- Overview of the Ethernet:
- The most popular network
in the US
- Ethernet was developed at
Xerox PARC
between 1973 and 1974
(Palo Alto Research Center Incorporated is a branch of the Xerox company)
- The Ethernet protocol was invented by
Robert Metcalfe:
click here
- Metcalfe was inspired by
Aloha
--- which he studied as part of his
PhD dissertation
- The most popular network
in the US
- Documentation on the
Ethernet (standard:
-
"The Ethernet, A Local Area Network. Data Link Layer and
Physical Layer Specifications" - 1980
-
click here
- Standardization dates:
- International standard in 1983
- The IEEE 802.3 standard on: June 23, 1983
-
"The Ethernet, A Local Area Network. Data Link Layer and
Physical Layer Specifications" - 1980
-
click here
- The Ethernet protocol is:
- A modified 1-persistent CSMA/CD protocol !!!
- The Ethernet protocol
flow chart:
Important note:
- Ethernet has a modified" back off algorithm which we will discuss in the next web page
- The Ethernet Frame format
Meaning of the fields:
Preamble: discussed below
Start: 10101011 (Fixed pattern)
Src: Ethernet address of the sender
Dest: Ethernet address of the intended receiver
Length: Length of the data field
Data: Data field
CRC: Check sum
- Pre-amble:
- The pre-amble
is used to pad
the Ethernet frame so that
the frame is long enough
for
collision detection
purposes
- The pre-amble
is used to pad
the Ethernet frame so that
the frame is long enough
for
collision detection
purposes
- Usage of the
pre-amble:
- A transmitting node can
only
detect
a collision
while the
node is
(still)
transmitting
- The pre-amble will make sure that simultaneous senders will transmit long enough that the simultaneous senders will detect a collision
- A transmitting node can
only
detect
a collision
while the
node is
(still)
transmitting
- Example:
how
nodes can
miss a
collission
- Nodes B and C
transmit very short frames:
- Their transmissions will collide:
- However: the
sending nodes
(B and
C) did
not detect the
collision:
And:
- Nodes B and C will not retransmit !!!!
The pre-amble will make sure that transmitting node will be transmitting long enough to detect a collission:
- Nodes B and C
transmit longer frames:
and hears the collision....
- Nodes B and C
transmit very short frames:
- Ethernet Address:
- Each nodes
on a Ethernet is
identified by a
unique
48 bits Ethernet address
- The Ethernet address is permanently inscribed in the Ethernet card of a computer
- Each nodes
on a Ethernet is
identified by a
unique
48 bits Ethernet address
- UNIX command to
find the Ethernet Address
on a computer:
cheung@aruba> ifconfig -a eth0 Link encap:Ethernet HWaddr 2C:41:38:8B:05:38 inet addr:170.140.150.36 Bcast:170.140.151.255 Mask:255.255.254.0 inet6 addr: fe80::2e41:38ff:fe8b:538/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:681868507 errors:0 dropped:0 overruns:0 frame:0 TX packets:628584348 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:335075333658 (312.0 GiB) TX bytes:540484678811 (503.3 GiB) Interrupt:20 Memory:fb000000-fb020000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:267964 errors:0 dropped:0 overruns:0 frame:0 TX packets:267964 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:107815976 (102.8 MiB) TX bytes:107815976 (102.8 MiB)Explanation:
Ethernet address = 2C:41:38:8B:05:38 (Hexadcimal digits) 2 C 4 1 3 8 8 B 0 5 3 8 = 0010 1100 0100 0001 0011 1000 1000 1011 0000 0101 0011 1000
-
Notice that:
- Ethernet frame:
There are:
- No Send sequence number
- No ACK sequence number
in Ethernet frames !!!
- Reason: (it confirms
what I said
before ---
see:
click here)
- The Ethernet (LAN) protocol does not provide reliability function !!!
- Note:
- Ethernet relies on
the Transport Layer
to ensure
reliable communication
- There is an ARQ protocol in the Transport layer !!!
- Ethernet relies on
the Transport Layer
to ensure
reliable communication
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- Ethernet:
- "Ethernet"
originally
referred to the protocol implementation
standardized by
Digital, Intel and Xerox.
- This standard is now known as the DIX standard.
- "Ethernet"
originally
referred to the protocol implementation
standardized by
Digital, Intel and Xerox.
- IEEE Standard for "Ethernet"
- The IEEE 802.3 protocol standard
describes
another
1-persistent
CSMA/CD protocol
that is very close to the
DIX Ethernet
standard.
- DIX Ethernet
and 802.3
differ slightly in the
frame format
and are in
most aspects identical.
- The only difference is the
length/EtherType field
See: click here
- The only difference is the
length/EtherType field
- The IEEE 802.3 protocol standard
describes
another
1-persistent
CSMA/CD protocol
that is very close to the
DIX Ethernet
standard.
-
Today, the term
"Ethernet"
refers generically to both:
- the DIX Ethernet implementation and
- the IEEE 802.3 standard.