Transcript LANs and Hi-speed LANs

NETE0510
LANs and Hi-speed LANs
Dr. Supakorn Kungpisdan
[email protected]
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Communications
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Outline
 LAN Overview
 Ethernet
 Token Ring
 FDDI
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LAN Topologies
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LAN Protocol Architecture
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IEEE 802 Layers
 Physical
 encoding/decoding of signals
 preamble generation/removal
 bit transmission/reception
 transmission medium and topology
 Logical Link Control
 interface to higher levels
 flow and error control
 Media Access Control
 on transmit assemble data into frame
 on receive disassemble frame
 govern access to transmission medium
 for same LLC, may have several MAC options
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LAN Protocols in Context
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Logical Link Control (LLC)
 transmission of link level PDUs between stations
 must support multi-access, shared medium
 but MAC layer handles link access details
 addressing involves specifying source and
destination LLC users
referred to as service access points (SAP)
typically higher level protocol
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LLC Services
 based on HDLC
 3 services provided:
Unacknowledged connectionless service
 Simple, no flow- and error control, no data delivery
guaranteed  rely on higher layer protocols
Connection-mode service
 Similar to that offered by HDLC
 Need connection setup, provide flow and error control
Acknowledged connectionless service
 Hybrid approach
 No connection setup required, but require
acknowledgement
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Media Access Control (MAC)
 MAC layer receives data from LLC layer
 fields
MAC control
destination MAC address
source MAC address
LLC
CRC
 MAC layer detects errors and discards frames
 LLC optionally retransmits unsuccessful frames
(link-to-link retransmission, not end-to-end)
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Outline
 LAN Overview
 Ethernet
 Token Ring
 FDDI
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Ethernet (CSMA/CD)
 most widely used LAN standard
 developed by
Xerox - original Ethernet
IEEE 802.3
 Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
random / contention access to media
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Pure VS Slotted ALOHA
Pure ALOHA
Slotted ALOHA
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ALOHA
 developed for packet radio nets
 when station has frame, it sends
 then listens for a bit over max round trip time (RTT)
 if receive ACK then fine
 if not, retransmit
 if no ACK after repeated transmissions, give up
 uses a frame check sequence (as in HDLC) to check for
errors
 frame may be damaged by noise or by another station
transmitting at the same time (collision)
 any overlap of frames causes collision
 max utilization 18%
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Slotted ALOHA
 time on channel based on uniform slots equal to frame
transmission time
 need central clock (or other sync mechanism)
 transmission begins only at the beginning of the slot
 So, frames either miss or overlap totally
 max utilization 37%
 both have poor utilization
 fail to use fact that propagation time (PT) is much less
than frame transmission time (TT)
 If PT >> TT, a station may succeed in transmitting a frame
 If TT >> PT, none of the stations may not succeed
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CSMA

1.
2.
3.
stations soon know transmission has started
so first listen for clear medium (carrier sense)
if medium idle, transmit
if two stations start at the same instant, collision




wait reasonable time
if no ACK then retransmit
collisions occur occur at leading edge of frame
max utilization depends on propagation time (medium
length) and frame length



shorter PT, longer frame, higher utilization
Also work well for the case that PT << TT
Collision can occur only more than one user begins
transmitting within PT
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CSMA Persistence and Backoff
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Non-persistent CSMA
 Non-persistent CSMA rules:
1. if medium is idle, transmit
2. if medium is busy, wait for amount of time drawn
from probability distribution (retransmission delay)
& retry
 random delays reduces probability of collisions
 capacity is wasted because medium will remain
idle following end of transmission even stations
are waiting to transmit frames
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1-persistent CSMA
 1-persistent CSMA avoids idle channel time
 1-persistent CSMA rules:
1. if medium idle, transmit;
2. if medium busy, listen until idle; then transmit immediately
 1-persistent stations are selfish
 if two or more stations waiting, a collision is
guaranteed
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P-persistent CSMA
 a compromise to try and reduce collisions and
idle time
 p-persistent CSMA rules:
1. if medium idle, transmit with probability p, and delay
one time unit (equal to max propagation delay) with
probability (1–p)
2. if medium busy, listen until idle and repeat step 1
3. if transmission is delayed one time unit, repeat step 1
 issue of choosing effective value of p to avoid
instability under heavy load
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CSMA/CD
 with CSMA, collision occupies medium for
duration of transmission
 better if stations listen whilst transmitting
 CSMA/CD rules:
1.
