July 2003© Peter Komisarczuk, VUW, 2003 Supplement 1 Ethernet LAN Technology Peter Komisarczuk...

July 2003© Peter Komisarczuk, VUW, 2003

Supplement 1 Ethernet LAN Technology

Peter KomisarczukMaterial used here is from various sources:A Tanenbaum, Computer Networks, 4th EditionW. Stallings, Data and Computer Communications, 6th EditionVarious web sources of information from Nortel Networks and Cisco Systems-- NTUST ( 加註紅大標題 --- 大哉問／考題型式 ) -2008Q2( 斜体的紅頁 --- 期中考暫不考，列入期末範圍 )- 04/14/2008


Corporate Overview

Corporate Local Area

NetworkHost-A

Host-B

Host-E

Host-D

Host-C

Corporate LAN

Corporate LAN

Internet Service Provider

StorageArea Network FC switch

Mainframe Storage Arrays

Storage

(Raid array/JBOD)

Servers

Tape

Router

IP based networkspredominate the LAN ……..


LAN Topologies


MAC Types

Defined by the topology the protocol must support LANs usually share the “medium” between a number of hosts

Most architectures could use either centralised or distributed control mechanisms Centralised:

• Good “access” control (e.g. not a recognised address)• Easy support of traffic priority, capacity and capability to override

normal operations • Simplifies the control logic (protocol) required at the controlled stations• Problem: Centralised controller is a potential single point of failure • Centralised controller is a potential bottleneck therefore reducing

throughput and increased response time Decentralised:

• Avoids single point of failure, but requires complex per node logic


MAC Types Defined by their “access” mechanism

Round Robin – one station at a time is given permission to transmit on the medium (e.g. by holding the “token”)

• This can be for an upper bound of time t or • by a defined number of frames: single or multiple frames, the time

between stations transmitting is based on maximum frame size Reservation – “time slots” are allocated to stations dependent

on • the type of source traffic (i.e. is it periodic, how much volume, time

constraints – e.g. end-to-end delay, jitter tolerance of the receiver) Contention – the access to the medium is not highly

controlled – works on statistics (e.g. CSMA/CD and various LAN and satellite variants)

(Defined by their signal – baseband or broadband)


Line Encoding

LAN encoding is often “baseband” - includes timing information in data stream Avoids multiple cables / complex interfaces (i.e. one

line for a clock - more pins to connect) Manchester encoding is typical

• Has a transition in the middle of each bit

0 1 0 0 1 1 0 0 0 1 1

Low to High Transition = 1High to Low Transition = 0


LAN Characteristics Typically a < 1 where a = Tp/Tf

Propagation delay Tp ? • For a speed of propagation = 2x108 meters per seconds, what is the

Tp for a network of 1km and 2.5km? Frame transmit time, Tf ? Minimum Ethernet frame size is 72 bytes

• At 10Mbps how long does it take to transmit a minimum frame? Max Ethernet frame size is 1518 bytes (1500 bytes data)

• How long does it take to transmit a maximum sized frame? “a” is dependent on Transmission Speed (bps) so how does “a”

change for different Ethernet transmission speeds? What is the typical Tf (Transmission delay) and ratio “a” at:

• 10Mbps ? 100Mbps ? 1Gbps ? 10Gbps ?• What value of frame size did you choose?


Physical Media LAN Cabling – usually developed in house – or as

infrastructure of rented buildings Inter-office cabling usually rented from telecommunications

providers (usually fibre) Intra-office copper – twisted pair (cat 5)

UTP – cheap and usually available (a telephony by-product) Older Ethernet systems used 50 ohm co-axial systems –

better for noisy environments LAN backbones most likely to be fibre based (issue on

distance for copper CAT 5 solutions – 100m max) Single mode fibre – solutions up to 120km Multimode fibre – up to a few km (typically < 2km)


Communication Reference Models

presentation

application

session

transport

network

data link

physical

OSIapplication

transport

internet

host tonetwork

TCP/IP

not present

tcp (connection) udp (connectionless)

ip (routes between networks)

smtp (email), http (web), ftp

IEEE 802.* Frame Relay, ATMFibre, copper (DSL)

HDLC


The LAN Protocol Stack IEEE produce(d) many

LAN standards Broke down link layer

into two sublayers: MAC (media/physical

and topology dependent) LLC (media and topology

independent) Followed ISO model

Our current study is limited to the LLC layer and below


LAN Frame Formats Overview

Ourcurrentscope

Today 90+% ofupper layer protocolsare IP

(a.k.a. FCS)A CyclicRedundancyChecksum

Generic link layer header

Topology/medium specific header


Networks and Devices

application

transport

network

data link

physical

Host

application

transport

network

data link

physical

Host

network protocol

data link

physical

Router

data link

physicaldata flow

MAC (data link)

physical

Bridge/Switch

physical

WAN LAN

e.g. Gigabit Ethernet- fibre optic cable

e.g. 100BaseTx-CAT 5, 802.11

e.g. PSTN – modem, satellite, TDM point-to-point leased lineCDMA (e.g. CDMA-1X), etc.


