Transcript Ch. 13 LAN Systems

Ch. 16 Ethernet
16.1 Traditional Ethernet
• IEEE 802.3 Medium Access Control
– Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
– The most commonly used medium access control
technique for bus/tree and star topologies.
– Original baseband version was developed by
Xerox and formed the basis IEEE 802.3
CSMA/CD standard.
16.1 Ethernet (p.2)
• ALOHA
– Developed for packet radio networks
(Abrahamson, 1970).
– If a station has something to send, it does so.
– Then the station listens for an ACK.
– If no ACK is received, then a collision is
assumed and the frame is retransmitted.
– Simple but maximum efficiency is only 18%.
16.1 Ethernet (p.3)
• Slotted ALOHA
– Stations always wait for the beginning of
a slot--this reduces collisions.
– Improves the performance --37%
maximum efficiency.
16.1 Ethernet (p.4)
• Carrier Sense Multiple Access (CSMA)
– Suppose propagation delays are small
relative to frame transmission times--then
stations usually know when the line is
being used.
– With CSMA, a station first listens--if the
medium is in use it will wait.
16.1 Ethernet (p.5)
• 1-persistent CSMA
– 1. If the medium is idle, transmit; otherwise, go
to step 2.
– 2. If the medium is busy, continue to listen until
the channel is sensed idle; then transmit
immediately.
• Other Approaches
– Nonpersistent--if busy, wait a random amount of
time.
– p-persistent--if idle, transmit with probability p.
16.1 Ethernet (p.6)
• CSMA/CD (Fig. 16.1)
– 1. If the medium is idle, transmit; otherwise, go to
step 2.
– 2. If the medium is busy, continue to listen until
the channel is idle, then transmit immediately.
– 3. If a collision is detected during transmission,
transmit a brief jamming signal to assure that all
stations know that there has been a collision and
then cease transmission.
– 4. After transmitting the jamming signal, wait a
random amount of time, then attempt to transmit
again (repeat from step 1).
16.1 Ethernet (p.7)
• How long does it take to detect a collision?
– Frames should be long enough to allow
collision detection prior to the end of
transmission (distance and speed are
factors also.)
• How long can segments be?
– The signal is attenuated and long segments
will cause problems in CD algorithm.
– IEEE standards recommend maximum
lengths.
16.1 Ethernet (p.8)
• IEEE 802.3 MAC Frame--Fig. 16.3
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Preamble--7-octet pattern of alternating 0s and 1s.
Start frame delimiter--pattern 10101011.
Destination address--48 bit address.
Source address--48 bit address.
Length--length of the LCC data field--2 bytes.
Pad--octets added to ensure that the frame is long
enough for proper CD operation.
– Frame check sequence--32-bit cyclic redundancy
check (does not cover preamble and SFD.)
16.1 Ethernet (p.9)
• Three types of MAC Frames
– Basic
– Q-tagged frame—supports 802.1 Q VLAN
– Envelope frame—allows additional prefixes
and suffixes to the data field (Provider Bridges
and MAC Security—IEEE 802.1 Working
group) –also ITU-T, and IETF (MPLS or
multiprotocol label switching)
16.1 Ethernet (p.10)
• IEEE 802.3 10-Mbps Specification (Ethernet)
– 10BROAD36—rarely used today
– 10BASE5
– 10BASE2
– 10BASE-T
– 10BASE-F (3 options)
– Table 16.2 summarizes the 10Mbps
alternatives.
16.2 High Speed Ethernet
• IEEE 802.3 100-Mbps (Fast Ethernet)
– Generic designation is 100BASE-T.
– All options use IEEE 802.3 MAC protocol and
frame format.
– 100BASE-X (TX and FX)
• Set of options that use a signal encoding scheme
defined for FDDI--4B/5B NRZI.
– 100BASE-T4
• Uses four twisted pair lines between nodes.
16.2 High Speed Ethernet (p.2)
• Gigabit Ethernet
– Uses the IEEE 802.3 MAC protocol and frame
format.
– A new medium and transmission specification
is defined.
– Fig. 16.5 illustrates a typical Gigabit Ethernet
application.
16.2High Speed Ethernet (p.3)
• Gigabit Ethernet--Media Access Layer
– Two enhancements for shared medium hub.
• Carrier extension--appends a set of special symbols to
the end of short MAC frames so that the resulting block
is as least 4096 bit-time in duration--this makes the
transmission time longer than the propagation time.
• Frame bursting--allows for multiple short frames to be
transmitted consecutively--avoids the overhead of carrier
extension when a number of small frames are ready for
transmission.
– The above are not needed in switching hubs.
16.2 High Speed Ethernet (p.4)
• Gigabit Ethernet--Physical Layer
– 1000BASE-SX--supports duplex links of up to 275
meters and 550 meters depending on multimode
fiber diameter--wavelengths are 770 to 860 nm.
– 1000BASE-LX--supports duplex links from 550 m
to 5km depending on fiber diameter--wavelengths
are 1270 to 1355 nm.
– 1000BASE-CX--2 shielded twisted pair (25m.)
– 1000BASE-T--4 CAT 5 unshielded twisted pair
(100m).
16.2 High Speed Ethernet (p.5 )
• 10-Gbps Ethernet
– Satisfies increased bandwidth demand.
– Initially used for backbone connectivity.
– Provides for connectivity between ISPs and
NSPs co-located facilities.
– Allows MANs to be constructed, and begins to
compete with ATM.
16.2High Speed Ethernet (p.6)
• 10-Gbps Ethernet (p.2)
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10GBASE-S--(up to 300 meters).
10GBASE-L--(up to 10 kilometers)
10GBASE-E--(up to 40 kilometers)
10GBASE-LX4--(up to 10 kilometers; uses
WDM)
– Figure 16.7.
16.2 High Speed Ethernet(p.7)
• 100 – G bps Ethernet
– IEEE 802.3ba standard was finalized in 2010.
– 40G bps and 100 G bps
– Expected to be deployed in switch uplinks
inside the data center as well as inter-building,
intercampus, MAN, and WAN connections.
16.3 IEEE 802.1 Q VLAN
Standard
• (2005) Defines the operation VLAN bridges
and switches that permits the definition,
operation and administration of VLAN
topologies.
• Fig. 16.10 shows the “tagged” IEEE 802.3
MAC Frame Format.