2.
3.
4.
if medium idle, transmit
if busy, listen for idle, then transmit
if collision detected, jam and then cease transmission
after jam, wait random time (backoff period) then retry
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CSMA/CD (cont’d)
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CSMA/CD Algorithm
1. Adaptor receives datagram from
4. If adapter detects another
net layer & creates frame
transmission while transmitting,
aborts and sends 48-bit jam
2. If adapter senses channel idle (no
signal
signal energy entering adapter for
96 bit times), it starts to transmit
5. After aborting, adapter enters
frame. If it senses channel busy,
exponential backoff: after the
waits until channel idle and then
mth collision, adapter chooses a
transmits
K at random from
{0,1,2,…,2m-1}. Adapter waits
3. If adapter transmits entire frame
K·512 bit times and returns to
without detecting another
Step 2
transmission, the adapter is done
with frame !
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CSMA/CD (cont’d)
Jam Signal: make sure all other
transmitters are aware of
collision; 48 bits
Bit time: time to send 1 bit of
data
= 0.1 microsec for 10 Mbps
Ethernet ;
for K=1023, wait time is
about 50 msec
(1023 x 512 x 0.1
= 52378 microsec
= 52.38 msec)
Exponential Backoff:
 Goal: adapt retransmission
attempts to estimated current
load
 heavy load: random wait
will be longer
 first collision: choose K from
{0,1}; delay is K· 512 bit
transmission times
 after second collision:
choose K from {0,1,2,3}…
 after ten collisions, choose K
from {0,1,2,3,4,…,1023}
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IEEE 802.3 Frame Format
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10Mbps Specification (Ethernet)
10BASE5
10BASE2
10BASE-T
10BASE-FP
Transmission
medium
Coaxial cable (50
ohm)
Coaxial cable (50
ohm)
Unshielded twis ted
pair
850-nm optical fiber
pair
Signaling
techni que
Baseband
(Manch ester)
Baseband
(Manch ester)
Baseband
(Manch ester)
Manches ter/on-off
Topology
Bus
Bus
Star
Star
Maximu m segment 500
length (m)
185
100
500
Nodes per segment
100
30
—
33
Cable diameter
(mm)
10
5
0.4 to 0.6
62.5/125 µm
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100Mbps Fast Ethernet
100BASE-TX
100BASE-FX
100BASE-T4
Transmission
medium
2 pair, STP
2 pair, Catego ry
5 UTP
2 optical fibers
4 pair, Catego ry
3, 4, or 5 UTP
Signaling
techni que
MLT-3
MLT-3
4B5B, NRZI
8B6T, NRZ
Data ra te
100 Mbps
100 Mbps
100 Mbps
100 Mbps
Maximu m
segme nt length
100 m
100 m
100 m
100 m
Networ k span
200 m
200 m
400 m
200 m
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100BASE-X
 uses a unidirectional data rate 100 Mbps over single
twisted pair or optical fiber link
 encoding scheme same as FDDI
 4B/5B-NRZI
 two physical medium specifications
 100BASE-TX
 uses two pairs of twisted-pair cable for tx & rx
 STP and Category 5 UTP allowed
 MTL-3 signaling scheme is used
 100BASE-FX
 uses two optical fiber cables for tx & rx
 convert 4B/5B-NRZI code group into optical signals
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MTL-3 Encoding
 An MLT-3 interface emits less electromagnetic
interference and requires less bandwidth than most other
binary or ternary interfaces that operate at the same data
rate
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100BASE-T Options
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Gigabit Ethernet Configuration
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Gigabit Ethernet – Physical
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10Gbps Ethernet Options
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Outline
 LAN Overview
 Ethernet
 Token Ring
 FDDI
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Token Ring
 Many types of token ring
technology:
 IBM’s Token Ring
 IEEE802.5 Token Ring
 FDDI (Fiber Distribution
Data Interface)
 IEEE802.17 Resilient Packet
Ring
 A token ring network consists
of nodes connected in a ring.