LAN and WAN Protocols

network protocol

data link

physical

Router

data link

physical

application

transport

network

data link

physical

Host

application

transport

network

data link

physical

Host

data flow

MAC (data link)

physical

Bridge/Switch

physical

WAN LAN

layer 2 = e.g. HDLC and its derivatives – PPPLayer 3 - Internet ProtocolLayer 4 – TCP and UDP

layer 2 = e.g. LLC/MACLayer 3 - Internet ProtocolLayer 4 – TCP and UDP

It’s about end-to-end communicationsand how it is achieved through a network


Ethernet Media

- options for different twisted pair types (UTP CAT 3/5, STP)- also point-to-point fibre optic connections (multimode/mono mode)


BUS LAN Example

BUS MAC types 802.3 Ethernet utilising CSMA/CD

All MAC protocols provide a means for addressing the computer interface 48 bit unique MAC address

per interface Includes a broadcast

address (FFFFFFFFFFFF)

Co-axial cable – 50 Ohms impedance Cable ends terminated to

absorb the frame/signals Nodes attached to T pieces


Ethernet MAC Sublayer Protocol

Frame formats (a) DIX Ethernet, (b) IEEE 802.3.

DIX = original (DEC, Intel, Xerox) = original 3Differences? Type field vs. Length field and preamble has

SOF delimiter

Why???SOF = 10101011 – to differentiate last byte of the preambleLength = better buffer utilisation but Type allows for protocol multiplexing(a LLC SAP function)


Collision Detection

Carrier SenseMultiple Accessw/ Collision Detection(CSMA/CD)

Based on radio/satelliteprotocols - aloha and slotted aloha

CSMA/CD has betterutilisation because ofsmall collision window(51.2 microseconds)


Binary Exponential Backoff!

Dynamically adapts to the number of stations trying to send Collision window based – determined by engineering of

original Ethernet as 2*max propagation delay on a max sized Ethernet network

Initial collision – backoff 0 or 1 collision windows. Each station chooses a random value “0” or “1”

If they collide again the random number is doubled and stations choose values, 0, 1 or 2

If a collision takes place again the the random variable is doubled again, and so on

The maximum backoff time is 1023*collision window If after 16 attempts the stations cannot get a packet through

they give up! The network congestion error is reported back up the protocol stack in the station(s)


Multiple LAN Segments - Repeaters

Bus segments - joined by repeaters Max thin co-axial cable run is

185m (approx 200m!) Max thick co-axial cable run

is 500m Repeater forwards a frame

from one segment to the other (is bidirectional)

Repeater regenerates clock but is otherwise transparent (small delay)

CSMA/CD operates over the whole network

Network “span” is between 1000 and 2500m => up to 5 segments can be connected

10Mbps(10Base2)

10Mbps(10Base5)

10Mbps(10Base2)


Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot time (51.2μs).

See the discussionin W Stallings 6/eAppendix 14B“PerformanceIssues”This picture is taken from A Tanembaum 4/e, Computer Networksto emphasise characteristic


Shared Medium Hub

10 Mbps The shared busis collapsed intothe hub backplane

Shared medium hub is similar to a A collapsed bus Works at one

speed (e.g. 10Mbps)

“Same” collision characteristics

Benefits: Makes use of

CAT 5 cabling Can be made to

switch off a station that is misbehaving


Hub LAN Architecture

The driver for hub LANs – using existing unshielded twisted pair for cheapestpossible LAN solution

StandardTelephonyCat 5 UTP

Typically 4 wire,Tx pair and a Rx pair – full duplex?

Source: W Stallings, 6th Edition

Issue: we are just creating a large broadcast domain – high probability of collisions

Solutions?


