Transcript Document

LOCAL AREA
NETWORS
•
A local area network (LAN) is a data communication
system that allows a number of independent devices to
communicate directly with each other in a limited
geographic area.
•
There are four basic architectures:
1. Ethernet
2. Token Bus
3. Token Ring
4. Fiber Distributed Interface (FDDI)
Ethernet, token bus, token ring are standards of the IEEE and
are part of its project 802; FDDI is an ANSI standard.
Three generations of Ethernet
• The logical link control (LLC) is the upper part sublayer of
the data link layer (logical addresses, control information,
an data)
• Media Access Control (MAC) sublayer resolves the
contention for the shared media.
ETHERNET
Access Method: CSMA/CD
• The Ethernet architecture does not have a regulated access
to the medium.
• 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 the
signals into unusable noise, are called collisions.
• As traffic increases on a multiple-access link, so do
collisions.
Access Method: CSMA/CD
• The access mechanism used in an Ethernet is called
carrier sense multiple access with collision detection
CSMA/CD
• In CSMA/CD system, any workstation wishing to transmit
must first listen for existing traffic on the line. A device
listens by checking for a voltage. If no voltage is detected,
the line is considered idle and the transmission is initiated.
During the data transmission, the station checks the line
for the extremely high voltages that indicate a collision.
• If a collision is detected, the station quits the current
transmission and waits a predetermined amount of time for
the line to clear, then sends its data again.
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Addressing
Each station on an Ethernet network has its own network
interface card (NIC). The NIC usually fits inside the
station and provides the station with a six-byte physical
address. The number on the NIC is unique.
Data Rate
Ethernet LANs can support data rates between 1 and
100Mbps.
Frame Format
•
IEEE 802.3 specifies one type of frame containing
seven fields:
802.3 MAC frame
• Preamble
The preamble contains seven bytes (56 bits) of alternating 0s
and 1s that alert the receiving system to the coming frame
and enable it to synchronize its input timing.
• Start Frame delimiter SFD
The SFD tells the receiver that everything that follows is data,
starting with the addresses.
• Destination Addresses DA
The DA field contains the physical destination address (six
bytes) of the packet’s next destination. If the packet must
across from one LAN to another to reach its destination,
the DA field contains the physical address of the router
connecting the current LAN to the next one.
• Source Address
The SA field contains the physical address ( six bytes) of the
last device to forward the packet. That device can be the
sending station or the most recent router to receive and
forward the packet.
• Length/type of Protocol Data Unit (PDU)
This field contains two bytes indicating the number of bytes
in the PDU. If the length of the PDU is fixed, this field can
be used to indicate type, or as a base for other protocols.
Protocol Data Unit PDU
The PDU is generated by the upper LLC sublayer, then linked
to the 802.3 frame. The PDU can be anywhere from 46 to
1500 bytes long, depending on the type of frame and the
length of the information field.
• CRC
The last field in the 802.3 frame contains the error detection
information, in this case CRC-32.
Implementation
• Ethernet LANs are configured as logical buses, although
they may be physically implemented in bus or star
topologies.
• Each frame is transmitted to every station on the link but
read only by the station to which it is addressed.
Categories of traditional Ethernet
10BASE5: Thick Ethernet
• 10Base5 is a bus topology LAN that uses based signaling
and has a maximum segment length of 500 meters.
• To reduce collisions, the total length of the bus should not
exceed 2500 meters (five segments).
• Also, the standard demands that each station be separated
from each neighbor by 2.5 meters.
10BASE5: Thick Ethernet
• The physical components includes coaxial cable, network
interface cards, transceivers, and attachment unit interface
(AUI) cables.
• RG-8 cable is thick coaxial cable that provides the
backbone of the IEEE802.3 standard.
• Transceiver
Each station is attached by an AUI cable to an intermediary
device called a medium attachment unit (MAU) or
transceiver.
10BASE5: Thick Ethernet
• The transceiver performs CSMA/CD function of checking
for voltages and collisions on the line and may contain an
small buffer. It also serves as the connector that attaches a
station to the thick coaxial cable itself via a tap.
• AUI cables
Each station is linked to its corresponding transceiver by an
attachment unit interface (AUI), also called a transceiver
cable. AUIs are restricted to a maximum length of 50
meters.