 Data always flows in a
particular direction around the
ring, with each node receiving
frames from its upstream
neighbor and then forwarding
them to its downstream
neighbor.
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Token Ring (cont’d)
 Different from Ethernet: ring-based VS bus
topology
 Same  single shared-medium network
 Two common features of Token Ring and
Ethernet
Involve a distributed algorithm that controls when
each node is allowed to transmit
All nodes see all frames; only the node identified in
a frame as the destination will save a copy of the
frame as it flows past.
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Token Ring specifications
 Data transfer rate is 4 or 16 Mbps
 Uses Twisted Pair cabling (Cat 3 for 4 MB/s, Cat 5 for 16
Mb/s) for IBM’s Token Ring, but not specified in
IEEE802.5
 Use Manchester encoding
 Access method is token passing
 Logical topology ring, physical topology is star
 Connector type is RJ-45
 Maximum attachments per segment is 250 (IEEE 802.5)
and 260 (IBM) per ring
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Token


Token is a special sequence of bits circulating around
the ring
Token Ring operation:
1.
2.
3.
4.

Each node receives and forwards the token
When a node that has a frame to transmit sees the token, it
takes the token off the ring, and insert its frame into the ring
Each node along the way simply forwards the frame, with the
destination node saving a copy and forwarding the message
onto the next node on the ring.
When the frame makes its way back around to the sender, this
node strips its frame off the ring and reinserts the token.
The media access algorithm is fair  the token
circulates around the ring, each node gets a chance to
transmit.
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Physical Properties
 Link or node failure would render the whole network
useless
 Solved by connecting each station into the ring using an
electromechanical relay.
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Physical Properties (cont’d)
 Several of these relays are usually packed into a single
box, known as a multi-station access unit (MSAU or
MAU)
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Token Ring Frame Format
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Token Ring Frame Format (Cont’d)
Access Control
 T is token bit, set to specify the token frame
 M is monitor bit, set by Active Monitor
Frame Status
 A =1, Address recognized
 C = 1, Frame copied
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Token Ring Media Access Control
 As the token circulates around the ring, any
station that has data to send may seize the
token by simply modifying 1 bit (T bit) in the
second byte token
 The first 2 bytes of the modified token now
become the preamble for the subsequent data
packet.
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Token Holding Time (THT)
 Specify how long a given node is allowed to hold
the token
How much data a given node is allowed to transmit each
time it possesses the token
 Time limit, data limit, or no limit?
 Default THT for IEEE802.5 is 10 ms
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Token Rotation Time (TRT)
 The amount of time it takes a token to traverse
the tine as viewed by a given node
TRT ≤ ActiveNodes x THT + RingLatency
 Where,
RingLatency denotes how long it takes to circulate
around the ring where no one has data to send,
ActiveNodes denotes the number of nodes that
have data to send
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Reliable Delivery
 802.5 provides a form of reliable delivery using 2 bits in
the frame status field, A and C bits




Initially A and C are 0s
When a destination station sees a frame, it sets A bit
When it copies the frame into its adaptor, it sets C bit
If the sending station receives the frame with A bit still 0,
the recipient is not functioning or absent
 If A bit is set, but C bit is 0, the destination could not
accept the frame (may be the buffer is full). The sender
may retransmit the frame
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Token Ring Priority
 The token contains a 3-bit priority field. It has certain
priority n at any time
 Each station that has data to send assigns priority to that
frame, and the station can only seize the token to
transmit a packet if the packet’s priority is at least as
great as the token’s
 The token’s priority changes over time due to 3
reservation bits in Access Control field
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Token Ring Priority (cont’d)
 For example, Station X waiting to send a priority
n packet may set the reservation bit to n if it
sees the a data frame going past an the bits
have not been set these bits to a higher value
 So, the station that currently holds the token
must reduce the priority of the token to n when it
releases the token.
