tcp/iptcp/ip dr. clincylecture1 ch3: underlying technologies (2 of 3) project 1 is posted on website...

Dr. Clincy Lecture 1

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Ch3: Underlying Technologies (2 of 3)

Project 1 is posted on website – due 2/23/11

Lecture #4


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Token Ring LAN

• Token Ring is a protocol defined by IEEE 802.5

• Use a token passing ACCESS method

Token Passing Method

• During idle times (network not being used), a token circulates

• The token is passed station to station until a station needs to send data

• When the station sends it’s data, it holds the token

• The data (or frame) circulates and get re-generated by each station

• The Rx takes in and COPY the frame (based on destination address)

• The data then continues back to the original Tx

• Token is then release to circulate


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Token Ring layers• Uses the same layers as Ethernet (MAC and LLC)

• LLC Layer – logical link control layer – performs the error and flow control routines (same as Ethernet)

• MAC Layer – media access control layer –it implements the Token Passing Access Method (versus Ethernet’s CSMA/CD access method)

Token Ring


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Token Ring Data frame• Token Ring frame defines 3 types of frames: data, token and abort

• Data Frame – carries a protocol data unit (actual data) and is addressed to a specific Rx (not broadcasted)

• Token Frame – is the placeholder frame (token) and uses only 3 of the 9 fields (SD, AC and ED)

• Abort Frame – doesn’t carry any info and is used to stop transmission

• SD – start delimiter – alert and synch the Rx

• AC – Access control - 3 bits set priority, 1 bit tells what type of frame, 1 bit is a monitor bit tells which station is monitoring or sending at the time, and 3 reservation bits for station wishing for access

• FC – Frame control – 1 bit tells if PDU (prot. Data Unit) is control info or data, 7 bits is used by Token Ring (ie. tells how to use AC field info)

• DA – Destination address

• SA – Source Address

• Data

• CRC – cyclic redundancy check – error checking

• ED – end delimiter – signals end of data

• FS – frame status – intermediate stations can set it letting the Tx know they read it, Rx can set it letting the Tx know it was copied and can be discarded now


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Token Ring Implementation• Token Ring is a series of shielded twisted pair transport medium linking each station into a ring

• Because the token needs to pass through each station with in the ring, if a station is down, it could be a problem

• Therefore, for each station, a switch is used to by pass the down (or disabled) station

• These bypass switches are packaged together as a MAU – multi-station access unit

NOTE: Don’t confuse the Ring Token technology with the Ring topology. With a ring topology approach, you would want to traverse in either direction (this is the main benefit of a ring topology).


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FDDI Ring• FDDI stands for Fiber Distributed Data

Interconnect• Data rate is the same as Fast Ethernet (100 Mbps)• Light signals versus electrical signals are used• Uses a token passing access method with self-

healing• What do we mean by “self healing” ? Ability to

detect and fix problems. The hardware automatically recognizes and fix problems


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FDDI Ring• How does the “self-healing” works ?• Two independent rings connecting all stations are used – dual

counter-rotating rings• The second ring is used only if a failure occurs• Functions like a Token Ring LAN until a failure (ie. fiber cut, node

failure)• In this case, the intermediate (non-Rx) nodes keep copies of the sent

frame too


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FDDI Ring• When the station detects it can’t communicate with the adjacent

station, it uses the second ring to bypass the adjacent station• Given a fiber cut or node failure, this station is bypassed and the

ring is closed


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FDDI Frame


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WIRELESS LANSWireless communication is one of the fastest growing technologies. The demand for connecting devices without the use of cables is increasing everywhere. Wireless LANs can be found on college campuses, in office buildings, and in many public areas. In this section, we concentrate on two

wireless technologies for LANs: 1) IEEE 802.11 wireless LANs,

sometimes called wireless Ethernet, 2) and Bluetooth, a technology for small

wireless LANs.


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Wireless Transmission (not in book)

• Wireless devices can transmit signals using radio frequency narrow band, infrared waves and radio frequency spread spectrum.

• The frequency spread spectrum technique is typically used for internet applications

• Two types of frequency spread spectrum techniques: (1) FHSS- Frequency Hopping Spread Spectrum and (2) DSSS – Direct Sequence Spread Spectrum


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FHSS- Frequency Hopping Spread Spectrum (not in book)

• Tx transmits at different carrier frequencies for the same period of time (rotates between a set of frequencies)

• The required bandwidth must be N times the original bandwidth, where N is the number of different carrier frequencies

• Tx and Rx must agree to the hopping pattern. In this case, the first bit signal is transmitted in spectrum 2.01-2.02Ghz, 2nd bit transmitted in the 2.03-2.04 Ghz spectrum, 3rd bit transmitted in the 2.04-2.05 GHz spectrum, etc..

