Components of DATA and INTERNET NETWORKING MODULE (MSc...

LANs, High Speed LANs & LAN Interconnections, 2002, Dr. Z. Sun 1

Components of DATA and INTERNET NETWORKING MODULE

(MSc EEM.din & Linked UG EE4.din)

LANs, High Speed LANs and LAN Interconnections

Dr Zhili SUNUniversity of Surrey

GuildfordSurrey

GU2 7XHTel: 01483 68 9493Fax: 01483 68 6011

Email: [email protected]


Sections

� 1. Local Area Networks (LANs): IEEE 802 standard LANs: Ethernet (803.3), Token Ring (802.5), Token Bus (802.4), MAC sublayer, LLC sublayer.

� 2. High Speed LANs (HSLANs): Fast Ethernet (IEEE802.3u), Hub, Switched Ethernet, gigabit Ethernet (IEEE802.3z), FDDI,

� 3. Wireless NetworksWireless LAN (IEEE803.11a, 11b & 11g), Broadband Wireless (IEEE802.16), Bluetooth (IEEE802.15)

� 4. LAN interconnection: Repeaters; Bridges - Transparent and Source Routing; Spanning Tree Algorithm, internetworking different LAN types, VLAN (IEEE802.1Q)

� Recommended text books


1. Local Area Networks (LANs)


Introduction

� LAN classification� IEEE 802 standard LANs:

• Ethernet (802.3) • Token Ring (802.5) • Token Bus (802.4)

� Medium Access Control (MAC) sublayer� Logical Link Control (LLC) sublayer


LAN Classification

Classifications are based on:� Physical Transmission Medium: Shielded/Unshield

Twisted Pair (STP/UTP) 10baseT, Co-axial Cable (10base2 thin 0.25 and 10base5 thick 0.5 inch diameter), optical Fibre 10baseF, wireless,

� Topology: Bus, Ring, Tree/Hub, Star

� Media Access Control: CSMA/CD, control token, fixed slots

� Standards bodies: IEEE, ISO


Standards - IEEE 802.x (ISO 8802.x)

� At start of LAN development many incompatible networks were developed

� In the early 1980’s an IEEE committee examined the various types, classified them and proposed standards to remove much of the incompatibility.

� LAN types thus became known by the IEEE 802 classification e.g. 802.3, 802.4, 802.5 ...

� ISO adopted IEEE 802 as ISO 8802


The framework of IEEE 802 Standards

802.1 Architecture, Management & Internetwork

802.2 Link Services

802.3 802.4 802.5 802.6 802.9

CSM

A/C

D

Tok

en B

us

Tok

en R

ing

MA

Ns

Inte

grat

ed

voic

e/da

ta

802.7 Broadband

LANs

802.8 FDDI

802.10 Secure LANs


IEEE 802 relevant to LANs� 802.1 — High layer LAN Protocol: Overview & architecture, bridging, VLAN� 802.2 — Logical Link Control services Working Group (WG) (inactive)� 802.3 — Ethernet (CSMA/CD) WG: access method & physical signalling� 802.4 — Token Bus WG: access method & physical signalling (inactive)� 802.5 — Token Ring WG: access method & physical signalling � 802.6 — Metropolitan Area Network - MAN (DQDB): WG (inactive)� 802.7 — Broadband Technical Advisory Group (TAG) (inactive)� 802.8 — Fiber-Optic Technical Advisory Group (FDDI-II): � 802.9 — Isochronous LAN WG: Integrated Service LAN� 802.10 — LAN Security Architecture WG: for all IEEE 802 at various level� 802.11 — Wireless LAN (HiperLAN in ETSI): access method & physical signalling � 802.12 — Demand priority WG. 802.13: not used. � 802.14 — Cable modem WG. 802.15 — Wireless personal area network WG. � 802.16 — Broadband wireless access Study Group (SG)� QoS / Flow control study group


LAN Protocol and OSI reference model

Application

Presentation

Session

TransportNetwork

Data Link

Physical

Logical Link Control

Medium Access Control

Physical

Service Access Points


Functions of LLC, MAC and PHY layers

� Logic Link Control (LLC) - Provide one or more Service Access Points (logical interfaces between adjacent layers)

� Media Access Control (MAC) - On transmission assemble data into frames with address and CRC; On reception disassemble frame and perform address recognition and CRC validation; and Manage communication over link

� Physical layer - Encoding/decoding of signals; Preamble generation/removal (for synchronization); and bit transmission/reception.