10BASE5: Thick Ethernet
• Transceiver tap
Each transceiver contains a connecting mechanism, called a
tap because it allows the transceiver to tap into the line at
any point.
10BASE2:Thin Ethernet
• Like 10Base5, 10Base2 is a bus topology LAN. The
advantages of thin Ethernet are reduced cost and ease of
installation.
• The disadvantages are shorter range (185 meters)
• The NICs in a thin Ethernet system provide all the same
functionality as those in a thick Ethernet, plus the functions
of the transceivers.
• The cable required to implement the 10Base2 standard is
RG-58. These cables are relatively easy to install and
move.
10Base-T: Twisted-Pair Ethernet
• A star-topology LAN using unshielded twisted pair (UTP)
cable instead of coaxial cable.
• It supports a data rate of 10Mbps and has a maximum
length (hub to station) of 100 meters.
• It requires a hub with a port for each station. The hub fans
out any transmitted frame to all to all its connected
stations.
1Base5: Star LAN
• Star-LAN is an AT&T product used infrequently today
because of its slow speed.
• Star-LAN allows as many as 10 stations to be linked, each
to the next, in a chain in which only the lead device
connects to the hub.
10Base-FL: Fiber Link Ethernet
10Base-FL uses a star topology to connect stations to a hub.
The standard is normally implemented using an external
transceiver called fiber-optic MAU. The station is
connected to the external transceiver by an AUI cable. The
transceiver is connected to the hub by using two pairs of
fiber-optic cables.
Switched Ethernet
• The 10Base-T Ethernet is a shared media network, which
means that the entire media is involved in each
transmission. Any frame produced by one station is
retransmitted by the hub to all stations in the network.
Thus, if two stations try to send frames simultaneously,
there is a collision.
Switched Ethernet
• If we replace the hub by a switch, a device that can
recognize the destination address and can route the frame
to the port to which the destination station is connected, the
rest of the media are not involved in the transmission
process.
Full-duplex switched Ethernet
•One of the limitations of 10Base5 and 10Base2 is that the communication is halfduplex.
•In Full-duplex switched Ethernet each station is connected to the switch
through two links: one to transmit and one to receive.
•The speed passes from 10Mbps to 20Mbps.
•There is no need for the CSMA/CD method.
Fast Ethernet
• Fast Ethernet operates to 100 Mbps.
• Star topology
100Base-TX
• The 100Base-TX design uses two category 5 unshielded
twisted-pair (UTP) or two shielded twisted-pair (STP)
cables to connect a station to the hub.
One pair is used to carry frames from the station to the hub
and the other to carry frames from the hub to the station.
• The distance between the station and the hub should be
less than 100 meters.
Fast Ethernet implementations
Fast Ethernet
100Base-FX
• The 100Base-FX design uses two optical fibers, one to
carry frames from the station to the hub and the other from
the hub to the station.
• The distance between the station and the hub should be
less than 2000 meters.
100Base-T4
• It requires four pairs of category 3 (voice grade) UTP.
Fast Ethernet
100Base-T4
• Two of the four pairs are bidirectional; the
other two are unidirectional. The 100
Mbps flow of data is divided into three
33.66-Mbps flows.
Gigabit Ethernet
•
•
•
It supports a data rate of 1 Gbps (1000
Mbps)
Usually serves as a backbone to connect
Fast Ethernet Networks.
Four implementations have been designed
for Gigabit Ethernet: 1000Base-LX,
100Base-SX, 1000Base-CX and
1000Base-T.
Token Bus
• Local area networks have a direct application in factory
automation and process control. In this case the nodes are
computers controlling manufacturing process.
• Ethernet (IEEE802.3) is not a suitable protocol for these
purposes because the number of collisions is not
predictable and the delay in sending data from a control
center (a computer in the network) to other devices or
computers along the “assembly line” is not a fixed value.
Token Bus
• Token Bus (IEEE 802.4) combines features of Ethernet (a
bus topology) and Token ring.
• Token bus is a physical bus that operates as a logical ring
using tokens.
• Stations are logically organized into a ring. A token is
passed among the stations. If a station wants to send data,
it must wait and capture the token.
