CSCD 218 : DATA COMMUNICATIONS AND NETWORKING 1

LECTURE 5 LOCAL AREA NETWORKs

LAN TECHNOLOGIES : Ethernet, Token Ring, FDDIMulti-Access Schemes : CSMA/CD, CSMA/CA and

Token Passing

LECTURER : FERDINAND KATSRIKU (PhD)

LAN Technologies : Ethernet Got its start in 1972 Xerox Palo Alto Research Centre(PARC) Commercial version released in 1975 with speed of 3Mbps Evolution through four generations :Standard,fast,gigabit

and ten-gigabit Ethernets. The most commonly used network architecture. Manchester Encoding. Uses CSMA/CD (carrier sense multiple access with collision

detection) Wait-and-listen network architecture.

Computers must contend for transmission time on the network medium.

While one computer transmits a frame, all other computers must wait

LAN Technologies : Ethernet

Main Advantage : It is one of the cheapest network architectures to implement.

Main Disadvantage : Relates to collisions on the network. The more the collisions,

the slower the network will run, and excessive collisions can even bring down the network.

Ethernet networks are typically found in a bus or star configuration, depending on the type of cabling used.

The architecture of Ethernet defines how network clients gain access to the medium, or network wire at the beginning of the communications process

Ethernet IEEE and Cable Specifications The IEEE provides specifications for a number of the

technologies related to networking computers, including Ethernet. Some specifications include : 802.3 Ethernet (CSMA/CD) LAN802.5 Token Ring LAN802.7 Broadband Technical Advisory Group802.8 Fibre-optic Technical Group802.9 Integrated Voice and Data Networks802.10 Network Security802.11 Wireless Networks

Ethernet IEEE and Cable SpecificationsEthernet Cabling :Ethernet has several different variations, each of which uses different cable types, topologies, and distance limitations.• The different types are:• 10 Base-5 (Thick Ethernet)• 10 Base-2 (Thin Ethernet)• 10 Base-T (UTP Ethernet)• 10 Base-FL• 100 Base-T• 100 Base-F• Gigabit Ethernet• 10-Gigabit Ethernet

10 Base-5 Thick EthernetThick Ethernet, officially known as 10 Base-5 10 Base-5 is laid out in a bus topology, with a single coaxial

cable connecting all nodes together. At each end of the coaxial cable is a terminator. Each node on the network physically connects to the

coaxial cable through a device called a transceiver and an AUI cable is connected between the node and the transceiver.

10 Base-5 Thick Ethernet

A single 10 Base-5 segment may be up to 500 meters (1650 feet) in length and may have up to 255 nodes connected to it. Please note that each node must be at least 2.5 meters (8.25 feet) apart.

10 Base-5 Thick EthernetAdvantage :Long Distances Possible

10 Base-5 allows distances up to 500 meters (1650 feet). Thismakes it very useful as a "backbone" technology for wiringtogether multiple locations within a building without the use of repeaters

Noise ImmunitySince 10 Base-5 uses a very heavily shielded cable, it can be

used in electrically noisy environments which can cause other network types to fail.Conceptually Simple

Since all devices on a 10 Base-5 network are simply chainedtogether on a common coaxial cable, it is a simple matter to

plan the routing of the cable.

10 Base-5 Thick EthernetDisadvantagesInflexible

10 Base-5 networks do not lend themselves well to installations where the setup of the network will

change much after the initial installation. It can be very difficult to add or move a node once it is connected to the coaxial cable.Fault Intolerant

Since 10 Base-5 uses a common physical cable to interconnect all the nodes, the failure of any part of the coaxial cable or any node has the ability to cause the collapse of the entire network.

10 Base-5 Thick Ethernet The most common use for 10 Base-5 was using it as a

"backbone“ technology. For example, a backbone can be used to connect 10 Base-T hubs to create one large network.

LAN Technologies : Token Ring Token Ring defines a method for sending and receiving data

between two network-connected devices. To communicate in a token-passing environment, any client

must wait until it receives an electronic token The token is a special frame that is transmitted from one

device to the next.

There are two types of token-passing architectures: – Token Bus is similar to Ethernet because all clients are

on a common bus and can pick up transmissions from all other stations

– Token Ring is different from Token Bus in that the clients are set up in a true physical ring structure.

– Eg : IBM Token Ring

LAN Technologies : Token Ring In Token Ring each client plugs into a device called a

Multistation Access Unit (MAU), which is similar to an Ethernet hub or switch• Each network interface has both transmit and receive ports

Token Ring topologies are physically wired in a star configuration using STP or UTP (Cat4 or higher for 16 Mbps) cabling.

LAN Technologies : FDDI Fiber Distributed Data Interface (FDDI) is another token-

passing environment that relies on a dual ring configuration for fault-tolerance.

