Underlying Technologies 4 Underlying Technologies Tutun Juhana Telecommunication Engineering School of Electrical Engineering & Informatics Institut Teknologi Bandung Computer Networks

WIRED LOCAL AREA NETWORKS

A local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus Most LANs today are also linked to a wide area network (WAN) or the Internet

Token Ring ATM LAN Token Bus FDDI Fiber Distributed Data Interface Ethernet is the Winner! IEEE Standard Project 802, designed to regulate the manufacturing and interconnectivity between different LANs

IEEE Standards Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols

The Ethernet Robert Melancton "Bob" Metcalfe (born April 7,1946) is an electrical engineer from the United States who co-invented Ethernet

Frame Format ‘Length” used by IEEE standard to define the number of bytes in the data field “Type” used by original Ethernet to define upper-layer protocol using the frame

Frame Length The minimum length restriction is required for the correct operation of CSMA/CD If the upper-layer packet is less than 46 bytes, padding is added to make up the difference Reason for the maximum length restriction: To reduce the size of the buffer (memory was very expensive when Ethernet was designed) It prevents one station from monopolizing the shared medium

Addressing Each station on an Ethernet network has its own network interface card (NIC) The NIC provides the station with a 6-byte physical address

Ethernet Address The address normally is referred to as the data link address, physical address, or MAC address

Example: 00-14-22 OUI for Dell 00-04-DC for Nortel 00-40-96 for Cisco 00-30-BD for Belkin

The address is sent left-to-right, byte by byte For each byte, it is sent right-to-left, bit by bit Example Show how the address 47:20:1B:2E:08:EE is sent out on line

Unicast, Multicast, and Broadcast Addresses A source address is always a unicast address the frame comes from only one station The destination address can be unicast, multicast, or broadcast

The broadcast address is a special case of the multicast address; the recipients are all the stations on the LAN

Define the type of the following destination addresses 4A:30:10:21:10:1A 47:20:1B:2E:08:EE FF:FF:FF:FF:FF:FF

Ethernet Evolution

Standard Ethernet

Access Method: CSMA/CD The IEEE 802.3 standard defines carrier sense multiple access with collision detection (CSMA/CD) as the access method for traditional Ethernet

Stations on a traditional Ethernet can be connected together using a physical bus or star topology, but the logical topology is always a bus Physical bus topology Physical star, logically bus topology The medium (channel) is shared between stations and only one station at a time can use it All stations receive a frame sent by a station (broadcasting) The real destination keeps the frame while the rest drop it

How can we be sure that two stations are not using the medium at the same time? If they do, their frames will collide with each other

To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed

Carrier sense multiple access (CSMA) requires that each station first listen to the medium (or check the state of the medium) before sending sense before transmit, or listen before talk CSMA can reduce the possibility of collision, but it cannot eliminate it The possibility of collision still exists because of propagation delay

Space/time model of a collision in CSMA

Collision of the first bit in CSMA/CD A transmits for the duration t4 - t1; C transmits for the duration t3 - t2  for the protocol to work, the length of any frame divided by the bit rate in this protocol must be more than either of these durations Before sending the last bit of the frame, the sending station must detect a collision, if any, and abort the transmission  because, once the entire frame is sent, station does not keep a copy of the frame and does not monitor the line for collision detection

The worst collision on a shared bus Minimum Frame Size The worst collision on a shared bus http://cnp3book.info.ucl.ac.be/lan/lan/

The frame transmission time Tfr must be at least two times the maximum propagation time Tp

Example In the standard Ethernet, if the maximum propagation time is 25.6 μs, what is the minimum size of the frame?

CSMA/CD flow diagram The station transmits and receives continuously and simultaneously (using two different ports)

Implementation

Fast Ethernet

IEEE created Fast Ethernet under the name 802.3u Fast Ethernet is backward-compatible with Standard Ethernet, but 10 times faster (100 Mbps) The goals of Fast Ethernet: Upgrade the data rate to 100 Mbps Make it compatible with Standard Ethernet Keep the same 48-bit address. Keep the same frame format. Keep the same minimum and maximum frame lengths

MAC Sublayer Keep only the star topology There are two choices: half duplex and full duplex In the half-duplex approach, the stations are connected via a hub in the full-duplex approach, the connection is made via a switch with buffers at each port The access method is the same (CSMA/CD) for the half-duplex approach For full-duplex there is no need for CSMA/CD The implementations keep CSMA/CD for backward compatibility with Standard Ethernet

Autonegotiation Autonegotiation allows two devices to negotiate the mode or data rate of operation It was designed particularly for the following purposes: To allow incompatible devices to connect to one another To allow one device to have multiple capabilities To allow a station to check a hub’s capabilities.