Topologies

Bus

Tree

Ring

Hub/Switch


Medium access control (MAC) techniques

� Round robin: With round robin, each station in turn is given the opportunity to transmit. During that opportunity, the station may decline to transmit or may transmit subject to a specified upper bound (maximum amount of data or time)

� Reservation: For stream traffic, reservation techniques are well suited.

� Contention: For bursty traffic, contention techniques are usually appropriate


Standardised MAC techniques

CSMA/CD (802.3)CSMA/CD (802.3)CSMA (802.11)

Contention

DQDB (802.6)Reservation

Request/priority (802.12)

Token ring (802.5, FDDI)

Token bus (402.4)Polling (802.11)

Round robin

Switched technology

Ring technology

Bus technology


Aloha and Slotted Aloha

Contention system:�Transmit whenever have data �Listen to the channel to see if the frame is OK�If not, back-off and re-transmit

� For slotted Aloha, transmit only at beginning of the slot to improve performance


The channel efficiency

� Throughput S = GP0

� Poisson distribution:P[k] = Gk e-G / k!

In 2 frame interval, the number of frames generated is 2G, thus P0 = e -2G

=> S = G e -2G

� Max. throughputS = 1/(2e), when G=1/2

� For slotted Alohathe vulnerable period is 1 frame period

(halved), thus P0 = e -G

=> S = G e -G


Carrier Sense Multiple Access (CSMA)

� 1-persistent: the station listens before sending. If the channel is busy, it waits until it idle. Transmit when the channel is idle. if collision, the station waits a random amount of time and start all over again

� non-persistent: If busy, the station does not continually sense. Instead, waiting for a random period, then repeating the algorithm

� p-persistent: It applies to slotted channel. If it is idle, it transmits with probability of p.


CSMA Persistence and Back-off

Channel Busy

Constant or Variable Delay

Ready p-persistent: transmit as soon as channel goes idlewith a probability p, otherwisedelay one slot and repeat process

1-persistent: transmitas soon as channel goes idle.If collision, back-off and try again.

non-persistent:transmit if idle, otherwise, delay and try again

Slot time


CSMA with Collision Detection (CD)

� Further improvement than persistent and non-persistence over Aloha, by aborting transmission as soon as stations detect a collision

� Contention period is 2τ where τ is propagation delay� Example: for a 1 km cable, the τ is about 5 µs (micro-

second)� Ethernet is one of this version� No MAC-sublayer protocol guarantees reliable delivery


Ethernet System

� The CSMA/CD system (as defined by IEEE 802.3) is based upon the original Ethernet specification defined by Xerox. The two are not identical.

CarrierSenseMultipleAccess CSMA/CDwithCollisionDetection

Attachment Unit Interface (AUI)

PC PC

Channel

PCTransceiver Medium Access Unit (MAU)

with network interface card (LANCE)


Ethernet Cabling

The most common kinds of Ethernet cabling.


Physical layer signal encoding

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.


Ethernet Frame Format

Preamble Destination address

Source address Data Pad Checksum

7 1 2 or 6 2 or 6 2 46–1500 0–46 4Bytes

Start of frame

delimiter

Length of LLC data fieldor Ethernet type field

Preamble: Receiver synchronization (10101011)Destination address: Identifies intended receiver Source address: Hardware address of sender Length/Type: Type of data carried in frameData: Frame payload CRC: 32-bit CRC code


� Retransmission scheduling uses the Truncated Binary Exponential Back-off algorithm

� On the Nth retransmission attempt the station chooses to retransmit after waiting a random integer number of ‘slot’ times, R, where:

0 <= R <= 2k

K = min(N, Back-off limit)

� Bit rate 10 Mbps� Slot time 512 bit times� Interframe gap 9.6 µs� Attempt limit 16� Backoff limit 10� Jam size 32 bits� Max frame size 1518 octets� Min frame size 64 octets

Back-off algorithm & Operational Parameters


ReceiveTransmit

Wait for a frame to transmit. Format frame for transmission.

