20: Other Technologies used at the Link Layer

5: DataLink Layer 5a-1

20: Other Technologies used at the Link Layer

Last Modified: 04/21/23 06:51 AM


Token Passing: IEEE802.5 standard 4 Mbps max token holding time: 10 ms, limiting frame

length

SD, ED mark start, end of packet AC: access control byte:

token bit: value 0 means token can be seized, value 1 means data follows FC

priority bits: priority of packet reservation bits: station can write these bits to prevent

stations with lower priority packet from seizing token after token becomes free


Token Passing: IEEE802.5 standard

FC: frame control used for monitoring and maintenance

source, destination address: 48 bit physical address, as in Ethernet

data: packet from network layer; checksum: CRC

FS: frame status: set by dest., read by sender set to indicate destination up, frame copied OK from

ring

limited number of stations: 802.5 have token passing delays at each station


Point to Point Data Link Control one sender, one receiver, one link:

easier than broadcast link:no need for explicit MAC addressingfull-duplex simultaneous bi-directional

operation = no need for media access control

e.g., dialup link, ISDN line popular point-to-point protocols:

PPP (point-to-point protocol)HDLC: High level data link control


PPP Design/Features

packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network

layer protocol (not just IP) at same time ability to demultiplex upwards

bit transparency: must carry any bit pattern in the data field

error detection (no correction) connection liveness: detect, signal link failure

to network layer network layer address negotiation: endpoint

can learn/configure each other’s network address


PPP non-requirements

no error correction/recovery no flow control no need to support multipoint links

(e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!|


PPP Data Frame

Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future

possible multiple control fields Protocol: upper layer protocol to which

frame delivered (eg. IP, PPP-LCP, IPCP, etc)


PPP Data Frame

info: upper layer data being carried check: cyclic redundancy check for

error detection


Byte Stuffing “data transparency” requirement: data field

must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte

Receiver: two 01111110 bytes in a row: discard first

byte, continue data reception single 01111110: flag byte


Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data


PPP Data Control ProtocolBefore exchanging

network-layer data, data link peers must

configure PPP link (max. frame length, authentication)

learn/configure network layer information

for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address


IP over Other Wide Area Network Technologies ATM Frame Relay X-25


ATM architecture

Adaptation layer (AAL): only at edge of ATM network roughly analogous to Internet transport layer

ATM layer: “network” layer Virutal circuits, routing, cell switching

physical layer


ATM Layer: ATM cell

5-byte ATM cell header 48-byte payload

Why?: small payload -> short cell-creation delay for digitized voice

halfway between 32 and 64 (compromise!)

Cell header

Cell format


ATM cell header

VCI: virtual channel IDwill change from link to link thru net

PT: Payload type (e.g. RM cell versus data cell)

CLP: Cell Loss Priority bitCLP = 1 implies low priority cell, can

be discarded if congestion HEC: Header Error Checksum

cyclic redundancy check


ATM: network or link layer?Vision: end-to-end

transport: “ATM from desktop to desktop” ATM is a network

technology

Reality: used to connect IP backbone routers “IP over ATM” ATM as switched

link layer, connecting IP routers


Datagram Journey in IP-over-ATM Network

at Source Host: IP layer finds mapping between IP, ATM dest address passes datagram to AAL5 AAL5 encapsulates data, segments to cells, passes to

ATM layer ATM network: moves cell along VC to destination (uses

existing one or establishes another) at Destination Host:

AAL5 reassembles cells into original datagram if CRC OK, datgram is passed to IP


X.25 and Frame Relay

Like ATM: wide area network technologies virtual circuit oriented origins in telephony world can be used to carry IP datagrams and

can thus be viewed as Link Layers by IP protocol just like ATM


X.25

X.25 builds VC between source and destination for each user connection

Per-hop control along patherror control (with retransmissions)

on each hopper-hop flow control using credits

•congestion arising at intermediate node propagates to previous node on path

•back to source via back pressure


IP versus X.25

X.25: reliable in-sequence end-end delivery from end-to-end“intelligence in the network”

IP: unreliable, out-of-sequence end-end delivery“intelligence in the endpoints”

2000: IP winsgigabit routers: limited processing

possible


Frame Relay

Designed in late ‘80s, widely deployed in the ‘90s

Frame relay service:no error controlend-to-end congestion control


Frame Relay (more) Designed to interconnect corporate customer

LANs typically permanent VC’s: “pipe” carrying

aggregate traffic between two routers switched VC’s: as in ATM

corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT)


Frame Relay (more)

Flag bits, 01111110, delimit frame Address = address and congestion control

10 bit VC ID field 3 congestion control bits

• FECN: forward explicit congestion notification (frame experienced congestion on path)

• BECN: congestion on reverse path• DE: discard eligibility

addressflags data CRC flags


Frame Relay -VC Rate Control Committed Information Rate (CIR)

defined, “guaranteed” for each VC negotiated at VC set up time customer pays based on CIR

DE bit: Discard Eligibility bit Edge FR switch measures traffic rate for

each VC; marks DE bit DE = 0: high priority, rate compliant frame;

deliver at “all costs” DE = 1: low priority, eligible for discard

when congestion


Summary

principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP

various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP ATM, X.25, Frame Relay

journey down the protocol stack now OVER!