• Good technique for security reasons – if someone tunes to one of the 5 frequency spectrums below, they would only get 1/5 of the info being transmitted.


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DSSS – Direct Sequence Spread Spectrum (not in book)

• Each bit sent by the Tx is replaced with a set of bits called a “chip code”

• For the time it takes to send the original single bit, it now will take more time to send the chip code

• Therefore, the data rate must be N times the original data rate, where N is the # of bits of the chip code

• Also, the bandwidth for the chip code should N times greater than the original bit stream’s BW Example of original bits

being transmitted as 6-bit chip codes


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ISM bands (not in book)

• In 1985, the FCC modified the radio spectrum to allow unlicensed devices (operating at 1 watt or less) to ISM bands – Industrial, Scientific and Medical bands

• Stimulated growth in wireless technology


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Wireless LANs Architecture

• IEEE 802.11 covers 2 services – (1) BSS - Basic service set and (2) ESS – Extended service set

• BSS – is the base architecture for a wireless LAN – it contains a stationary or mobile stations and a central access point (optional)

• Without central access point, the BSS can’t transmit to other BSS’s

example ? example ?


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Wireless LANs Architecture - ESS• Contains 2 or more BSS’s with central access points

• The BBS’s central access points are connected via a distribution system (could be a wired LAN) – this network is called an Infrastructure network

• BBS’s within reach of one another can communicate

• BBS’s not within reach have to communicate via the central access points


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Wireless LANs Access Method• Wireless LANs use an access method similar to CSMA/CD access method discussed

last lecture

• The access method is called CSMA/CA (vs CSMA/CD) and stands for carrier sense multiple access with collision avoidance

• With CSMA/CA, all nodes have equal access and the medium is sensed before data is sent

• However, collision detection is not applicable because the environment is wireless – THEREFORE, COLLISIONS MUST BE AVOIDED.

CSMA/CA Process

• Each station determines how long it needs the medium and all other stations refrain from using it

• After the Tx detects the medium is free, it sends a RTS (request to send) and it contains the amount of time

• The Rx acknowledges the request by issuing a CTS (clear to send) to all stations

• Tx sends data

• Rx acknowledges the receipt of data

Example


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CSMA/CA and NAV

SIFS

SIFS

All other stations

• • •

Source Destination

TimeTime Time Time

DIFS

RTS1

SIFS

CTS 2

Data3

ACK 4

NAV(No carrier sensing)

The way collisions are prevented is: when the Tx issues a RTS, a timer called Network Allocation Vector (NAV) is created for the duration of time for (1) to (4) above – all stations affected by this transmission uses the NAV in letting it know when it can check the channel for idleness

If free, Tx waits amount of time called distributed interframe space (DIFS)

After Rx receive RTS, it waits amount of time called short interframe space (SIFS), before send a CTS


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Frame formatDefines the frame type and some control info

For wireless, some time the protocol recommends “fragmentation” due to corrupted frames (the smaller the better) or frames being too large

Carries the NAV value or ID of the frame

Depends on To DS and From DS fields

Defines sequence # of frame for flow control

Can carry up to 2312 bytes

CRC-32 error detection

DS- Distribution System


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Frame Types• Wireless LANs have three categories of frames: (1) Management Frames, (2)

Control Frames, and (3) Data frames

• Mgmt frames – used for the initial communications between stations and access points

• Data Frames – used for carrying data and control info

• Control Frames – used for accessing the channel and acknowledging frames


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Bluetooth

• Wireless LAN technology designed to:– connect devices of different functions (ie phone, camera, printer,

etc)– spontaneously form (devices find each other)– connect to the Internet– be small by nature – large size will cause chaos– Handle data rate of 1 Mbps with 2.4 GHz of bandwidth– Can be interference between 802.11b wireless LAN and Bluetooth

LANS (802.15)

• The networks are called “Piconet”• Defined by standard 802.15 (PAN)


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Bluetooth Architectures (2 types)• Can have up to 8 stations

• One station is the primary and the rest are secondary stations

• all secondary stations synch their clock to the primary station.

