1. Local Area Networks

8/13/2019 1. Local Area Networks

1/48

BROADBAND DIGITAL NETWORKS

By Aftab A. Memon

Mehran University of Engineering and Technology,Jamshoro.

10TL-BATCHThis class will meet at: 10.00a.m.-12.00 noon (Tuesdays & Wednesdays)9.30 a.m. 11.00 a.m. (during Ramadan)

Today's Lecture:

Local Area Networks Overview


2/48

LAN (Local Area Networks)

A LAN consists ofShared transmission medium

now so valid today due to switched LANs

set of hardware and software for the interfacing

devices

regulations for orderly access to the medium


3/48

Why High Speed LANs?

Office LANs used to provide basic connectivityConnecting PCs and terminals to mainframes and

midrange systems that ran corporate applications

Traffic patterns were light

Emphasis was on file transfer and electronic mail Speed and power of PCs has risen

Graphics-intensive applications and GUIs

Client/server computing is now dominant

architecture in business environment Computing over network

Frequent transfer of large volumes of data


4/48

LAN Protocol Architecture

Corresponds to lower two layers of OSI model IEEE 802 reference model

Logical link control (LLC)

Media access control (MAC)Physical


5/48

IEEE 802 Protocol Layers vs.

OSI Model


6/48

IEEE 802 Layers - Physical

Signal encoding/decoding Preamble generation/removal

for synchronization

Bit transmission/reception Specification for transmission medium and

topology


7/48

802 Layers - Medium Access

Control & Logical Link Control

OSI layer 2 (Data Link) is divided into two in 802 Logical Link Control (LLC) layer Medium Access Control (MAC) layer

MAC layerAssembly of data into frame with address and error detection

fields (for transmission) Disassembly of frame (on reception)

Address recognition

Error detection

Govern access to transmission medium

Not found in traditional layer 2 data link control

LLC layer Interface to higher levels

flow control

Based on classical Data Link Control Protocols


8/48

LAN Protocols in Context


9/48

Generic MAC & LLC Format

Actual format differs from protocol to protocol MAC layer receives data from LLC layer

MAC layer detects errors and discards frames

LLC optionally retransmits unsuccessful frames


10/48

LAN Topologies

Bus Ring

Star


11/48

Bus Topology Stations attach to linear medium (bus)

Via a tap - allows for transmission and reception

Transmission propagates in medium in both directions

Received by all other stations

Terminator absorbs frames at end of medium

Need to identify target station Each station has unique address

Destination address included in frame header

Need to regulate transmission To avoid collisions

If two stations attempt to transmit at same time, signals will overlapand become garbled

To avoid continuous transmission from a single station. If one stationtransmits continuously access blocked for others

Solution: Transmit Data in small blocks frames


12/48

Frame Transmission - Bus LAN


13/48

Ring Topology

Repeaters joined by point-to-point links in closed loopReceive data on one link and retransmit on another

Links unidirectional

Stations attach to repeaters Data transmitted in frames

Circulate past all stations

Destination recognizes address and copies frame

Frame circulates back to source where it is removed

Medium access control determines when stationcan insert frame


14/48

Frame

Transmission

Ring LAN


15/48

Star Topology

Each station connecteddirectly to central nodeusing a full-duplex

(bi-directional) link

Central node can broadcast (hub)Physical star, logically bus

Only one station can transmit at a time

Central node can act as frame switchretransmits only to destination

todays technology


16/48

Medium Access Control (MAC)

In LANs data is broadcastthere is a single medium shared by different users

We need MAC sublayer fororderly and efficient use of broadcast medium

This is actually a channel allocation problem Synchronous (static) solutions

everyone knows when to transmit

Asynchronous (dynamic) solutionin response to immediate needsTwo categories

Round robin

Contention


17/48

Dynamic Channel Allocation

Categories

ContentionAll stations contend to transmit

No control to determine whose turn is it

Stations send data by taking risk of collision (with

others packets) however they understand collisions by listening to the

channel, so that they can retransmit

There are several implementation methods

In general, good for bursty traffic which is the typical traffic types for most networks

Efficient under light or moderate load

Performance is bad under heavy load


18/48

Ethernet (CSMA/CD)