In addition to its ability to recover from a primary ring failure FDDI also functions at 100 Mbps.

An FDDI network contains two complete rotating rings – Dual counter-rotating ring :

The primary ring is used to send data when everything is working correctly. The secondary ring provides an alternative data path

when a fault occurs in the primary ring.

LAN Technologies : FDDI FDDI is commonly used as a backbone network architecture

because of its failover capability. If the primary ring fails for any reason, the nearest

DAC/DAS will wrap the signal on the wire.

FDDI network is called self-healing because the hardware can detect acatastrophic failure and recover automatically.Hardware in the stations adjacent to a failure detect the disconnectionand reconfigure so they loop incoming hits back along the reverse path

Multi Access Schemes : CSMA All computers attached to an Ethernet participate in a distributedcoordination scheme called Carrier Sense Multiple Access (CSMA).

The scheme based on electrical signal activity on the cable to determine status.

When no computer is sending a frame, the cable does not contain electrical signals. During frame transmission, a sender transmits electrical signals used to carry encode bits. We can determine whether the media is being used by detecting the presence of a carrier.

If no carrier is present, the computer can transmit a frame. If a carrier is present, the computer must wait for the sender to finish before proceeding.

Multi Access Schemes : CSMA CSMA cannot prevent a computer from interrupting anongoing transmission (conflicts). If two computers at opposite ends of an idle cable both

have a frame ready to send simultaneously, a collision occurs.

Multi Access Schemes : CSMA/CD Carrier Sense Multiple Access with Collision Detect (CSMA)

is the formal name for access method that can monitor a cable during transmission to detect collisions.

Multi Access Schemes : CSMA/CD After a collision occurs, there will be a corrupted Ethernet frame

and the computer must wait for the cable to become idle again before transmitting a frame.

The sender will send a noise burst to all stations to abort transmission.

Ethernet requires each computer to delay for an arbitrary time t1 (where 0 ≤ t1 ≤ d) after a collision before attempting to retransmit.

If another collision occurs, the computer will wait for t2 (where 0 ≤ t2 ≤ 2d) and double range for successive collision.

In general, the wait is between 0 and 2r -1 where r is the number of collision.

This is called Binary Exponential Back off algorithm. The computer will freeze the time interval after 10 attempts and

stop trying after 15 attempts.

Multi Access Schemes : CSMA/CD

Multi Access Schemes : CSMA/CDMinimum Frame Size : For CSMA/CD to work, we need a restriction on the frame

size. Sender must detect a collision,if any,before sending the last

bit of the frame. Station does not keep copy of frame nor monitor the line

for collosion. Hence Tfr must be at least two times the max Tp. Thus Tp

for signal transfer to receiver, another Tp for collision effect to receiver.

So the sender must still be transmitting at 2TpSo to calculate for the minimum frame size,=> 2Tp * bandwidth of network ; units is in bits or bytes

Multi Access Schemes : CSMA/CDExample :A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal) is 25.6, what is the minimum size of the frame?

Solution :The frame transmission time is Tfr = 2 x Tp =51.2. This means, in the worst case, a station needs to transmit for a period of 51.2 to detect the collision. The minimum size of the frame is 10 Mbps x 51.2 =512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet.

Multi Access Schemes : WLAN’s and CSMA/CA

Problem of limited coverage:- not all computers receive all transmissions hence cannot use CSMA/CD. In the above example, if the maximum transmission distance is d, computer A does not receive computer C’s message.When A transmits to B and C also transmits to B, the frames will collide at B – hidden terminal problem.When C hears the transmission from B to A, C may falsely concluded that it cannot transmit to D – exposed terminal problem.

Multi Access Schemes : CSMA/CA A set of wireless LAN technology are available that use a

modified form of CSMA/CD known as Carrier Sense Multiple Access with Collision Avoidance (CSM/CA); basis for IEEE

802.11 LAN. The basic idea is that the sender simulates the receiver into

outputting a short frame before transmitting data so that station nearby can detect this transmission and avoid transmitting for the duration of the upcoming (large) data frame.

Control messages are much shorter than data frames, the probability of a second collision is much lower than with

conventional Ethernet. Collisions of control messages can still occur and they can be handled with backoff algorithm.

Multi Access Schemes : CSMA/CA

The operation is as follows:-• B wants to send message to C• B send a Request To Send (RTS); 30 bytes packet to C.

This packet contains the length of the data frame that will eventually follow.

• C replies by sending a Clear To Send (CTS) also containthe length of the data. B is clear to send data after

receiving the CTS.• All stations know the length of the data and their relative position to the transmitter / receiver and they can act accordingly.

Multi Access Schemes : CSMA/CACollisions are avoided through the use of CSMA/CA’s three strategies : interframe space, the contension window and acknowledgements.