Carrier sense signal on?

Wait interframe gap time. Start transmission.

Collision detected?

Complete transmission and set status as done.

Set status as attempt limit exceeded. Attempt limit reached?

Compute and wait backoff time

Transmit jam sequence. Increment attempts count.

yn

n

y

y

n

Incoming signal detected?

Set carrier sense signal on. Get bit sync and wait for SFD.

Receive frame.

FCS and frame size OK?

Destination address matches own or group address?

Pass frame to higher layer for processing.

Discard frame.

n

n

y

y

y

n

CSMA/CD Operational Parameters


Token Ring

� The Token Ring is described by the IEEE 802.5 specification. This is based upon the original work by IBM and now defines both 4Mbps and 16 Mbps rings.

� Suitable for real time� Delimiter, access control,

frame control � Sources, destination address,

and checksum are the same as the IEEE 802.3 PC

PCPC

SD EDAC

Token


Token ring


Token Ring Frame Formats

DASD AC FC SA DATA FCS ED FS

SD AC ED

JK 0J K 00 0

JK1JK1 I E

PPP RRRT M

FF ZZZZZZ

ACxx ACxx

1 1 1

1 1 1 2 or 6 2 or 6 < 5000 4 1 1

Bytes

Start delimiter (SD)

End delimiter (ED)

Access Control

Frame Control (FC)

Frame Status (FS)

Token Format

Frame format

32-bit CRC: Frame Check Sequence (FCS)


Token Ring Operation

Transmit Receive

Waiting for token to be received

Frame waiting to be transmitted?

Token priority <= frame priority?

Token holding timer expired?Forward token

with correct priority

Transmit waiting frame. Remove frame after

circulating ring. Pass A and C bits from tail

of frame to higher layer

R bits < frame priority?

Forward token with correct priority

Set R bits to frame priority

n

y

n

yn

y

y

n

Token?

Set A and C bits at tail of frame. Pass stored frame to higher layer.

Store frame contents and repeat at ring interface

Frame addressed to me?

Wait for a frame to be received

Enter transmit routine.

Discard frame.

n

y

y

n


Ring Management

� In addition to tokens and data frames the Token Ring protocol employs a number of special frame types to facilitate ring management.

� Ring management is required for:• Initialization• Active monitoring• Standby monitoring• Fault diagnosis

� The different ring management frames include:• Duplicate Address Test

(DAT)• Active Monitor Present

(AMP)• Standby Monitor Present

(SMP)• Claim Token (CT)• Purge (PRG)• Beacon (BCN)


Token ring control frames


Ring Fault Detection and Recovery

A B C D

EFGH

Standby ring

A B C D

EFGH

TCUFailure


Token bus


Token bus frame


Token bus control frames


IEEE 802.2 Logical link control

� Hide difference between the various 802 networks by providing a single format and interface to the network layer

� Based on HDLC, provide 3 service options as the link layer

� Error control using acknowledgement

� Flow control using a slide window

� All 802 LANs and MAN offer best-efforts service


Logical Link Control (LLC)� The various IEEE 802 MAC

sublayers are all designed to work with the 802.2 Logical Link Control (LLC) sublayer

� The LLC is based upon HDLC and is thus similar to the LAPB protocol used in X.25

� The LLC sublayer can optionally support three types of service:• Type 1: Unacknowledged

connectionless• Type 2: Connection-oriented• Type 3: Acknowledged

connectionless� LLC frames are carried in the data

field of the MAC sublayer’s frame.