Segmenting Ethernet Networks: LAN Bridging Basics

A bridge is a MAC layer device that interconnects 2 or more LAN segments (usually of the same MAC type)

Stallings gives 4 reasons for using a bridge: Reliability of a LAN network is increased – any single error no

longer affects the whole network Performance of a single large LAN is improved by clustering

related devices onto a single LAN segment such that inter-LAN traffic is minimised

• Reduces collision domain in IEEE 802.3• Minimises token rotation time in IEEE 802.5

Security – different concerns within an organisation can be physically separated by bridges and inter LAN communications limited

Geography – sites within an organisation can be linked with remote bridges to create one larger LAN


Basic Learning Bridge Operation


Bridge Functions and Design Aspects

A bridge is actually two “half” bridges – one on each LAN Read all frames on local LAN and learn all “local” addresses (table

of MAC addresses and port – may also be statically provided by network administrator)

• Any unknown addresses (or known on the “other” LAN(s)) are passed to the “other” half bridge

Using the MAC for the other half bridge, retransmit each frame Bridge makes no modifications to the MAC frame – except for the

regeneration of the MAC FCS Bridge contains frame buffer space to meet peak traffic demands Bridge contains layer 2/MAC routing intelligence as well as

addressing functionality Routing can be static (allocated by administrator), source routed or

dynamic (e.g. the Spanning Tree Protocol STP) Links are given a “cost” as a metric of their importance in the

network, or to avoid their use (e.g. a backup satellite link) Bridges may be multiport (an Ethernet switch is the ultimate

multiport bridge)


Local LAN bridge configuration


Remote LAN bridge configuration

WANMAC PPPPhy. Phy.

PPP MACPhy. Phy.

t4 t5

E.g. E1 or T1 leased line

t1 t8

t2 t7

t3 t6

t4 t5

User Data

User DataLLC-H

User DataLLC-HMAC-H MAC-T

User DataLLC-HMAC-H MAC-TPPP-H PPP-T


W Stallings example network Note LAN B and C are

just to connect bridges Look at some sample

frame routes ……… There is one network

loop in the network – where is it?

There are two mechanisms to overcome network loops

LAN Example


Without The Spanning Tree Algorithm or a Static Routing Table

Station A sends a frame to Station B Both bridges learn

A is on LAN X Both bridges Tx

frame on LAN Y Station B receives

2 copies of the frame

Both bridges see a frame from A on LAN Y and learn A is on Y

Result: station ABecomes unreachable from LAN Y


Static Routing

Set-up by the network administrator Requires all MAC addresses to be programmed into bridges Requires each bridge to be programmed with a routing table

Contains MAC address and destination port/LAN Inflexible, if a fault occurs there is no standard mechanism for

rapid reconfiguration of the routing tables Suitable for small networks which are stable (no changes)

Two IEEE standards developed for dynamic routing: Spanning Tree Protocol (STP) – used by Ethernet/

IEEE802.3 LANs Source Routing – Token Ring/IEEE802.5


Spanning Tree Protocol (STP) Algorithm consists of three mechanisms:

Address Learning• Use source MAC address field in each frame• Frame has come from the LAN on which it was received (i.e.a

port on the bridge)• To adapt to reconfiguration of the network each address has a

timer. When timer expires the address is removed from the database

Frame Forwarding• Forwarding database is created for each port attached to a LAN• A frame with matching destination address is forwarded through

the port (if port is not disabled by Spanning Tree Algorithm)• If address is unknown forward frame to all other active ports

except for the port the frame came in on Spanning Tree Algorithm ………


Spanning Tree Algorithm Based on graph theory: nodes (LANs) and edges (bridges)

For any connected graph there is a spanning tree of edges connecting pairs of nodes, that maintains the connectivity of the graph but contains no closed loops

One bridge is selected as route (can be set by network administrator, or use unique numbers burned into each bridge at manufacture) Calculate the paths to each bridge from the root Root determines the network topology based on the path

information collected The algorithm is dynamic – hello messages every t seconds

between bridges maintain topology information about the network: i.e. which bridge is down or which LAN is down After 3 consecutive missed hello’s the LAN/bridge is “down”


Questions

When would a bridge contain an instance of a LLC? Go back to the example network (5 slides back taken

from W Stallings 6th edition, figure 3.16) which bridge interface should be placed into an “administrative down state” to remove the loop from the network? Discuss the criteria you chose. Note there are two possible network configurations that

could be created What “cost” would you place on a link in a remote bridge

in a network? Discuss which network parameters would be used to

make your choice of cost.