A bit about physical connections


Rating wide area internet connections T0, DS0 – 1 voice channel, 65 Kbps

What homes get for 1 telephone line T1 (Level 1 transmission line) or DS1

1.544 Mbps, 24 voice channels at 64 Kbps T3 or DS3 = 28 T1 lines, 44.746 Mbps OC3 = 3 DS3s OC12 = 12 DS3s OC48 = 48 DS3s, 2488 Mbps OC192 = 192 DS3s


SONET and SDH Higher data rates often achieved using

synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) SONET in the US and Canada and SDH in the

rest of the world Transport over optical fiber using lasers/

LEDs Transporting large amounts of telephone

calls and data traffic over the same fiber without synchronization problems


T0 = typical phoneline connection DS3 delivered native on a copper trunk or

converted to an optical fiber run when needing longer distances between termination points DS3 transported over SONET is encapsulated

in a STS-1 SONET channel Still analog when delivered over fiber

When delivering data over an OC3 or greater SONET is used. OC-3 SONET link contains three STS-1s, and

therefore may carry three DS3s. Likewise, OC-12, OC-48, and OC-192 may

carry 12, 48, and 192 DS3s respectively.5: DataLink Layer 5a-29

More on SONET

Designed to carry multiple real-time, uncompressed, circuit-switched voice lines encoded in Pulse-Code Modulation (PCM) format

Also multiple digital bit streams of differing origin within single framing protocol Multiplex circuit mode communications (T1, T3,

DS1, DS3,etc.) from a variety of different sources over same fiber

Emphasis is on merging many different flow into one quickly


STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission format for SDH.

STM-1 frame is transmitted in exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit


Protocol neutral Not communications protocols in and of

themselves Generic, all-purpose transport containers for

moving both voice and data. Used to carry ATM, Ethernet, TCP/IP etc.


SONET standard defined by Telcordia and American National Standards Institute (ANSI) standard T1.105 and T1.119

SDH standard specified in International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803 SDH originally defined by the European

Telecommunications Standards Institute (ETSI)


Carrier Pricing

Two simple components: local loop and port

Local loop = cost to transport the signal from the end user's central office (CO) to the point of presence (POP) of the carrier Local loop cost based on geography/distance

from CO to POP Port = cost to access the network through

the carrier's network Port cost based on access speed and yearly

commitment level


Fiber cable runs

One example from the North Country


Undersea cables


multiple SONET signals can be transported over multiple wavelengths on a single fiber pair by means of wave length-division multiplexing, including dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM).

DWDM circuits are the basis for all modern submarine communications cable systems and other long-haul circuits.


Other

Satellite Links Pros and Cons



Outtakes


IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

NAV: Network Allocation Vector

802.11 frame has transmission time field

others (hearing data) defer access for NAV time units


IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for

DISF sec. then transmit entire frame

(no collision detection)-if sense channel busy

then binary backoff

802.11 CSMA receiver:if received OK return ACK after SIFS


IP-Over-ATMClassic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses

IP over ATM replace “network” (e.g.,

LAN segment) with ATM network

IP addresses -> ATM addresses just like IP addresses to 802.3 MAC addresses!

ATMnetwork

EthernetLANs

EthernetLANs


ARP in ATM Nets

ATM network needs destination ATM address just like Ethernet needs destination Ethernet

address IP/ATM address translation done by ATM ARP

(Address Resolution Protocol) ARP server in ATM network performs

broadcast of ATM ARP translation request to all connected ATM devices

hosts can register their ATM addresses with server to avoid lookup


Access Control

802.11 working group considered 2 proposals for a MAC algorithm Distributed access protocols Centralized access protocols


Distributed Access Protocols

Distribute the decision to transmit over all the notes

Like Carrier-sense mechanisms in Ethernet

Makes sense especially for an ad hoc network of peer workstations

Can also be good for busty traffic


Centralized Access Protocols

Regulation of transmission by a centralized decision maker

Natural for networks with a base station Especially good if network is highly

utilized ( avoid fighting it out among peers)

