Lecture 6 11.4.2012

Special Course in Computer Science: Local Networks

Roadmap of the Course So far Basic telecom concepts General study of LANs Ethernet

Today An alternative of Ethernet at a time: token networks Token bus Token ring

An ITU-T local network, ATM LAN

Token Bus

Using LANs Direct application in Factory automation Process control

The nodes are computers controlling the manufacturing process

Real-time processing needed Minimum, predictable delays

Bus topology needed An assembly line resembles a bus

Token bus IEEE 802.4 Standard sustained by General Motors Published in 1990, reviewed in 1995, 1997 Currently withdrawn

Combines Physical configuration of Ethernet (bus topology) Collision-free feature of token rings (=> predictable

delays)

Physical vs. logical topology Physical topology: bus Logical topology: ring Formed based on physical addresses of stations in

descending order A station has a Successor: the station with immediate lower address;

referred as next station Predecessor: the station with immediate higher address;

referred as previous station

More on the ring To form the ring Station with highest address has as predecessor

the station with lowest address Station with lowest address has as successor the

station with highest address

Bus/Ring illustration

Token passing Token: a small frame circulating from station to

station in the logical ring It controls the access to the shared medium When a station has data to send: keeps the token and

sends the data frames There are timers to prevent monopolization Token contains the address of the next station The ring is logical Physical topology is bus => every station gets the token but only

the logical “next” can use it

Token passing illustration 1

Token passing illustration 2

Service classes Data to send can be classified in priority classes:

0, 2, 4, or 6 (6 – highest priority) If a station does not classify its data, then it

implicitly belongs to class 6 When a station uses the classification It has 4 queues, one per class Data arranged in corresponding queue by MAC

layer

A station with queues

Timers There are 4, to handle the different classes THT (token holding timer) Maximum time a station can send class 6 data Same for all stations

TRT4, TRT2, TRT0 (token rotation timers) Maximum time a token takes to circulate the ring + the

time to send respective class data

Ring management The whole token bus depends on the logical ring

management

Management frames There are 7 Token Claim-token Set-successor Solicit-successor-1 Solicit-successor-2 Resolve-contention Who-follows

Removing stations

If a station leaves the ring, its predecessor should now bypass it To reform logical ring

A station may leave Voluntarily Unexpectedly (due to malfunctioning)

Example: token bus with stations A, B, C, D, E, in this logical order Assume station C is leaving the ring

Voluntary leaving

C waits until it gets the token (from B) C sends a set-successor frame to B, identifying D as

the future successor of B B updates its record with the above C sends token to D C leaves the ring

Unexpected leaving B sends token to C and listens for activity on bus

If C is functioning, it sends data/token on the bus and B hears about it, so nothing more happens

If C is not functioning, B hears nothing and then waits a preset amount of time (response window time)

If nothing happens B sends token again, to be sure there is no transmission problem

If after the 2nd response window still nothing happens, B assumes C does not function and sends a who-follows frame with C address in it

D replies with a set-successor frame specifying itself as B’s future successor

B changes its record to the above B sends token to D

Adding stations Stations in the ring provide opportunities for

stations needing to join the ring Periodical testing

A station cannot add itself to the ring Waits for invitation

Two cases 1. The address of joining station within the range of

ring station addresses 2. The address of joining station outside the range

of ring station addresses

Case 1 A token-holding station Sends invitation: solicit-successor-1 frame, with its

address and its successor’s Creates a response window of twice the propagation

delay A station with address in specified range answers

with a set-successor frame If token-holding station gets no answer during the

window, it closes it and sends token to its successor

Case 1 (2) If token-holding station gets an answer (set-

successor frame) during the window Changes its successor to answering station Sends token to the newly added station

The newly added station Sets predecessor and successor accordingly Becomes part of the ring

Case 1 (3) If there are more than one answer => there is a

collision Token-holding station sends a resolve-contention frame Waits for answer for 4 response window times

Each answering station replies in one of the 4 response windows, according to their first 2 address bits