Ethernet Switch10 Mbps, 100 Mbps, 1 Gbps Switch recognises

where traffic is going to (address table) Efficient for whole

network Up-stream LAN

connections can be faster

Collisions are avoided

No change required to station software/hardware

Throughput of the switch could be up to Σ port speed*nos ports

Two types of switches are available (a) store and forward (buffers whole frame) and (b) cut-through switch (uses early detection of MAC address to select output port – minimises buffer size and packet latency)

Multiple “trunk” linksCan be provided forhigh resiliency


Gigabit Ethernet Network Example

Typical hierarchical LAN configuration.

Local servers per workgroup

Smaller switches orhubs for each workgroup

Large switch in core with central servers GbE connections from core switch (often duplicated)


Fast Ethernet, Gigabit Ethernet

The “original” Fast Ethernet cabling.

The “original” Gigabit Ethernet cabling.


Switched EthernetAn example of an Ethernet switch

(modular system – 8 slots for line cards)

High density 10/100BaseT and 100BaseFX ports (up to 96*100BaseT is typical per linecard)

Redundant Power Supply Units (PSU)

High capacity switching cards (duplicated)(typical capacity is 64Gbps or greater for this size of box)

Gigabit Ethernet ports – typically 8 ports per linecardOften deployed in pairs for redundancy


Ethernet Trunking

If there are multiple connections between bridges the STP will turn off all but one connection to avoid circular routes within the network

If some of the connections are between the same two machines we can fool the STP into thinking multiple links are effectively a single higher capacity Ethernet connection This is good as multiple Ethernet links provide a resilient

network configuration in case fibre is cut or a port fails There are several proprietary methods and a standards

based method from the IEEE MLT (Multi-Link Trunking – Nortel Networks), PAgP

(Port Aggregation Protocol – Cisco Systems), IEEE 802.3ad LACP (Link Aggregation Control Protocol) – the standard


Ethernet Trunking Example What Cisco says:

Link Aggregation Control Protocol (LACP) is part of an IEEE specification (802.3ad) that allows you to bundle several physical ports together to form a single logical channel.

LACP allows a switch to negotiate an automatic bundle by sending LACP packets to the peer. It performs a similar function as Port Aggregation Protocol (PAgP) with Cisco EtherChannel.

• (There are two forms of EtherChannel Fast EtherChannel for bundling up 100Mbps ports and Gigabit EtherChannel for bundling up Gigabit Ethernet ports)

http://www.cisco.com/en/US/tech/tk389/tk213/tk833/tech_protocol_home.html

Hash DA/SA to a portPAgP management protocolDetects any port/fibre failureand re-computes hash to use remaining ports/fibres


Questions – switch architecture

Under what circumstance might a cut through switch need to store a whole frame? Advanced question: Where in the switch is the buffering

required? What error condition may be propagated in a cut-through

switch hub? What is the traffic capacity of a switch with 24*10Mbps ports

and 2*100Mbps ports if all ports can work at full rate? Identify the “data” that is required to be managed in the

packet forwarding table of a switch. Will it be the same for both cut-through and store-and-

forward switched hubs? What is the latency introduced by a store-and-forward

switch? Contrast that to the latency of a cut-through switch?


Repeaters, Hubs, Bridges, Switches

What is the available bandwidth per network? Per station?What is the growth capacity of each network? How could they be extended/expanded? What congestion/contention characteristics does each network exhibit?

R H B S

GigabitEthernet

10 or 100BaseTx

10Base5

10Base2

Either all are 10BaseTxor 100BaseTx

(a) (b) (c) (d)


Advanced Ethernet Capabilities

The following information is provided to complete a discussion of Ethernet: The IEEE developed an optional addition to the

Ethernet MAC frame• VLAN – IEEE802.1q = allows traffic to be segregated

by arbitrary allocation of stations to workgroups – as if they were on separate LANs

• Priority – IEEE802.1p = allows traffic to be prioritised at stations, bridges and switches

• E.g. time sensitive information is characterised as high priority to minimise its latency through the network

• Best effort traffic (email, file transfer, etc.) has the lowest priority


The IEEE 802.1P/Q Standard

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

12 bit VLAN identifier (4095)

4 bytes

3 bits forpriority –8 levels

A special value-means the nextfields are 802.1 p/q


Virtual LANs

(a) Four physical LANs organized into two VLANs (gray and white) by two bridges.

(b) The same 15 machines organized into two VLANs by switches.


Transition to VLANs

Transition from legacy Ethernet to VLAN-aware Ethernet.

The shaded symbols (host and switch) are VLAN aware.

The white symbols (hosts) are not VLAN aware.