• The communications with the primary can be 1-to-1 or 1-to-many

• Can have an unused 8th secondary – must be activated to use and some existing secondary must be deactivated

• Multiple piconets combined is a Scatternet

• A secondary station in one piconet can be a primary station in a 2nd Piconet


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Internet – Underlying Technologies• Recall the various types of interconnected networks comprising the Internet:

LANs, Point-to-Point WANs and Switched WANs

• We have covered LANS: Ethernet, Token Ring, Wireless and FDDI Ring

• Let’s cover the Point-to-Point WANsPoint-to-Point WANS

• Connect devices via a public network line (ie. telephone company)

• Telephone company – physical layer

• Point-to-Point WAN – data link layer and up

• Company services provided to make the connection:

• Modem (modem to switching station to ISP)

• DSL

• Cable Modem

• T Lines (ie. T1, T3)

• SONET (optical carriers)


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Telephony/56K Modem

Uploading,quantization noise

Downloading,no quantization noise

Sampled 8000 times per sec with 8 bits per sample (1 bit for control) = 56 kps

Digital Signal

Analog Signal

Digital Data


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P-to-P: DSL – Digital Subscriber Line• DSL – a set of technologies used to provide high-speed data service over copper wires that

connect between the central office and local residences/businesses without expensive repeaters.

• How is DSL implemented ? – high-speed DIGITAL WAN between COs – link between subscriber and the network is analog (becoming more and more digital though)

• How does DSL work ? – divides the given bandwidth into 3 bands and offer phone service on one band and up and down stream traffic on the other 2 – phone service can occur with NO interruptions.

What does POTS stands for ??Ranges changed for 4th Book Ed


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P-to-P: Other DSL Services• RADSL – rate adaptive asymmetric DSL – scales back the

speed of ADSL based on the quality of the wire and distance between the CO and user.

• Side Note: A newer version of ADSL called Universal ADSL (or UADSL) is being deployed in an attempt to standardize ADSL to a set of standard speeds – speeds vary across the Country

• HDSL – high bit rate DSL – an digital alternative to T-1 analog service (T-1 contains multiple high-speed analog lines)

• SDSL – symmetric DSL – same as HDSL however only 1 line is provided (is full-duplex)

• VDSL – very high bit rate DSL – similar to ADSL however, in addition to using twisted-pair, coaxial and fiber-optic can be used in getting a much higher bit rate


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P-to-P: Cable Modem• Still talking about point-to-point WANS• Uses the cable TV network• How does it work ?. some of the bandwidth dedicated to television signals is used for

data traffic.• How does it work ?. The data signals are modulated into sine waves and placed on analog

channels• How does it work ?. Typically, the BW in a neighborhood (or certain proximity) is shared

(like a LAN in an office). Therefore, you never know if you have access to all of the BW. The more people using cable modems the worst the performance.

• Some cable companies can dedicate some BW for phone service therefore offering voice, video/TV and data services on one cable

• Cable modems are faster than computer modems because they are not limited by the 3000 Hz BW of the telephone line

• The newer cable systems uses digital cable boxes and digital networks can send/receive data on separate digital channels

• (draw picture of typical cable/video network – briefly explain history)


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P-to-P: T1/T3 Service• Transport carriers originally designed for voice (672

circuits ???).

• Typical “long haul” or “back bone” network – also used to interconnect WANs we mentioned

• T1 line can send 8000 193-bit frames in one second

• T3 line can send 224,000 193-bit frames in one second or be treated as 28 T1 lines (we called this a channel T1)

• Fractional T lines – several customers sharing a T1 line – their data is multiplexed onto a single T1.


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P-to-P: SONET• SONET means Synchronous Optical Networks – it’s a

standard that defines a high-speed fiber-optic data carrier.

• Electrical signals (called STSs – synchronous transport signals) are converted to light or optical signals (called optical carriers)

• Comes in different rates, OC-1, OC-3, OC-9 ….. OC-48 ….OC-192

• The lowest data rate for SONET (OC-1) is greater than a T3’s data rate – Wow !


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PPP frame• To make a point-to-point connection, a protocol is needed at the data link layer

(there are multiple protocols)

• Well known protocol called PPP (point-to-point protocol) is used

• PPP is the protocol of choice when connecting IP networks over telephone lines

• Flag – bounds the PPP frame

• Address – broadcast address (recall a point-to-point connection)

• Control – frame sequencing info could go here – although most LANS don’t need a sequence number on frames (no routing)

• Protocol – tells what type of data is in the data field

• Data field – actual data

• FCS – frame check sequence – used for error detection

We also have LCP and NCP. Link Control Protocol – the PPP’s data field carry info regarding the mgmt of the link itself. Network Control Protocol – provides PPP the ability to carry actual IP packets in it’s data field.

tcp/iptcp/ip dr. clincylecture1 ch3: underlying technologies (2 of 3) project 1 is posted on website...

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