DSAP address

SSAP address Control Information

1 1 1 or 2 >=0Bytes:

LLC Frame format (LLC-PDU)


LLC Control Field

0

11

1

N(S) N(R)

N(R)

P/F0

S P/F

P/FM M

Information frame

Supervisory frame

Unnumbered frame

N(S),N(R) = send / receive sequence numbers (7 bits)S = supervisory function bitM = modifier function bitp/f = poll/final bit


Summary

� LANs � IEEE 802 standard LANs:� Ethernet (802.3) � Token Ring (802.5) � Token Bus (802.4)� MAC sublayer protocols� LLC sublayer


2. High Speed LANs


Introduction

� Fast Ethernet (IEEE802.3u)• Ethernet shared medium Hub• Switched Ethernet

� Gigabit Ethernet (IEEE802.3z)� FDDI


Ethernet Shared Medium Hub

Hub

DTE DTE DTE


Switched Ethernet

A simple example of switched Ethernet.


Switched Ethernet

� Add electronics to Hub so hub can recognise (remember) the addresses of DTE connected to each port thus only need to transmit frames intended for that DTE (need FIFO to buffer until DA appears)

� Arrange backplane with one line/port - steer frames to specific line - several frames can be transit at same time

� Can have higher speed port to link to server DTE -recipient of all unrecognised frames

� Collision Detection only required when frame received for a port already receiving a frame


Fast Ethernet Specifications

IEEE 802.3 (100 Mbit/s)

100BASE-X

100BASE-TX

• 2 STP

• 2 Category 5 UTP

100BASE-FX

100BASE-T4

• 2 Optical fibre• 4 Category 3

• 4 Category 5

15

IEEE 802.3 concise notation:<rate in Mbit/s> <signalling method> <Max Length in 100m>


Fast Ethernet: IEEE802.3u

� They must use hubs or switches


Fast Ethernet 100base4T

� Main problem is to transmit 100Mb/s over Twisted pair!� The 100base4T scheme uses all 4 pairs in parallel � Uses 8B6T (8bits translated to 6 ternary symbols) to

reduce baud/pair to below 30Mbaud(100 x (6/8))/3 = 25 MHz

DTE Hub


Fast Ethernet 100baseX

� 100baseX uses single, higher specified twisted pair (or optical fibre - hence X)

� For FX - optical fibre cable,4B5B is used as used in FDDI.� For TX - Shielded twisted pair (STP), MTL-3 signalling

scheme is used.


Gigabit Ethernet – IEEE802.3z

The strategy for Gigabit Ethernet is the same as that for fast Ethernet:� New media and transmission specification

• Point to point• Carrier extension• Frame bursting

� The same CSMA/CD protocols and Ethernet format and backward compatible

(a) A two-station Ethernet. (b) A multistation Ethernet.


Gigabit Ethernet cabling

� 1000BASE-SX: uses short-wavelength� 1000BASE-LX: uses long-wavelength, � 1000BASE-CX: use two pairs specialise shielded

twisted-pair (STP) cable � 1000BASE-T: use four pair of Category 5 UTP� New encoding rules are used on fibres (10B/10B)� IEEE802.3ae is under studies for 100 gigabit Ethernet


Fibre distributed data interface (FDDI)

n Modelled on IEEE 802.5n High speed LAN 100 Mbit/sn synchronised frame every 125 µs, support 96

PCM channels (4xT1 or 3xE1)n Bit error rate 2.5x10E-10n 4B/5B encoding


Fibre Distributed Data Interface (FDDI)

� ISO 9314, 100 Mbps, � 200Km, 1000 nodes� Token passing� Dual ring structure� Intended primarily as a

means of interconnecting LANs (via relays) but individual nodes can be attached.

� Two station types:• Single Attach Station (SAS)• Dual Attach Station (DAS)

A

B A

B

A B

M

S S

M

wiring concentrator

DASDAS

SASSAS

inserted bypassed

slave key

master keyprimary ring

primary ring

secondary ring

inserted bypassed

optical coupling unit


FDDI frame formats

Pre SD FC DA SA data FCS ED FS

Pre SD FC ED

Frame check sequence coverage

2>16 2 4 / 12 8 1/2 34 / 12

symbols

token

<=9000


FDDI – Physical Layer Signalling

� To enable clock synchronisation FDDI uses a 4 of 5 group code at the physical layer.