Implicit priority given to 00-stations

Response to resolve-contention frame

First two bits in address

Responding window Waiting time

00 1 None

01 2 One response window time

10 3 Two response windows time

11 4 Three response windows time

Case 1 (4) When two/more stations with same first 2 bits

answer in the same response window then there is collision again

Inviting station sends resolve-contention frame again Only the colliding stations answer based on their

second two address bits Step above repeated until there is one station

succeeding to transmit

Case 2

Station-to-join waits invitation from station with lowest address

When station with lowest address gets token Sends solicit-successor-2 frame with its address and the

largest address in ring Station with address smaller than the lowest or bigger

than the highest answers Next steps as in case 1

Token recovery In what case can there be no token? When ring is first set up When token-holding station fails When a token is corrupted in transit and discarded

by receiving station A new token has to be generated and put in the

ring

Token recovery procedure Every station has a token-recovery timer When this timer expires and there is no activity in

the bus, a station knows there is no token in the network

A station detecting this sends a claim-token frame Indicates that station is willing to generate a new

token

Token recovery procedure 2 When more than one timer expires at the same

time All corresponding stations are token-claimants A collision of their claim-token frames occurs

To solve collision Each station pads the data field of the claim-token

frame with 0/2/4/6 × time slot, depending on the first 2 address bits

Claim-token frame padding

First two bits Length

00 0

01 2 × time slot

10 4 × time slot

11 6 × time slot

Token recovery procedure 3 All stations sending padded claim-token frames

continue listening to the bus If a station senses a frame longer than its own, it

gives up the claim The station sending the longest frame regenerates

token If collision again, procedure repeated based on

second two bits, and third two bits, etc Implicit priority given to 11-stations

Removing duplicate tokens If a token-holding station detects activity in the

bus It knows that other station holds the a token too It drops its token and moves to listening state

Ring initialization At startup, each station knows its own address

only and is connected to the bus It can receive broadcast messages It does not know a predecessor / successor There is no token in the ring

Token-recovery and adding-station procedures are combined

Ring initialization steps New token generated using the token-recovery

procedure The winning station becomes the token-holder The ring is thus formed with one station

The token-holder adds stations to ring using the usual procedure For adding a 2nd station to ring, token-holder defines itself

as both predecessor and successor (case 2)

Token bus layers

MAC sublayer

Access method token passing over a bus topology, as discussed

MAC frame fields Preamble: 1/more bytes of a predefined pattern,

used for synchronizing sender and receiver Added by the physical layer

Start delimiter (SD): 1 byte, serves as a flag to alert the receiver to a frame arrival The N bits stand for non-regular encoding of binary 1 This encoding performed by PhL => implementation

dependent Data field transparency ensured

MAC frame fields 2 Frame control (FC): 1 byte, defines frame type

MAC frame fields 3 Destination address (DA): 2-6 bytes, the physical

address of frame’s next destination Frame Destination address

Claim-token Ignored

Solicit-successor-1 Ignored

Solicit-successor-2 Ignored

Who-follows Ignored

Resolve-contention Ignored

Token Address of successor

Set-successor Address of station that sent solicit-successor frame or of predecessor

Data Address of data recipient

MAC frame fields 4 Source address (SA): 2-6 bytes, the physical

address of sending station Present in all frame types

Data: 0 - 8190 bytes For a data frame: data coming from LLC layer For a control frame: some extra information

Data field contents Frame Data

Claim-token Length of frame

Solicit-successor-1 Address of successor

Solicit-successor-2 Address of successor

Who-follows The address of not-responding successor

Resolve-contention Missing

Token Missing

Set-successor New address of successor

Data Data coming from LLC layer

MAC frame fields 5 Frame check sequence (FCS): 4 bytes, a CRC-32

error detecting sequence End delimiter (ED): 1 byte, serves as a flag to alert

receiver to a frame termination The N bits stand for non-regular encoding of binary 0 This encoding performed by PhL => implementation

dependent Data field transparency ensured Last bit can be used to show if there are more frames to

send

Physical layer

Carrier band, phase continuous Based on carrier band modulation Analog modulation Uses the whole medium capacity No multiplexing

Signaling Data first encoded with Manchester encoding Manchester encoding then modulated with FSK 2 frequencies, a high (H) and a low (L) one Specific modulation technique: 0 bit as HL, 1 bit as LH

