Transcript EEE449 Computer Networks - Universiti Sains Malaysia

EEE449 Computer Networks
Local Area Network (LAN)
LAN
• Topology
• The way in which the end points attached to the
network are interconnected
• Common topologies are bus, tree, ring and star
LAN Topologies
LAN Topology
• Bus
– Use of a multipoint medium
– All stations are attached directly to a linear
transmission medium through a tap
– Full duplex operation between the station and the tap
allows data to be transmitted onto the bus and
received from the bus
– A transmission from any station propagates the length
of the medium in both directions and can be received
by all other stations
– At each end of the bus is a terminator which absorbs
any signal and remove it from the bus
LAN Topology
• Tree
– The transmission medium is a branching cable with
no closed loops
– The layout begins at a point known as the headend
– One or more cables start at the headend, and each of
these may have branches
– The branches in turn may have additional branches
– A transmission from any station propagates
throughout the medium and can be received by all
other stations
LAN Topology
• Ring
– The network consists of a set of repeaters joined by
point-to-point links in a closed-loop
– Links are unidirectional
– Each stations attached to the network at a repeater
and transmit data onto the network through the
repeater
– A transmission circulates past all the other stations
until it returns to the source station, where it is
removed
– Need medium access control
LAN Topology
• Star
– Each station is directly connected to a
common central node typically via two pointto-point links
Transmission
Transmission
LAN interconnections
• Bridges and routers interconnect LANs and
connect LAN to WAN
• Bridge for interconnecting LANs
• Routers – interconnecting variety of LANs and
WANs
• Bridge
– Between LANs that use identical protocols for the
physical and link layers
• reasons for use: reliability, performance,
security, geography
LAN interconnect
LAN interconnections
• Bridge
– no modification to frame content or format
– no encapsulation
– exact bitwise copy of frame
– Large buffer space for minimal buffering to
meet peak demand
– contains routing and address intelligence
– may connect more than two LANs
– bridging is transparent to stations
LAN interconnections
• Router
– Connect two networks that may or may not be
similar
– Employs internet protocol
– Network layer (layer 3) device
– More later
LAN interconnections
• Hubs
– active central element of star layout
– each station connected to hub by two lines
– hub acts as a repeater
– limited to about 100 m
– optical fiber may be used out to 500m
– Can have multiple levels involving a header
hub and intermediate hubs
LAN interconnections
• Layer 2 switches
–
–
–
–
Has replaced hub in popularity particularly for high-speed LANs
aka a switching hub
Multiplying capacity of LAN
store-and-forward switch
• accepts frame on input line, buffers briefly, routes to destination port
• see delay between sender and receiver
• better integrity
– cut-through switch
• use destination address at beginning of frame
• switch begins repeating frame onto output line as soon as
destination address recognized
• highest possible throughput
• risk of propagating bad frames
LAN interconnections
LAN interconnections
• Layer 2 switches
– no change to attached devices to convert bus
LAN or hub LAN to switched LAN
• e.g. Ethernet LANs use Ethernet MAC protocol
– have dedicated capacity equal to original LAN
– scales easily
• additional devices attached to switch by increasing
capacity of layer 2
LAN interconnections
• Layer 3 switch
– Implements packet-forwarding function of the router in
hardware
– packet by packet
• operates like a traditional router
– flow-based switch
• enhances performance by identifying flows of IP packets with
same source and destination
• by observing ongoing traffic or using a special flow label in
packet header (IPv6)
• a predefined route is used for identified flows to speed up
flow
LAN interconnections
LAN Protocol
LAN Protocol
• Includes physical, MAC and LLC layers
• Physical layer –
– Encompasses topology and transmission medium
• MAC
– Control access to the medium for an orderly and
efficient use of the capacity
– Centralised or decentralised control
– Synchronous (dedicated capacity) or asynchronous
(on demand)
– Synchronous access is not suitable for LAN due to
unpredictable station demand
LAN Protocol
• MAC
– Asynchronous access can be based on round
robin, reservation or contention
• Round robin
– Efficient when many stations have data to
transmit over an extended period of time
– Considerable overhead in passing turns when
only few stations transmit
LAN Protocol
• Reservation
– Time on the medium is divided into slots
– Stations can reserved future slots
– Suitable for streaming
• Contention
– All stations contend for the time slots
– Suitable for bursty traffic
LAN Protocol
• LLC
– transmission of link level PDUs between
stations
– must support multiaccess, 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
LAN protocol
Bridge protocol
•
•
•
•
IEEE 802.1D
Station address is designated at the MAC level
bridge does not need LLC layer
can pass frame over external comms system
–
–
–
–
–
capture frame
encapsulate it
forward it across link
remove encapsulation and forward over LAN link
e.g. WAN link
Bridge protocol
High-Speed LANs
– Ethernet (IEEE 802.3 10-Mbps)
– Fast Ethernet (IEEE 802.3 100-Mbps)
– Gigabit Ethernet
– 10-Gbps Ethernet
Ethernet
• most widely used LAN standard
• developed by IEEE 802.3
• IEEE 802.3 MAC – use CSMA/CD
– Station continues to listen to the medium while transmitting
– If the medium is idle, transmit
– Otherwise if the medium is busy, continue to listen until the
channel is idle and then transmit immediately
– 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
– After transmitting the jamming signal, wait a random amount of
time (Backoff) then attempt to transmit again
Ethernet
Ethernet
Preamble: A 7-octet pattern of alternating 0s and 1s used by the receiver to establish
bit synchronization.