� Symbols have at most 2 consecutive 0’s thus when using NRZI (signal transition on each ‘1’) at most two bits between transitions.

� J and K symbols break 2 ‘0’s rule and are used as end-markers

� IDLE has all ‘1’s and thus max clock transitions - used in preamble

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

11110 01001 10100 10101 01010 01011 01110 01111 10010 10011 10110 10111 11010 11011 11100 11101

IDLE J K T R S

QUIET HALT

11111 11000 10001 01101 00111 11001 00000 00100

Control Symbols

Data Symbols

4-bit data 5-bit symbol


Latency

n SD field of 2 symbols (5bits) requires a buffer at receiver before SA known thus introduces latency of 0.08µs

n Generally rounded as 1µs / ring interfacen Thus latency = propagation delay (5µs/km) +

N x Station latency (1µs)n 20km ring with 200 stations = 300µs

or 30,000bits


Timed Token Rotation Protocol

n FDDI uses same approach as used in Token Ringn Preset parameter - Target Token Rotation Timen For each rotation of token station computes time since

last received token - TRT (Token Rotation Time)n TRT is a measure of loading on ringn The station computes THT = (TTRT-TRT)

this THT is Token Hold Time and determines how long the station may hold the token and thus continue to transmit frames


Max Throughput or Untilisation

N stations Τ = ring latencyi.e. time taken to pass token around loopTTRT = Timed token rotation timeτ = time required to transmit token

A

B

C

Assume heavily loaded ring - all stations have frames to transmitConsider B - when token first passed will not be able to transmit astimed out - thus need complete token rotation (Τ)On completion B transmits with holding time TTRT-ΤIt then has to transmit token time τ which takes Τ/n to reach C


Max Utilization

TTRT-Τ

(TTRT-Τ) + Τ + τ + Τ/n Umax =

TTRT >> τ thus

n(TTRT-Τ)

nTTRT+ΤUmax = thus -> 1 as TTRT increases


Maximum Access Delay

Have to wait for all other stations to transmit + token rotation times

Amax = (n-1) (TTRT-Τ) + nΤ + Τ

= (n-1)TTRT + 2Τ


Summary

� High speed LANs and technologies� Ethernet Hubs and Switches� Fast Ethernet� Gigabit Ethernet� FDDI


3. Wireless networks


Outline

� Wireless LAN (IEEE802.11a, 11b & 11g)• The 802.11 Protocol Stack• The 802.11 Physical Layer, MAC Sublayer Protocol,

Frame Structure, and Services� Broadband Wireless (IEEE802.16)

• The 802.16 Protocol Stack• The 802.16 Physical Layer, MAC Sublayer Protocol,

and Frame Structure� Bluetooth (IEEE802.15)

• Bluetooth Architecture• Bluetooth Applications, Protocol Stack, Radio Layer,

Baseband Layer, L2CAP Layer and Frame Structure


The 802.11 Protocol Stack

Part of the 802.11 protocol stack.


The 802.11 MAC Sublayer Protocol

(a) The hidden station problem.(b) The exposed station problem.


The 802.11 MAC Sublayer Protocol (2)

The use of virtual channel sensing using CSMA/CA.



A fragment burst.



Interframe spacing in 802.11.


The 802.11 Frame Structure

The 802.11 data frame.

Data,Control,management

RTSCTSACK

More fragments

PowerManage

More frames

•WEP used &•Order for processing


802.11 Services

� Distribution services• Association: connect mobile station to base stations• Disassociation: release connection• Reassociation: move rom one cell to another• Distribution: route frame from the base station• Integration: frame needs to be sent through non-802.11

� Intracell services• Authentication: authenticate the mobile station• Deauthentication: the station leaves the network• Privacy: Encryption using RC4• Data delivery: data transmission


Broadband wireless (IEEE802.16)

The 802.16 transmission environment.


The 802.16 Protocol Stack

The 802.16 Protocol Stack.