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Token Release
 Early release or delayed release
 Early release allows better bandwidth utilization
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Active and Standby Monitors
 Every station in a token ring network is either an active
monitor (AM) or standby monitor (SM) station.
 However, there can be only one active monitor on a ring
at a time.
 Becoming an AM is chosen by election. Once an AM is
chosen, every other station becomes a standby monitor.
All stations must be capable of becoming an active
monitor station if necessary.
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Active Monitor Election
 Electing AM is done when the ring is first connected or
on the failure of the current AM.
 The active monitor is chosen through an election or
monitor contention process.
 a loss of signal on the ring is detected,
 an active monitor station is not detected by other stations on
the ring, or
 when a particular timer on an end station expires such as the
case when a station hasn't seen a token frame in the past 7
seconds.
 The station that detects the above situation will try to
become a new AM by performs the following:
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Active Monitor Election (cont’d)
1.
2.
3.
The station sends a “claim token” frame, saying it
wants to become a new AM. This frame contains its
MAC address
If that token circulates back to the sender, it is
assumed that it can become a new AM
If other stations also want to become a new AM, they
also send the claim tokens. The station with highest
MAC address will become a new AM.
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Active Monitor
 The active monitor performs a number of ring
administration functions:
 Operate as the master clock for the ring in order to provide
synchronization of the signal for stations on the wire.
 Insert a 24-bit delay into the ring, to ensure that there is
always sufficient buffering in the ring for the token to
circulate.
 Ensure that exactly one token circulates whenever there is
no frame being transmitted, and to detect a broken ring.
 Token may vanish for several reasons e.g. bit error
 Responsible for removing circulating frames from the ring.
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Detecting A Missing Token
 AM watches for a passing token and maintains a
timer equal to the maximum possible token
rotation time. The interval equals:
NumStations x THT + RingLatency
If the timer expires without the AM seeing a
token, it creates a new token
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Detecting Errors
 AM checks for corrupted or orphaned frames
 Dealing with corrupted frames
 The corrupted frame is the frame with checksum error or invalid
format. Without the AM intervention, it could circulate forever
 The AM removes it and reinsert a new token
 Dealing with orphaned frames
 The orphaned frame is a normal frame whose “parent” died 
the sending station is down after sending the frame
 This frame can be detected by using “monitor” bit in Access
Control field
 Initially the monitor bit is 0. it is set to 1 for the first time it passes
the AM. If the AM detects this frame with this bit set, it knows that
this frames is going by for the second time.
 Then the AM drains the frame off the ring
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Outline
 LAN Overview
 Ethernet
 Token Ring
 FDDI
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FDDI
 Fiber Distribution Data Interface
 Requently used as high-speed backbone
technology because of its support for high
bandwidth and greater distances than copper.
 An implementation on copper is called CDDI
 An FDDI network consists of a dual ring
transmitting data in opposite directions: primary
and secondary rings
 Tolerate a single break in the cable or the failure
of one station
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FDDI (cont’d)
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FDDI Specifications
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Station-attachment Types
 Because of the expense of dual-ring configuration, some
node connects with a single cable  single attachment
station (SAS); their dual-connected counterpart is called
dual attachment station (DAS)
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Concentrator
 A concentrator attaches several SASs to the dual ring 
analogous to MSAU used in 802.5
 If an SAS fails it uses an optical bypass to isolate the
failed SA, thereby keeping the ring connected
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Concentrator (cont’d)
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Dealing With Failures
Cable failure
Station failure
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Optical Bypass Switch
 Provides continuous dual-ring operation if a device on
the dual ring fails.
 Prevent ring segmentation and eliminate failed stations
from the ring.
 The optical bypass switch performs this function using
optical mirrors that pass light from the ring directly to the
DAS device during normal operation.