Encoding and signaling for CBPCont

Carrier band, phase continuous 2 Transparency Refers to how the SD and ED fields represent their N bits Also refers to padding representation Achieved through a use of L and H frequencies that are not

used in data N bits are modulated in pairs (NN) Use LLHH

Padding: HHLL

Carrier band, phase continuous 3 Parameter Value

Encoding Manchester

Modulation FSK (3.75, 6.25 Mhz)

Data rate 1Mbps

Topology Bidirectional bus

Medium Coaxial cable

Medium interface T connector and drop cable up to 35 cm in length

Carrier band, phase coherent Based on carrier band modulation Analog modulation Uses the whole medium capacity No multiplexing

Signaling Data first encoded with Manchester encoding Manchester encoding then modulated with FSK 2 frequencies, a high (H) and a low (L) one L = data rate, H = 2 × data rate 0 bit as H, 1 bit as L

Encoding and signaling for CBPCoh

Carrier band, phase coherent 2 Transparency Refers to how the SD and ED fields represent their N

bits N bits are modulated in pairs (NN) Half-period of H One period of L Half-period of H

Carrier band, phase coherent 3 Parameter Value

Encoding Manchester

Modulation FSK (5/10, 10/20 Mhz)

Data rate 5 or 10 Mbps

Topology Bidirectional bus

Medium Coaxial cable

Medium interface Taps and drop cable up to 300 m in length

Broadband Specifies a broadband transmission With multiplexing

Signaling Data first encoded with multilevel duobinary 3 levels: 0, 2, 4 0 bit encoded as signal value 0, 1 bit encoded as signal level 4

Encoded data then modulated with a combination of ASK/PSK

Encoding and signaling for Broadband

Broadband 2 Transparency Refers to how the SD and ED fields represent their N

bits N bits use the third level (2)

Broadband 3 Parameter Value

Encoding Multilevel duobinary

Modulation ASK / PSK

Data rate 1, 5 or 10 Mbps

Topology Unidirectional bus, with frequency converter / dual bus

Medium Coaxial cable

Medium interface Taps and drop cable

Optical fiber

Specifies a broadband transmission With multiplexing

Signaling Data first encoded with Manchester encoding Manchester encoding then modulated with ASK 1 bit as a high amplitude 0 bit as a zero amplitude

Encoding and signaling for Optical fiber

Optical fiber 2 Transparency Refers to how the SD and ED fields represent their N

bits N bits use another amplitude level

Optical fiber 3 Parameter Value

Encoding Manchester

Modulation ASK (on/off)

Data rate 5, 10, or 20 Mbps

Topology Star

Medium Optical fiber

Bandwidth 10, 20, or 40 MHz

Distributed queue, dual bus (DQDB)

Designed to be used in MANs Each station connected to two (unidirectional)

backbone links Access method: distributed queues

Token Ring

Token ring motivation Medium access mechanism of Ethernet (CSMA/CD)

may result in collisions Data may be sent multiple times before transmission

makes it to the link Unpredictable delays Especially if traffic is heavy

Unpredictable occurrence of collisions Token ring designed to solve the above

unpredictability

Token ring IEEE 802.5 Standard sustained by IBM Published in 1989, reviewed in 1991, 1998, 2000,

2001 Still in use at some IBM sites, virtually nowhere else Work progressing on a GB version (802.5v)

Uses a token passing access method

Basic idea Stations take turns in sending data Turns are regulated by a token placeholder frame that passes from station to

station Created and regenerated by a monitor station

When a station has the token and data to send, it attaches the data to the token One station sends only one data frame at a time

Token passing illustration 1

Token passing illustration 2

Token passing illustration 3

Token passing illustration 4

Token passing steps 1. When network unoccupied, a 3-byte token circulates from station to

station 2. Token is captured by the first station with data to send 3. Station keeps token and sends a data frame 4. Data frame proceeds in the ring, being regenerated at each station 5. Intermediate stations examine the destination address and relay frame 6. The recipient recognizes its address, copies the message, checks for

errors and changes 4 bits in last byte of frame This indicates address recognized and frame copied

7. Frame then continues to the sender 1. Sender recognizes itself in the source address 2. Sender releases token into the ring 3. Sender examines the modified bits and if those are set it discards the data

frame. If not, data should be resent in the next token capture

Token ring layers

MAC sublayer Responsible for Token passing Reservation operations Creation and delivery of frames to physical layer