The Physical Layer and service classes

Frames and time slots for time division duplexing.

• Service Classes• Constant bit rate service• Real-time variable bit rate service• Non-real-time variable bit rate service• Best efforts service


The 802.16 Frame Structure

(a) A generic frame. (b) A bandwidth request frame.

EC: Encryption, type: packet or fragment, CI: present or not of checksum, EK: encryption key used, length: frame length, connection id: the packet belong to,


Bluetooth Architecture (IEEE802.15)

Two piconets can be connected to form a scatternet.


Bluetooth Applications

The Bluetooth profiles.


The Bluetooth Protocol Stack

The 802.15 version of the Bluetooth protocol architecture.


The Bluetooth Frame Structure

A typical Bluetooth data frame.

Identify the master

Identifydevices

Frametypes

F: flow control by slaveA: ack is used by piggybackS: sequence to detect re-transmission


Summary

� Wireless LAN (IEEE802.11a, 11b & 11g)� Broadband Wireless (IEEE802.16)� Bluetooth (IEEE802.15)


3. LAN interconnections


Introduction

� Repeaters� Bridges � Transparent Routing; � Spanning Tree Algorithm� Source Routing; � Internetworking different LAN types� VLAN


Repeaters, Bridges & Routers and Gateways

(a) Which device is in which layer.(b) Frames, packets, and headers.


Hubs, Bridges and Switches

(a) A hub. (b) A bridge. (c) a switch.


Repeater and Bridge functions

Transport

Network

LLC

MAC

Physical

LAN Segment 1

Transport

Network

LLC

MAC

Physical

End Station End Stati

LAN Segment 2

MACPhysical

MACPhysical

Relay

BridgeTransport

Network

LLC

MAC

Physical Repeater

LAN Segment 1

Transpor

Network

LLC

MAC

Physical

End Station End Stat

LAN Segment 2

Repeater Bridge


Why Bridges ?

� Removal of physical constraints such as length, number of stations, segments, etc (geographical separated and physical distance limit)

� Buffering allows mix of LAN types (Different department have different LANs initially)

� Transparent to protocols above MAC� Easier management and security� Partitioning improves overall reliability� Accommodate the load


Bridge Types

� Transparent (or Spanning Tree)• bridges make all routing decisions• IEEE 802.1(D) standard

� Source Routing• end-stations make major routing decisions• part of IEEE 802.5 - token ring - standard


Transparent Bridges

LLC Entities

MAC Entity

LLC Entities

MAC Entity

LAN 1 LAN 2

MAC Relay Entity

Higher Layer Entities (Bridge Protocol Entity, Bridge Management, etc.)•Bridge uses a

database so that frames can be forwarded to correct port

•Need someway for this database to be dynamically created and maintained


Bridge Architecture

ForwardDataBase

Stationaddress

PortNo.

Port management

BridgeProtocol

MAC Memory MAC

LAN seg A LAN seg B


Bridge Learning

� If LAN segments + bridges arranged in a tree structure (i.e. only 1 possible route from any station to any other station) then possible for Bridge to learn station addresses by monitoring traffic on each port. During this learning phase frames are forwarded on all ports (‘flooding’)

� By including an inactivity timer to limit remembrance of address then DTE allowed to move around


Multiple Paths

� If however there are two paths between any segments then the entry in the Forward DataBase will be continually updated

1

2

1

2

receive

flood

flood


Need to agree single path

� Need to arrange single ‘logical’ path between any two segments

� Bridges regularly exchange special framesBridge Protocol Data Units

� Each Bridge allocated priority value + unique identifier� Special Root Bridge dynamically chosen

(with highest priority and smallest id


Spanning Tree Algorithm

� A single bridge is dynamically chosen to be the Root Bridge.

� Each bridge decides which of its ports has the least Path Cost to the Root (RPC). This port is treated as the Root Port (RP).

� For each LAN segment a single bridge port is selected for forwarding frames. This is known as the Designated Port (DP).

� RPs cannot also be DPs.� Bridge ports which are neither RPs

nor DPs are ‘switched off’, i.e. a port can either be in a forwarding state or a blocking state.