Implementation

UTP Colour code

Gigabit Ethernet

Gigabit Ethernet (IEEE 802.3z) The goals of the Gigabit Ethernet: Upgrade the data rate to 1 Gbps Make it compatible with Standard or Fast Ethernet Use the same 48-bit address Use the same frame format Keep the same minimum and maximum frame lengths. To support autonegotiation as defined in Fast Ethernet

MAC Sublayer A main consideration: keep the MAC sublayer untouched  To achieve a data rate of 1 Gbps, this was no longer possible Gigabit Ethernet has two distinctive approaches for medium access Half-duplex Full-duplex Almost all implementations of Gigabit Ethernet follow the full-duplex approach

Full-Duplex Mode There is a central switch connected to all computers or other switches Each switch has buffers for each input port in which data are stored until they are transmitted There is no collision in this mode  CSMA/CD is not used The maximum length of the cable is determined by the signal attenuation in the cable, not by the collision detection process

Half-Duplex Mode A switch can be replaced by a hub  a collision might occur  CSMA/CD is used  the maximum length of the network is totally dependent on the minimum frame size Three solutions have been defined: Traditional Carrier extension Frame bursting

Traditional approach Keep the minimum frame length 512 bits The maximum network length only 25 m (because the length of a bit is 1/100 shorter than in standard Ethernet) It may not even be long enough to connect the computers in one single office

Carrier Extension Increase the minimum frame length  512 bytes (4096 bits)  8 times longer It forces a station to add extension bits (padding) to any frame that is less than 4096 bits The maximum length of the network can be increased 8 times to a length of 200 m This allows a length of 100 m from the hub to the station Frame RRRRRRRRRRRRR Carrier Extension 512 bytes

Carrier Extension is very inefficient if we have a series of short frames to send (each frame carries redundant data)

Frame Bursting To improve efficiency, frame bursting was proposed Instead of adding an extension to each frame, multiple frames are sent To make these multiple frames look like one frame, padding is added between the frames (96 bits) so that the channel is not idle  The method deceives other stations into thinking that a very large frame has been transmitted Frame Extension Frame Frame Frame 512 bytes Frame burst Maximum frame burst is 8192 bytes

Gigabit Ethernet Implementation

Ten-Gigabit Ethernet

Ten-Gigabit Ethernet standard : IEEE802.3ae The goals : Upgrade the data rate to 10 Gbps. Make it compatible with Standard, Fast, and Gigabit Ethernet. Use the same 48-bit address. Use the same frame format. Keep the same minimum and maximum frame lengths. Allow the interconnection of existing LANs into a metropolitan area network (MAN) or a wide area network (WAN) Make Ethernet compatible with technologies such as Frame Relay and ATM.

Implementation Ten-Gigabit Ethernet operates only in full duplex mode  no need for contention  CSMA/CD is not used

Wireless LANs

Topics Discussed in the Section IEEE 802.11 MAC Sublayer Addressing Mechanism Bluetooth

Basic service sets (BSSs)

Extended service sets (ESSs)

CSMA/CA flow diagram

CSMA/CA and NAV 2CTS

Hidden station problem

Note The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

Use of handshaking to prevent hidden station problem

Exposed station problem

Use of handshaking in exposed station problem

Piconet

Scatternet

POINT-TO-POINT WANs

65K Modems DSL Technology Cable Modem T Lines/E lines SONET/SDH PPP Topics Discussed in the Section 65K Modems DSL Technology Cable Modem T Lines/E lines SONET/SDH PPP

56K modem

Note ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses.

Bandwidth division

ADSL and DSLAM

Cable Modem

Cable modem configuration

T1/E1 Carrier System

SONET/SDH

PPP (point-to-point protocol)

Switched WANs

Topics Discussed in the Section X.25 Frame Relay ATM

X.25 Speed is about 64 kbps

Frame Relay Speed 56Kbps to 1.544Mbps

ATM (Asynchronous Transfer Mode) Note A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-size block of information.

ATM multiplexing

Architecture of an ATM network

Virtual circuit

Note A virtual connection is defined by a pair of numbers: the VPI and the VCI.

ATM layers

Use of the layers

The IP protocol uses the AAL5 sublayer. Note The IP protocol uses the AAL5 sublayer.

CONNECTING DEVICES

Topics Discussed in the Section Repeaters Bridges Routers

Connecting devices

Repeater or hub

A repeater forwards every bit; it has no filtering capability. Note A repeater forwards every bit; it has no filtering capability.

A bridge has a table used in filtering decisions. Note A bridge has a table used in filtering decisions.

A bridge does not change the physical (MAC) addresses in a frame. Note A bridge does not change the physical (MAC) addresses in a frame.

Bridge

Learning bridge M M M M

A router is a three-layer (physical, data link, and network) device. Note A router is a three-layer (physical, data link, and network) device.

A repeater or a bridge connects segments of a LAN. Note A repeater or a bridge connects segments of a LAN. A router connects independent LANs or WANs to create an internetwork (internet).

Routing example

A router changes the physical addresses in a packet. Note A router changes the physical addresses in a packet.