Three frame types Data frame Token frame Abort frame

Data frame The only frame that Carries a PDU Is addressed to a specific destination

Data frame fields Start delimiter (SD): 1 byte, alerts the receiver to a frame

arrival, allows for synchronization The J, K bits stand for non-regular encoding of binary 1

J: both differential Manchester encoding transitions are cancelled K: middle differential Manchester encoding transition is cancelled

This encoding performed by PhL => implementation dependent Data field transparency ensured

Data frame fields 2 Access control (AC): 1 byte priority subfield: 3 bits token bit subfield: when set, says this is a data frame monitor bit subfield reservation subfield: set by stations wishing the token

Data frame fields 3 Frame control (FC): 1 byte type bit subfield: control or data PDU special info subfield: 7 bits, info used by token ring

logic E.g., how to use the AC field info

Data frame fields 4 Destination address (DA): 6 bytes, physical

address of frame’s destination Source address (SA): 6 bytes, physical address

of frame’s source Data: 0 - 4500 bytes, contains the PDU PDU length/type not included

CRC: 4 bytes, CRC-32 error detecting sequence

Data frame fields 5 End delimiter (ED): 1

byte, indicates the end of sender’s data and control info PhL handles it Transparency

ensured with the J,K violations

Frame status (FS): 1 byte, set by receiver when frame was read and by the monitor when frame has been around the ring

Token frame Token: placeholder and reservation field Token frame: SD, AC, ED fields AC indicates that the frame is a token AC includes the priority and reservation fields

Abort frame Carries no information at all, only SD and ED Can be generated by Sender: to stop its own transmission Monitor: to purge an old transmission from the network

Addressing Each data frame carries two addresses Source address and destination address 2 or 6 bytes

Addresses can be local/global Destination address can be Unicast: first bit of first byte is 0 Multicast or broadcast: first bit of first byte is 1 Broadcast: all address bits are 1

Physical layer Bit and byte ordering Byte ordering: like in Ethernet Bit ordering: first bit of first byte first

Implementation: series of STP (shielded twisted pairs) sections linking stations to their neighbors

Switch Without switches disabled/disconnected nodes can stop the traffic

flow With switches Each station connected to automatic switch Switch can bypass an inactive station Station disabled: switch closes the ring without it Station comes in: NIC signals the switch and station is

brought back to the ring

Switch illustration

Multi-station access unit (MAU) Individual switches are combined into a hub

called MAU MAU can support up to 8 stations

Combining MAUs

Other issues Coding: differential Manchester Special code violations for some signal sequences

Cabling: UTP/STP UTP has data rate of 4 Mbps STP has data rate of 4 to 16 Mbps

Stations With STP: up to 250, with UTP: up to 72

Ring management Damaged frame By noise Destination station informs sender of the error Sender station should retransmit the frame

Errant (orphan) frame Frame that repeatedly circulates the ring Occurs when sender station crashes before frame

comes back Solved by the monitor station It sets the monitor bit in AC field to 1 the first round When (in a subsequent round) it finds a frame with monitor

bit 1, it drains that frame

Ring management 2 No frame/token When a station receives a frame and crashes

before forwarding it / generating a new one Solved by monitor station Monitor has timer When a frame passes the monitor, timer is set When timer expires with no frame back to monitor, this one

creates a new token

Priority and reservation levels Optional priority mechanism defined in 802.5 Allows the overriding of the equal opportunity right Allows different priority levels to station => stations

with higher priority get access to token earlier Stations with high priority get data in real time When (almost) all stations involved, no real time

3 levels Station priority level (StPr): 0 – 7 Defined by network manager, is part of station

Current priority level (CurPr) at any moment, the ring operates at one specific

priority level, named CurPr Current reservation level (CurRv) Value of last reservation made by a station

Current reservation level - more When a frame circulates, station with data to

send can make a reservation If StPr > CurRv Different strategies if frame is data/token

Hence, a station can override a reservation made by a station with lower StPr

Implementing priorities

Data frame sent by another station arrives

2nd condition: StPr > CurRv Λ StPr ≠ CurPr

Implementing priorities 2

Data frame originated by station arrives

Implementing priorities 3

Token frame arrives

If this is the responsible station, then LOWER priority (not raise)