� The Spanning Tree Algorithm works dynamically and in a distributed fashion.

� Bridges exchanging Bridge Protocol Data Units (BPDUs) between themselves.

� All the bridges have a unique MAC group address for exchanging BPDUs.

� Each BPDU contains a number of fields including:• Bridge ID of the Root• RPC to the root from the bridge• ID of the bridge sending the BPDU• ID of the port sending the BPDU


Example of spanning tree bridges


Source Routing Bridges

� Can be used with any LAN type though primarily used with Token Ring LANs

� The end station perform the routing function. The routeing information is inserted into the frame and is used by each bridge.

� Routeing information comprises of segment-bridge, segment-bridge, ...., identifiers.

� Each end station need to know the routeing information.� If a destination unknown, discovery frame is broadcasted to

find the routeing information� Spanning tree is used to avoid frame explosion.


Token Ring Frame Format (Routing)

SD AC FC Destinationaddress

IG

Sourceaddress

Routinginformation

Data FCS ED FS

1= Routing information field present0= No routing available

1 1 1 2 or 6 2 or 6 variable variable 4 1 1

Frametype

Maximumframe size

Routingfield length

Segmentidentifier

Bridgeidentifier

Routingcontrol

Routedesignator 1

Routedesignator 2

Routedesignator N

Routinginformation

2 2 2 2


Source Routing Frame Explosion

A B• • • •

Bridge

Ring

1 2 N1 Frame 3 Frames 3 N-1 Frames


Source Routing vs Transparent

� Routing PhilosophyUser chooses vs Bridge cooperation

� Quality of RoutesSpanning tree avoid loops but doesn’t guarantee best;

with source routing can find all paths and choose best for specific destination

� Use of available bandwidthsource routing could load balance

� Route forwarding overheadsTransparent bridge has to keep large route table -

slow search especially with high speed LANs


Source Routing vs Transparent (cont.)

� Route finding efficiencyConcerning with cost of determining route -

Transparent scheme needs 1 frame/branch of tree from a bridge node; source routing although initial route frames also follow tree subsequent all-routes broadcast frames do not

� ReliabilityTransparent regularly check for bridge/link failures - in

source routing this task moved to stations which may not have information


Remote bridges

� It connect two or more distant LANs

� Put a bridge in each LAN and connect the bridge pair-wise with point to point lines (such as leased telephone line)

� Various protocols can be used on the point to point lines (such as data link protocol) putting complete MAC frame in the payload

� Or strip off the MAC header and tailor at the sources and put it back at the destination


Mixed LAN Bridging

Unfortunately this is not as straight forward is it may at firstappear. Problems are caused by:

� Different frame formats: frames need reformatting - not too much of problem

� Different data rates: bottlenecks fast->slow involves queuing

� Different maximum frame sizes: potential problem as different MACs have different sizes - segmentation not part of 802.1(D) - get into Bridge-Routers

� Priorities• Token Ring’s A and C bits• Token Bus’s temporary token handoff feature


IEEE 802 MAC Frame Formats (1/2)

PreambleStart

delimiterAccess control

Frame control DA, SA Length Data Pad FCS

End delimiter

Frame status

802.3

802.4

802.5


IEEE 802 MAC Frame Formats (2/2)

The IEEE 802 frame formats. The drawing is not to scale.


Virtual LANs (802.1Q)

(a) Four physical LANs organized into two VLANs, gray & white, by two bridges. (b) The same 15 machines organized into two VLANs by switches.


The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.


The IEEE 802.1Q frame

The 802.3 (legacy) and 802.1Q Ethernet frame formats.


Summary

� Repeaters;� Bridges � Transparent Routing; � Spanning Tree Algorithm� Source Routing; � Internetworking different LAN types� VLAN


Recommended Books

� Tanenbaum A., “Computer Networks - 4rd edition”, Prentice Hall 1996, 0-13-066102-3.

� Stalling W., “Data and Computer Communications - 6th edition”, Prentice Hall 2000, 0-13-086388-2.

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