Start Frame Delimiter (SFD): The sequence 10101011, which indicates the actual
start of the frame and enables the receiver to locate the first bit of the rest of the frame.
Destination Address (DA): Specifies the station(s) for which the frame is intended.
It may be a unique physical address, a group address, or a global address.
Source Address (SA): Specifies the station that sent the frame.
Length/Type: Length of LLC data field in octets, or Ethernet Type field,
LLC Data: Data unit supplied by LLC.
Pad: Octets added to ensure that the frame is long enough for proper CD operation.
Frame Check Sequence (FCS): A 32-bit cyclic redundancy check, based on all fields
except preamble, SFD, and FCS.
Ethernet IEEE 802.3 10Mbps
10BASE5
10BASE2
10BASE-T
10BASE-FP
Transmission
medium
Coaxial cable (50
ohm)
Coaxial cable (50
ohm)
Unshielded twisted
pair
850-nm optical fiber
pair
Signaling
te chnique
Baseband
(Manchester)
Baseband
(Manchester)
Baseband
(Manchester)
Manchester/on-off
Topology
Bus
Bus
Star
Star
Maximum se gment 500
length (m)
185
100
500
Nodes pe r se gment 100
30
—
33
5
0.4 to 0.6
62.5/125 µm
C able diamete r
(mm)
10
<data rate in Mbps> <signaling method><max segment length in hundreds of meters>
Ethernet IEEE 802.3 10Mbps
• Alternatives for 10-Mbps are:
• 10BASE5: Specifies the use of 50-ohm coaxial cable and
Manchester digital signaling. The maximum length of a
cable segment is set at 500 meters. Can extend using up to
4 repeaters.
• 10BASE2: lower-cost alternative to 10BASE5 using a
thinner cable, with fewer taps over a shorter distance than
the 10BASE5 cable.
• 10BASE-T: Uses unshielded twisted pair in a star-shaped
topology, with length of a link is limited to 100 meters. As
an alternative, an optical fiber link may be used out to 500
m.
• 10BASE-F: Contains three specifications using optical fibre
Fast Ethernet (IEEE 802.3 100Mbps)
•a low-cost, Ethernet-compatible LAN operating at 100 Mbps
•All of the 100BASE-T options use the IEEE 802.3 MAC protocol and frame format.
100BAS E-TX
100BAS E-FX
100BAS E-T4
Transmi ssion
medium
2 pair, STP
2 pair, Category
5 UTP
2 optical fibers
4 pair, Category
3, 4, or 5 UTP
S ignali ng
te ch n ique
MLT-3
MLT-3
4B5B, NRZI
8B6T, NRZ
Data rate
100 Mbps
100 Mbps
100 Mbps
100 Mbps
Maxi m um
segm en t l en gth
100 m
100 m
100 m
100 m
Network span
200 m
200 m
400 m
200 m
Fast Ethernet (IEEE 802.3 100Mbps)
•100BASE-X refers to a set of options that use two physical links between nodes;
one for transmission and one for reception.
•100BASE-TX makes use of shielded twisted pair (STP) or
high-quality (Category 5) unshielded twisted pair (UTP).
•100BASE-FX uses optical fiber.
•For all of the 100BASE-T options, the topology is similar to that of 10BASE-T,
namely a star-wire topology.
Gigabit Ethernet
• defines a new medium and transmission specification
• retains the CSMA/CD protocol and Ethernet format
of its 10-Mbps and 100-Mbps predecessors.
• compatible with 100BASE-T and 10BASE-T,
preserving a smooth migration path.
• As more organizations move to 100BASE-T, putting
huge traffic loads on backbone networks, demand for
Gigabit Ethernet has intensified.
Gigabit Ethernet
A 1-Gbps switching hub provides backbone connectivity for central servers and
high-speed workgroup hubs.
Each workgroup LAN switch supports both 1-Gbps links, to connect to the backbone
LAN switch and to support high-performance workgroup servers, and 100-Mbps links,
to support high-performance workstations, servers, and 100-Mbps LAN switches.
Gigabit Ethernet
Gigabit Ethernet
• 1000BASE-SX: This short-wavelength option supports duplex links of up
to 275 m using 62.5-µm multimode or up to 550 m using 50-µm multimode
fiber. Wavelengths are in the range of 770 to 860 nm.
• • 1000BASE-LX: This long-wavelength option supports duplex links of up
to 550 m of 62.5-µm or 50-µm multimode fiber or 5 km of 10-µm singlemode fiber. Wavelengths are in the range of 1270 to 1355 nm.
• • 1000BASE-CX: This option supports 1-Gbps links among devices
located within a single room or equipment rack, using copper jumpers
(specialized shielded twisted-pair cable that spans no more than 25 m).
Each link is composed of a separate shielded twisted pair running in each
direction.
• • 1000BASE-T: This option makes use of four pairs of Category 5
unshielded twisted pair to support devices over a range of up to 100 m.
10-Gbps Ethernet
Higher-capacity backbone pipes will help relieve congestion for workgroup switches,
where Gigabit Ethernet uplinks can easily become overloaded, and for server farms,
where 1-Gbps network interface cards are already in widespread use.
The goal for maximum link distances cover a range of applications: from 300 m to 40 km