$1 -> ≥ $2 -> >

Other procedures in the reservation process

Making a reservation CurRv := StPr, in AC field of received frame

Resetting CurRv CurRv := 0

Sending a data/token frame

Raising CurPr Responsible station Keeps track of 2 levels, the

old CurPr and the present CurPr

Uses 2 stacks for this

Lowering CurPr

Only by the responsible station

An extra step 2 missing: CurRv := 0

Example A has 2 data frames to send to C B has 2 data frames to send to A C has one data frame to send to D Token is first captured by A, when CurPr=1 and

CurRv=0.

Exp cont 1) A captures token and sends data to C (can do it, since its priority (2) is bigger

than CurPr) CurPr=1, CurRv=0 2) B forwards data frame and makes reservation (its priority bigger than CurRv) CurPr=1, CurRv=4 3) C copies data, sets frame (those 4 bits) and forwards data frame (cannot make

reservation, since its priority is smaller than CurRv) CurPr=1, CurRv=4 4) D forwards data frame CurPr=1, CurRv=4 5) A gets its frame back, drains it, then modifies CurPr to 4, CurRv to 0 (and so

becomes responsible station: OldPrSt is 1, CrtPrSt=4); it then releases token CurPr=4, CurRv=0 6) B captures token and sends data to A CurPr=4, CurRv=0 7) C forwards data frame and makes reservation (its priority bigger than CurRv) CurPr=4, CurRv=3

Exp cont 8) D forwards data frame CurPr=4, CurRv=3 9) A copies data, sets frame and forwards it; cannot make reservation CurPr=4, CurRv=3 10) B gets its frame back, drains it, releases the token CurPr=4, CurRv=3 11) C forwards token CurPr=4, CurRv=3 12) D forwards token CurPr=4, CurRv=3 13) A gets token and is responsible station so it lowers the priority, CurPr

:= 3 (=max(OldPrSt, CurRv)), CurRv:=0; OldPrSt is 1, CrtPrSt=4, 3 CurPr=3, CurRv=0 14) B gets token and sends data to A (can do it, since its priority (4) is

bigger than CurPr) CurPr=3, CurRv=0

Exp cont 15) C does not need reservation (only a free token) so it forwards data frame CurPr=3, CurRv=0 16) D forwards data frame CurPr=3, CurRv=0 17) A copies data, sets frame and forwards it; and makes reservation CurPr=3, CurRv=2 18) B gets its frame back, drains it, releases the token CurPr=3, CurRv=2 19) C captures token and sends data to D CurPr=3, CurRv=2 20) D copies data, sets frame and forwards it CurPr=3, CurRv=2 21) A forwards data frame CurPr=3, CurRv=2 22) B forwards data frame CurPr=3, CurRv=2

Exp cont 23) C gets its frame back, drains it, releases the token CurPr=3, CurRv=2 24) D forwards token CurPr=3, CurRv=2 25) A responsible to lower priority: CurPr:=2 (=max(OldPrSt, CurRv)),

CurRv:= 0; A sends data to C; OldPrSt is 1, CrtPrSt=4, 3, 2 CurPr=2, CurRv=0 26) B forwards data frame CurPr=2, CurRv=0 27) C copies data, sets frame, forwards it CurPr=2, CurRv=0 28) D forwards it CurPr=2, CurRv=0 29) A gets its frame back and drains it; it’s responsible so it sets CurPr :=

1, CurRv:= 0, and pops both stacks, no longer responsible CurPr=1, CurRv=0

Token ring requirements To be used in commercial and light industrial

environments There is a somewhat large installed base,

especially with IBM-affiliated organizations Not as popular as Ethernet More complex Eventually Ethernet caught up in terms of speed

FDDI Fiber distributed data interface 100 Mbps token ring LAN/MAN specification Introduced before Fast Ethernet Never made it to the desktop, used as backbone

network A standard on its way out Copper version abbreviated CDDI

ATM LAN

ATM LANs ATM: asynchronous transfer mode Cell relay protocol designed by ATM forum, adopted

by ITU-T Mainly a WAN Technology can be adapted to LANs

Plan We first discuss ATM technology We then study how the above applies to LANs

ATM Introduced in the early 90s, very advertised In current use WAN links inside telephone networks Broadband access lines such as DSL