 If a failure of the DAS device occurs, e.g. a power-off, the
optical bypass switch will pass the light through itself by
using internal mirrors and thereby will maintain the ring's
integrity.
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Optical Bypass Switch (cont’d)
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Dual Homing
 Critical devices, such as
routers or mainframe
hosts, can use a faulttolerant technique called
dual homing to provide
additional redundancy
and to help guarantee
operation.
 In dual-homing situations,
the critical device is
attached to two
concentrators.
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FDDI Physical Characteristics
 Limit to 500 stations in a network, span over 100
km.
 FDDI uses 4B/5B encoding
 Primary ring offers the rate up to 100 Mbps
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Timed Token Algorithm
 Token Holding Time (THT) is calculated the same as that
of 802.5
 Target Token Rotation Time (TTRT): the amount of
time whereby all nodes agree to live within
 Measured TRT (MTRT): the time where each node
measures between successive arrivals of the token
 If MTRT > TTRT, the token is late, the node does not
transmit any data
 If MTRT < TTRT, the token is early, the node is allowed
to hold the token for the difference between TTRT-MTRT
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FDDI Frame Format
 The FDDI frame format is similar to the format of
a Token Ring frame.
 FDDI frames can be as large as 4,500 bytes.
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FDDI Frame Format (cont’d)
 Preamble—Gives a unique sequence that prepares
each station for an upcoming frame.
 Start delimiter—Indicates the beginning of a frame by
employing a signaling pattern that differentiates it from
the rest of the frame.
 Frame control—Indicates the size of the address fields
and whether the frame contains asynchronous or
synchronous data, among other control information.
 Destination address—Contains a unicast (singular),
multicast (group), or broadcast (every station) address.
FDDI destination addresses are 6 bytes long.
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FDDI Frame Format (cont’d)
 Source address—Identifies the single station that sent
the frame. FDDI source addresses are 6 bytes long.
 Data—Contains either information destined for an upperlayer protocol or control information.
 Frame check sequence (FCS)— error detection
 End delimiter—Contains unique symbols; cannot be
data symbols that indicate the end of the frame.
 Frame status—Allows the source station to determine
whether an error occurred; identifies whether the frame
was recognized and copied by a receiving station.
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Timed Token Algorithm (cont’d)
 However, if a node sees the token but it has lots of data
to send, its MTRT > TTRT, it cannot transmit data
 To account for this possibility, FDDI defines 2 classes of
traffic: synchronous and asynchronous
 For synchronous data, a node is allowed to send after
receiving a token, no matter it is early or late
 Synchronous: traffic is delay sensitive, e.g. voice or video
 The total amount of data to send is bound by TTRT
 For asynchronous data, the token must be early
 Asynchronous: more suitable for non-delay-sensitive
data e.g. file transfer
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Timed Token Algorithm (cont’d)
 Question: how a node determines if it can
send asynchronous traffic?
Answer: A node can send if MTRT < TTRT
 Question: what if TTRT is too small so that the
node cannot transmit the full message without
exceeding TTRT?
Answer: the node is allowed to send the frame
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Token Maintenance
 All nodes monitor the time to ensure that the
token has not been lost
 Each node should see a valid transmission: data
frame or the token
 The idle time that each node can experience is
equal to the ring latency plus the time it takes to
send a full frame  normally a bit less than 2.5
ms
 Normally each node sets a timer event to 2.5 ms
 If the timer expires, the node sends a “claim”
frame
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Electing Active Monitor
 Bidding for TTRT  the node with lowest TTRT wins.
This node can hold the token and can transmit a frame
 If nodes have equal TTRTs, the node with higher
address wins
 If a node receives a claim frame, it checks to see if the
TTRT bid in the frame is less than its own.
 If so, the node reset its local TTRT and forward the frame to
the next node
 If its TTRT < the bid TTRT, remove the claim frame and
putting its own claim frame on the ring
 If the claim frame is back to the sender, it can safely claim the
token
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Questions?
Next Lecture
ISDN
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Communications
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