Carriers Sense Multiple Access with CollisionDetection

is the underlying technology

Xerox Ethernet (1976) by Metcalfe

IEEE 802.3 standard (1983)

Contention technique that has basis in famous

ALOHA network


19/48

ALOHA

Packet Radio (applicable to any shared medium) initially proposed to interconnect Hawaiian Islands

by Norman Abramson of Univ. of Hawaii (early 70s)

When station has frame, it sends

collisions may occur Station listens for max round trip time

If no collision, fine. If collision, retransmit after arandom waiting time

Max channel utilization is 18% - very bad


20/48

Slotted ALOHA

Divide the time into discrete intervals (slots) equal to frame transmission time

need central clock (or other sync mechanism)

transmission begins at slot boundary

Collided frames will do so totally or will not collide

Algorithm If a node has a packet to send, sends it at the beginning of the

next slot

If collision occurred, retransmit at the next slot with aprobability p

Max channel utilization is 37% doubles Normal ALOHA, but still low


21/48

CSMA (Carrier Sense Multiple

Access)

First listen for clear medium (carrier sense) If medium idle, transmit

If busy, continuously check the channel until it is idleand then transmit

If collision occurs Wait random time and retransmit

Collision probability depend on the propagation delay Longer propagation delay, worse the utilization

Collision occurs even if the propagation time is zero. WHY?

1-persistent CSMA

Better utilization than ALOHA


22/48

CSMA/CD (IEEE 802.3 Ethernet)

With CSMA, collision occupies medium forduration of transmissionit is inefficient to complete the transmission of a

collided packet

As in 1-persistent CSMAIf medium idle, transmit

If busy, listen for idle, then transmit

Stations listen while transmitting

If collision detected (due to high voltage onbus), cease transmission and wait random timethen start again random waiting time is determined using binary

exponential backoff mechanism


23/48

CSMA/CD

Operation


24/48

Binary exponential back off

random waiting period but consecutive collusionsincrease the mean waiting time mean waiting time doubles in the first 10 retransmission

attempts

after first collision, waits 0 or 1 slot time

if collided again (second time), waits 0, 1, 2 or 3 slots if collided for the ith time, waits 0, 1, , or 2i-1 slots

the randomization interval is fixed to 0 1023 after 10th

collision

station tries a total of 16 times and then gives up if cannot

transmit low delay with small amount of waiting stations

large delay with large amount of waiting stations


25/48

IEEE 802.3 Frame Format

>= >=

Preamble is alternating 0s and 1s (for clock synchronization)

SFD is 10101011

FCS excludes Preamble and SFD

Addresses are uniquely assigned by IEEE to manufacturers.

Why unique?


26/48

Interconnection Elements in

LANs

Bridges Hubs

Switches


27/48

Bridge Operation Example


28/48

Bridge Protocol Architecture

IEEE 802.1D operates at MAC level

Station address is at this level

Bridge does not need LLC layer


29/48

Shared Medium Hub

Central hub Hub retransmits incoming signal to all outgoing

lines

Only one station can transmit at a time With a 10Mbps LAN, total capacity is 10Mbps


30/48

Layer 2 Switches

Central repeater acts as switch Incoming frame switches to appropriate

outgoing lineUnused lines can be used to switch other traffic

More than one station transmitting at a timeEach device has dedicated capacity equal to the LAN

capacity, if the switch has sufficient capacity for all


31/48

Types of Layer 2 Switch

Store and forward switchAccept input, buffer it briefly, then output

Cut through switch

Take advantage of the destination address being atthe start of the frame

Begin repeating incoming frame onto output line assoon as address recognized

May propagate some bad frames

WHY?

2


32/48

Problems with Layer 2

Switches (1)

As number of devices in LANs grows, layer 2 switchesshow some limitations

Broadcast overload

In LANs some protocols (e.g. ARP) work in broadcast manner

Lack of multiple links

Set of devices and LANs connected by layer 2 switchesshare common MAC broadcast address

If any device issues broadcast frame, that frame is delivered toall devices attached to network connected by layer 2 switches

and/or bridges In large network, broadcast frames can create a significant

overhead

P bl ith L 2


33/48

Problems with Layer 2

Switches (2) and Solution

Current standards for bridge protocols dictateno closed loopsOnly one path is allowed between any two devices

Limits both performance and reliability.

