEEE 802.2 is the Link layer standard.
Differences between Ethernet and 802.3
1-There are some electrical and connector
differences; most
equipment uses IEEE 802.3.
2-There is difference in the header. DIX uses
TYPE,
802.3 uses LENGTH. SInce the frame is limited
in size, the two coexist.
IEEE & OSI
PHY
MAC
LLC
Data link
1-
Ethernet
Why Ethernet became so popular
↗ Easy to understand, implement, manage, and maintain
↗ Low-cost network implementations
↗ Topological flexibility for network installation
Frame Format
Ethernet
↗ Preamble:Starts with 7 bytes of 1010101 to get timing synchronized
↗ SFD: indicate the frame is starting (10101011) , 1- byte
↗ PDU or Frames: headers (addresses + length/type) ↗ 802.3 standard substitutes length with type field
↗ The Length/EtherType: In 802.3 it indicates the number of bytes of data in the frame from 0 to 1500 bytes. Frames must be at least 64 bytes long, not including the preamble, so, if the data field is shorter than 46 bytes, it must be compensated by the
Pad field (padding bytes).
↗ Type field is the first thing in data portion
↗ Data (46-1500), add bits if necessary to get to 46 bytes. ↗ CRC-32 FCS for error checking
Preamble SFD Destination Address
Source
Address Length Data + Pad FCS
7 1 6 6 2 46-1500 4
Frame Format (Ethernet & 802.3)
Ethernet
Ethernet Addresses
↗ MAC address, short for Media Access Control address, a hardware address that uniquely identifies each node on a network
↗ When a manufacturer creates a network capable piece of hardware they will assign the MAC address which will usually begin with a code that is tied to the manufacturer
↗ It will be unique to every device, even two devices of the same type.
↗ A device’s MAC address is composed of six pairs of hexadecimal numbers ↗ The numbers are separated by colons as in the following example:
↗ 6E:51:F5:C1:11:00
Ethernet
Ethernet Address
↗ The first three octets identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI)
↗ The following three octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness
Ethernet
Ethernet Addresses Types
↗ Unicast; unique, 48-bit address assigned to each adapter ↗ Examples:
↗ 01:23:45:67:89:AB ↗ 08:00:07:A9:B2:FC ↗ Broadcast: all 1s
Normal Ethernet Operation
A B D Data C Address mismatchpacket discarded Address mismatchpacket discarded
Address match packet processed Send data
to node D
Transmitted packet seen by all stations on the LAN
Ethernet Collisions
A
B C
D
Collision
Ethernet
• Two stations (2 and 4) communicating using a single bus
Ethernet
How Ethernet Works
Shared Media Problem
↗ Problem: demands can conflict, e.g., two hosts send simultaneously ↗ Solution is a medium access control (MAC) algorithm
Ethernet
Need for an Access Method
↗ Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other
↗ Such overlaps which turn signals to noise are called Collisions ↗ As traffic increases on multiple-access link, so do collisions
↗ Such a network therefore needs a mechanism to coordinate traffic, minimize the number of collisions and maximizes the number of frames that are delivered successfully
Ethernet
Ethernet MAC –
CSMA/CD
(Carrier Sense Multiple Access/Collision Detection)
↗ Multiple access
↗ Nodes send and receive frames over a shared link ↗ Carrier sense
↗ Nodes can distinguish between an idle and busy link ↗ Collision detection
Ethernet
Ethernet MAC –
CSMA/CD
MAC Algorithm
↗ If line is idle (no carrier sensed) ↗ Send immediately
Ethernet
Ethernet MAC –
CSMA/CD
MAC Algorithm
↗ If line is busy (carrier sensed)
↗ Wait until the line becomes idle and then transmit immediately ↗ Called 1-persistent (special case of p-persistent)
↗ If collision detected
Ethernet MAC (CSMA/CD)
Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yesattempts < 16
attempts == 16
Ethernet’s CSMA/CD (more)
Jam Signal:
make sure all other transmitters are aware of
collision; 48 bits;
Exponential Backoff:
•
If deterministic delay after collision, collision will occur
again in lockstep
•
If random delay with fixed mean
–
Few senders
needless waiting
–
Too many senders
too many collisions
Ethernet
Switched Ethernet
↗ No need Of MAC
↗ No collision because of the micro level segmentation ↗ Switches performs the switching between hosts
Ethernet
Ethernet Flavors
10Base5 10BaseT 10baseFL 100BaseTX 100BaseFX Media Thick coax
RG-8, RG-11 TIA/EIA UTP Cat 3,4,5,5e,6 (2 pair) 62.2/125 m Multi-mode Fiber TIA/EIA UTP Cat 5 and up (2 pair) 62.2/125 m Multi-mode Fiber Or Single mode fiber Speed 10 Mbps 10 Mbps 10 Mbps 100 Mbps 100 Mbps
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