Also good if some data is time sensitive/high priority


Distributed Foundation Wireless MAC Compromise was Distributed

Foundation Wireless MAC (DFWMAC) Distributed Access control mechanism

with an optional centralized control layer on top of that Distributed Coordination Function (DCF) on

top of physical layer On top of that is optional Point Coordination

Function (PCF) that provides contention free service

Access Control


CSMA

DCF uses Carrier Sense Multiple Access (CSMA) CSMA means listen before you send to make sure

the medium is idle No Collision Detection - Not CSMA/CD like Ethernet

CD based on listening while you send to make sure you hear only your signal

Wireless HW not made to send and listen at same time Large dynamic range of possible signals – cannot

effectively distinguish incoming weak signals from noise and the effects of its own transmission

Medium Access Control Logic

IFS = interframe space

Each time failincrease time towait before send


Interframe Space (IFS) Values

Actually three different IFS values Short IFS (SIFS)

Shortest IFS Used for immediate response actions

Point coordination function IFS (PIFS) Midlength IFS Used by centralized controller in PCF scheme when using

polls Distributed coordination function IFS (DIFS)

Longest IFS Used as minimum delay of asynchronous frames

contending for access


Priority

Stations using SIFS have “priority” over others because they will test for idle faster find and then start transmitting

Others that wait longer will find the channel busy when they listen after PIFS or DIFSs


IFS Usage

SIFS Acknowledgment (ACK) Clear to send (CTS) Poll response( for PCF)

PIFS Used by centralized controller in issuing

polls (for PCF) Takes precedence over normal contention

traffic DIFS

Used for all ordinary asynchronous traffic


Contention Periods/ Contention-Free Periods The DCF and PCF respectively operate in

Contention Periods (CPs) and Contention Free Periods (CFPs)

In CPs, stations compete with each other to win channel access

In CFPs, an Access Point (AP) grants the opportunity of transmission to stations by polling


Polling

Since PIFS smaller than DIFS, coordinator can seize coordinator and lock all traffic ( at least traffic that obeys the rules) while it polls and receives responses

When polling coordinator sends a poll to a station, it can respond using SIFS ( beating the next PIFS and any DIFS)


Polling

In a CFP, a PC polls the first station in its polling list, and it may also piggyback some data to the polling frame.

The polled station responds either with an ACK or a data frame piggybacked to the ACK frame.

An SIFS separates the polling and responding frames.

Once the frame exchange sequence with the first station is done, the PC waits for a PIFS and then polls another station in its polling list.


Superframes

CPs and CFPs alternate in a superframe A superframe is an interval between two beacon

frame transmissions. A beacon frame is broadcasted by APs in BSSs or

random stations in IBSSs. It carries management information to the stations.


IEEE 802.11 MAC TimingPCF Superframe Construction


Superframe

Point coordinator would lock out asynchronous traffic by issuing polls

Superframe interval defined During first part of superframe interval, point coordinator polls round-

robin to all stations configured for polling Point coordinator then idles for remainder of superframe Allowing contention period for asynchronous access

At beginning of superframe, point coordinator may seize control and issue polls for given period Time varies because of variable frame size issued by responding

stations Rest of superframe available for contention-based access At end of superframe interval, point coordinator contends for

access using PIFS If idle, point coordinator gains immediate access

Full superframe period follows If busy, point coordinator must wait for idle to gain access Results in foreshortened superframe period for next cycle


Acknowledgements

When station received frame addressed directly to it ( not broadcast or multicast) it replies with an ACK after waiting SIFS

ACKs allow for recovery from collision since no collision detection

Use of SIFS allows for efficient delivery of an LLC data unit that requires multiple MAC frames Just get SIFS between ACK and then next frame No one else will gain control of the channel until the

entire LLC if over


802.11 Physical Layer Standards

Op. Freq. Data RateTypical/Max(Mbit/sec)

RangeIndoor/Outdoor(meters)

Legacy802.11-1997

2.4 GHz ½ ?

802.11a (1999)

5 GHz 25/54 15-30

802.11b (1999)

2.4 GHz 5.5/11 45-90

802.11g(2003)

2.4 GHz 25/54 45-90

802.11n(2009)

5 and 2.4 GHz 144/600 91/182


802.11b was the first, followed by 802.11a ( higher BW, less popular)

802.11g higher BW, directly compatible with b

802.11n – even higher BW, backwards compatible with b and g


RC4 WEP uses RC4 a stream cipher

Stream ciphers are vulnerable to attack if the same key is used twice (depth of two) or more.

Say we send messages A and B of the same length, both encrypted using same key, K. The stream cipher produces a string of bits C(K) the same length as the messages. The encrypted versions of the messages then are: E(A) = A xor C E(B) = B xor C where xor is performed bit by bit.

Say an adversary has intercepted E(A) and E(B). He can easily compute:

E(A) xor E(B) However xor is commutative and has the property that X

xor X = 0 (self-inverse) so: E(A) xor E(B) = (A xor C) xor (B xor C) = A xor B xor C xor

C = A xor B

20: Other Technologies used at the Link Layer

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