• Token Bus is limited to factory automation and process
control and has no commercial applications in data
communication.
Token Ring (IEEE 802.5)
• Token ring requires that stations take turns sending data.
• Each station may transmit only during its turn and may
send only one frame during each turn.
• The mechanism that coordinates this rotation is called
token passing.
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Figure 12-15
Token Passing
Figure 12-15-continued
Token Passing
Figure 12-15-continued
Token Passing
Figure 12-15-continued
Token Passing
Token Ring Frame
52
Token Ring (IEEE 802.5)
Access Method: Token passing
• Whenever the network is unoccupied, it circulates a simple
three-byte token.
• This token is passed from NIC to NIC in sequence until it
encounters a station with data to send.
• That station waits for the token to enter its network board.
If the token is free the station may send a data frame.
• This data frame proceeds around the ring regenerated by
each station.
Token Ring (IEEE 802.5)
• Each intermediate station examines the destination address,
if the frame is addressed to another station, the station
relays it to its neighbor.
• If the station recognizes its own address, copies the
message, checks for errors, and changes four bits in the
last byte of the frame to indicate address recognized and
frame copied.
• The full packet then continues around the ring until it
returns to the station that sent it.
Token Ring (IEEE 802.5)
• The sender receives the frame and recognizes itself in the
source address field. It then checks the address- recognized
bits. If they are set, it knows that the frame was received.
• The sender then discards the used data frame and releases
the token back to the ring.
Priority and reservation
• A busy token can be reserved by a station waiting to
transmit regardless of that station’s location on the ring.
• Each station has a priority code. As a frame passes by, a
station waiting to transmit it may reserve the next open
token by entering its priority code in the access control
(AC) field of the token or data frame.
• A station with a higher priority may remove a lower
priority reservation and replace it with its own.
Monitor stations
Several problems may occur to disrupt the operation of a
token ring network.
1. A station may neglect to retransmit a token
2. A token may be destroyed by noise
3. A sending station may not release the token once its turn
has ended
4. A sending station may neglect to remove its used data
frame from the ring
In the cases 1,2,3 there is no token on the ring and no station
may send data.
To handle these situations, one station on the ring is
designated as monitor station.
• The monitor sets a timer each the token passes. If the token
does not reappear in the allotted time, it is presumed to be
lost and the monitor generates a new token and introduces
it into the ring.
• The monitor guards against perpetually recirculating data
frames by setting a bit (status bit) in the AC (access
control) field of each frame.
• If the status bit has been set, it knows that the packet has
already been around the ring and should be discarded. The
monitor destroys the frame and puts a token into the ring.
Addressing
Token ring uses a six-byte address, which is imprinted on the
NIC card.
Data Rate
Token ring supports data rates of up to 16 Mbps.
Frame formats
The token ring protocol specifies three types of frames:
1. Data/command Frames
2. Token Frames
3. Abort Frames
• The data/command frame is the only one of the three types
of frames that can carry a PDU and is the only addressed to
a specified destination.
• The nine fields of the frame are start delimiter (SD),
access control (AC), frame control (FC), destination
address (DA), source address (SA), PDU, CRC, en
delimiter (ED), frame status (FM).
• The AC field is one byte long and includes four subfields:
Priority ( 3 bits), Token ( 1 bit), Monitor (1bit), reservation
(3bits)
• Token bit: 1 indicates that the frame is a data/command
frame; 0 indicates a token or abort frame.
• The frame control (FC) field is one byte long and contains
two fields: type and special information.
•
Type is one bit field used to indicate the type of
information contained in the PDU ( control information or
data).
• Special information field is a 7 bits field. This field
contains information used by the Token ring logic.
• The last byte of the frame is the Frame status field (FS).
It can be set by the receiver to indicate that the frame has
been read, or by the monitor to indicate that the frame has
already been around the ring.
Token frame
• This frame includes only three fields: the SD, AC, and ED.
The SD indicates that a frame is coming. The AC indicates
that the frame is a token and includes the priority and
reservation fields.
Abort frame
An abort frame carries no information at all, just starting and
ending delimiters. It can be generated either by the sender
to stop its own transmission or by the monitor to purge an
old transmission from the line.