The 802.1P Mechanism

8 queues – one per priority type is ideal Priority 0 is reserved for network management traffic Priority 1 through 8 is user defined Priority 1 often given to voice over IP (VoIP)

Minimises the latency for voice packets as they pre-empt other packets

Priority 8 usually given to “best effort” traffic Who sets 802.1p bits?

The station can set them – latest O/S can do this based on the application/protocol (e.g. RTP)

The bridge/switch can set them – using a technique called deep packet classification


Priority Queuing – Nortel PP8600


Going Up The LAN Protocol Stack Logical Link Control User = higher layer

protocol, e.g. IP layer, mgnt Identified by SAP (Service

Access Point) at Tx and Rx – i.e. service oriented

LLC based on HDLC – flow control + error recovery

LLC is designed for Multi-access shared medium – any type of IEEE MAC

MAC layer deals with actual transmission, error detection, access to the medium etc.

LLC = IEEE802.2


LLC Service Perspective 3 LLC “service” types are defined

Unacknowledged connectionless service (type 1)• No flow or error control – errored frames are detected & dumped• Minimal logic required – used when higher layer protocols provide

error control and flow control Connection mode service (type 2)

• Extended HDLC format (address format and sequence numbers)• Logical connection is set-up between nodes, with full flow and error

control (as HDLC)• (Used in minimal systems with small connectivity requirements)

Acknowledged connectionless service (type 3)• No logical connection is set-up between stations• Each datagram is acknowledged (window size = 1)

LLC passes frames to MAC to be transmitted LLC receives non-errored frames from the MAC


MAC and LLC frame formats


LLC frame formats DSAP and SSAP identify the network layer protocol

Just like the port number in TCP e.g. HTTP on port 80 The DSAP can be identified as a single or group

value C/R – Command/Response bit identifies frame type

LLC Control Format Based on an extended mode HDLC format with 7

bit sequence numbers:

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 80 N(S) P/F N(R)


HDLC Frames


HDLC Frames

Type 1 only 3 frame types are required (UI, XID, TEST, white arrows)

Type 2 “SABME” is used (green arrows show typical frames) 127 (7 bit addresses) Send and Receive SAP supported

7 bit sequence numbers are mandatory

Requires a “connection table” – per connection status, state, variables

Type 3 - new packet type: Acknowledged Connectionless (AC) AC Information and AC response PDUs, using a 1 bit sequence number


Type 1 Simplified Protocol Operation

Network LayerProtocol

Type1

SAP

MAC

LLC

SAP_ACTIVATION_REQUEST*SAP_DEACTIVATION_REQUEST*UNITDATA_INDICATIONUNITDATA_REQUEST

Transmit FrameReceive Frame

Deals with the six + input events (SAP + MAC)Classifies the received frames (address, SAP, type, …Formats data frames, sends them up/down the stackFormulates response frame to received XID/TEST

Type 1 is effectively an unrestricted simplex protocol* Also deals with SAP GROUP activation and deactivation


Type 1 Protocol Operation

Taken from IEEE standard 802.2-1998

Don’t learn thisState diagram!


Type 2 Protocol Operation (States)

Don’t learn these state transitiondiagrams Its just an example

Type 2 LLCStates and transitions as defined in the IEEE standard 802.2-1998.

Taken from IEEE standard 802.2-1998


So where does the LLC and MAC fit?

Socket App

Winsock

TCP/IP WinSock

Provider

TCP/IP Transport

Driver

NDISMiniport

NIC

UserUser

KernelKernel

NDIS = Network Driver Interface SpecificationNDIS = Network Driver Interface Specification

- allows an arbitrary interface to a number of NIC, allows an arbitrary interface to a number of NIC,

USB interfaces, etc. USB interfaces, etc.

WinSockWinSock

Traditional Microsoft ModelTraditional Microsoft Model

NICNIC

MAC and LLC (type 1) implemented in NICMAC and LLC (type 1) implemented in NIC


Questions

What “data” uniquely identifies the LLC logical connection? What is the function of the Test PDU? What is the function of the XID PDU? What is the maximum amount of data that can be

outstanding with type 3 data transfer? (e.g. in terms of the number of frames or in bytes [for Ethernet])

What is the maximum amount of data that can be outstanding with type 2 data transfer? (e.g. in terms of the number of frames or bytes [for Ethernet])

What is the amount of data outstanding with type 1 data transfer? (e.g. in frames or bytes [for Ethernet])

July 2003© Peter Komisarczuk, VUW, 2003 Supplement 1 Ethernet LAN Technology Peter Komisarczuk...

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