Designed to Optimize the use of high-rate transmission media Provide an interface with existing systems Move functions from software to hardware domain

Problems with existing systems Packet networks Packet: overhead bits + data Various/variable sizes and intricacies of packets Packets of 65545 bytes coexist with packets of 200

bytes Consequence Unpredictable traffic Devices have elaborate software to manage

packets Internetworking among different packet networks is

slow + expensive at best, impossible at worst

Cell networks Cell Small data unit of fixed size Basic unit of data exchange

Advantages Cell routing done in hardware (=> fast) Hardware can be set up to handle broadcast Small cells do not block lines for long => quality of

service Cells appear as delivered continuously and in order

=> real-time transmission

An ATM cell

Switches ATM is a cell-switched network End points: user access devices Switches UNI: user network interface Connects end points to switches

NNI: network-to-network interface Connects the switches

Virtual circuits ATM networks are connection-oriented Sending a data requires sending 1st a packet to set

up the connection Connections = virtual circuits In analogy to physical circuits used in telephone systems Have unique identifiers

Permanent virtual circuits allowed in ATM networks Similar to leased lines of telephones

A virtual circuit

Elements of a virtual connection Transmission paths (TP) Physical connection between switches or a switch

and an end point Intuitively: set of all highways between 2 cities

Virtual paths (VP) Provides a connection/set of connections between

switches Intuitively: one highway connecting 2 cities

Virtual circuits (VC) Basis of cell networks Relays all cells of a message, in order Intuitively: highway lane

TP, VP, VC

Identifiers Virtual connection defined by a pair of IDs A virtual path identifier (VPI) A virtual circuit identifier (VCI)

For UNI interface VPI is 8 bits, VCI 16 bits

For NNI interface VPI is 12 bits, VCI 16 bits

Routing Switches route the cells between the end points 2 types of switches (for efficiency) VP switches VPC switches

Allow for hierarchical routing Mostly VP switches (use only VPI for routing) Switches interacting with end points: VPC

ATM reference model

ATM layers usage End points Use all three levels

Switches Use only the 2 bottom layers

ATM layers usage illustration

Physical layer Defines: transmission medium, voltages, bit

transmission, encoding, electrical-to- optical transformation

Not a particular set of rules ATM cells could be sent by themselves or packaged

inside the payload of other carriers => ATM independent of the transmission medium

ATM layer Deals with cells and transport Layout of cells Management of connections and traffic Routing Congestion control

Accepts 48-byte segments from AAL (application adaptation layer), adds header (5 bytes) and produces the cells (53 bytes)

Application Adaptation Layer (AAL) Most applications do not want to work directly

with cells AAL allows users to send packets larger than a

cell

High data rate in ATM 155 and 622 Mbps, higher speeds also supported 155 Mbps: what is needed for high-definition TV 622 Mbps: four 155 Mbps channels Attractive speed to be used in a LAN

Other ATM advantages for a LAN ATM technology supports different types of

connections between end users Permanent and temporary connections

ATM technology supports multimedia communication Variety of bandwidths for different applications Guarantees a bandwidth of several Mbps for real-

time video Easily adaptable for expansion

ATM LAN architecture

Pure ATM Architecture ATM switch connects stations of a LAN Like in Ethernet

Stations Can communicate at 155/622 Mbps Use VPI and VCI instead of source/destination address

Drawback System has to be built from scratch Existing LANs cannot be upgraded into pure ATM LANs

Pure ATM

Legacy LAN architecture ATM technology is used as a backbone to

connect traditional LANs Stations on the same LAN can use format and

data rate of their own LAN Stations on different LANs use a converter to

change the frame format Output from several LANs can be multiplexed to

create a high data rate input to ATM switch

Legacy LAN

Mixed architecture Best solution: mix the pure and the legacy Existing LANs are kept New stations can be connected directly to the ATM switch Gradual migration of legacy LANs onto ATM LANs

Question How stations in traditional Ethernet communicate with

stations directly connected to the switch (or vice versa)

Mixed architecture ATM LAN

We continue ATM LANs next time!