Solution: break up network into subnetworksconnected by routers (that operate at IP layer)MAC broadcast frame limited to devices and switches

contained in single subnetwork

IP-based routers employ sophisticated routingalgorithms Allow use of multiple paths between subnetworks going

through different routers

P bl ith R t d


34/48

Problems with Routers and

Layer 3 Switches

Routers are designed to be implemented at the gatewayand only process packets to/from outer networks

outside traffic is less than the internal traffic

High-speed LANs and high-performance layer 2 switches pumpmillions of packets per second

the same router may create a performance bottleneck in theheart of a LAN

Solution: layer 3 switches

Implement packet-forwarding logic of router in hardware

faster Two categories

Packet by packet

Flow based


35/48

Layer 3 Switch Categories

Packet by packetOperates in same way as traditional router

but much faster

Flow-based layer 3 switch tries to enhanceperformance by identifying flows of IP packetsthat have same source and destination

Done by observing ongoing traffic or using a special

flow label in packet header (IPv6)Once flow is identified, predefined route can be

established to speed-up the forwarding process

T i l L l N t k


36/48

Typical Local Network

Configuration


37/48

Gigabit Ethernet Physical

1000Base-SXShort wavelength, multimode fiber

1000Base-LX

Long wavelength, Multi or single mode fiber

1000Base-CX

A special STP (


38/48

Gigabit Ethernet Medium

Options (Log Scale)


39/48

10-Gbps Ethernet Data Rate and

Distance Options (Log Scale)


40/48

Fibre Channel - Background

I/O channel Direct point to point or multipoint comms. link

Hardware based

High Speed

Very short distance

User data moved from source buffer to destination buffer

Network connection

Interconnected access points

Software based protocol

Flow control, error detection & recovery

End systems connections


41/48

Fibre Channel

Best of both technologies Channel oriented

Data type qualifiers for routing frame payload

Link level constructs associated with I/O ops

Protocol interface specifications to support existingI/O architectures

e.g. SCSI

Network oriented

Full multiplexing between multiple destinationsPeer to peer connectivity

Internetworking to other connection technologies


42/48

Fibre Channel Requirements

Full duplex links with two fibers per link 100 Mbps to 800 Mbps on single line Full duplex 200 Mbps to 1600 Mbps per link

Up to 10 km

Small connectors

High-capacity utilization, distance insensitivity Greater connectivity than existing multidrop channels

Broad availability i.e. standard components

Multiple cost/performance levels

Small systems to supercomputers Carry multiple existing interface command sets for existing channel

and network protocols

Uses generic transport mechanism based on point-to-point links anda switching network

Supports simple encoding and framing scheme

In turn supports a variety of channel and network protocols


43/48

Fibre Channel Elements

End systems - Nodes Switched elements - the network or fabric

Communication across point to point links


44/48

Fibre Channel Network


45/48

Fibre Channel Physical Media

Provides range of options for physical medium,the data rate on medium, and topology ofnetwork

Shielded twisted pair, video coaxial cable, andoptical fiber

Data rates 100 Mbps to 3.2 Gbps

Point-to-point from 33 m to 10 km


46/48

Fibre Channel Fabric

General topology called fabric or switched topology Arbitrary topology includes at least one switch to

interconnect number of end systems

May also consist of switched network

Some of these switches supporting end nodes Routing transparent to nodes

Each port has unique address

When data transmitted into fabric, edge switch to which nodeattached uses destination port address to determine location

Either deliver frame to node attached to same switch ortransfers frame to adjacent switch to begin routing to remotedestination


47/48

Fabric Advantages

Scalability of capacityAs additional ports added, aggregate capacity of networkincreases

Minimizes congestion and contention

Increases throughput

Protocol independent Distance insensitive

Switch and transmission link technologies may changewithout affecting overall configuration

Burden on nodes minimized Fibre Channel node responsible for managing point-to-pointconnection between itself and fabric

Fabric responsible for routing and error detection

Five Applications of Fibre


48/48

Five Applications of Fibre

Channel

1. Local Area Networks

Documents