Communication Networks/Ethernet

Ethernet
Ethernet was invented in 1973 at Xerox Corporation's Palo Alto Research Center(PARC) by a researcher named Bob Metcalfe. Bob Metcalfe was asked to build a networking system for the computers at PARC. Xerox wanted such a network because they were designing the world's first laser printer and wanted all of the computers there to be able to print using it. There were two challenges he faced, he had to make it fast enough for the laser printer as well as be able to connect hundreds of computers in the same building together.

ETHERNET is a frame based technology used in the Local Area Networking(LAN). The LAN market has seen several technologies, but the most dominant today is ETHERNET. The original Ethernet was created in the 1976 at xerox's Palo Alto Research Center(Parc). A computer connected via a LAN to the Internet needs all five layers of the Internet model. The Three uper layers(Network, Transpot and Application) are common to all LANS. The Data Link layer is divided into Logic Link Control(LLC) sublayer and the Medium Access Control(MAC) sublayer. The LLC is desined for all LANs. While the MAC sublayer is slightly different for each Ethernet version.

802.3 MAC Frame
The 802.3 Ethernet frame consists of seven fields: Preamble, SFD, DA, SA, Length/type of protocol data unit, upper layer data and CRC.


 * 1) Preamble: Consists of 7 bytes of alternating 0s and 1s that alerts the receiver about the coming frame and enables synchronization of receiver data clock.
 * 2) Start Frame Delimiter(SFD): Consists of 1 byte (10101011), signals end of synchronization bits, and the start of frame data.
 * 3) Destination Address(DA): Consists of 6 bytes. This is the physical address of the destination.
 * 4) Source Address(SA): Consists of 6 bytes. This is the physical address of the sender of the frame.
 * 5) Length/type field: Consists of 2 bytes. as of 802.3-1997 this field contains the etherType OR length (see EtherType)
 * 6) Data & Padding: Its minimum length is 46 bytes and maximum is 1500 bytes.
 * 7) Cyclic Redundancy Check (CRC): The last field contains the error detection information, in this case its size is 4 bytes.

Manchester Encoding


Encoding is mean transform the information in to the signals. You can send your data by converting them in to the signals. If you are sending a binary bit pattern 10000000, then there may be chance at the receiver side, it will consider as 00001000 or 00100000. So we have to find out some exact method by which receiver will determine the start, end or middle of whicheach bit withoutrefernce to an external clock. Two methods are there(1)Manchester Encoding (2)Differential Manchester Encoding.

Manchester Encoding
With this method each bit block is divided in to two equal intervals. A binary 1 bit is sent by having the voltage set high during the first interval and low in the second interval, a binary 0 is just the reverse: first low and then high. By using these type of pattern every bit period has a transition in the middle, so it would become very easy for the receiver to synchronize with sender. A drawback of this method is that it requires twice as much bandwidth as straight binary encoding because the pulse is half the width.

Differential Manchester Encoding
It is a variation of Manchester encoding method. In this, a 1 bit is indicated by the absence of a transition and 0 is indicated by the presence of the transition at the start of the interval. By doing this we can overcome by the drawback of this previous method.

But all Ethernet systems use Manchester Encoding method due to its simplicity, Ethernet does not use differential Manchester encoding due to its complexity.

CSMA/CD
Most of the Ethernet uses 1-persistent Carrier Sense Multiple Access (CSMA)/Collision Detection (CD) method, basically an algorithm for arbitration. CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:


 * 1) A device with a frame to send listens until Ethernet is not busy.
 * 2) When the Ethernet is not busy, the sender begins sending the frame.
 * 3) The sender listens to make sure that no collision occurred.
 * 4) Once the sender hears the collision, they each send a jamming signal, to ensure that all stations recognize the collision.
 * 5) After the jamming is complete, each sender randomizes a timer and waits that long.
 * 6) When each timer expires, the process starts over with Step 1.

So, all devices on the Ethernet need to use CSMA/CD to avoid collisions and to recover when inadvertent collisions occur.

The minimum length restriction is required for the correct operation of CSMA/CD. If there is a collision before the physical layer sends a frame out of a station, it must be heard by all the station. An Ethernet frame must therefore have a minimum length of 64 bytes.

Addressing
Each Station on an Ethernet network has its own network interface card. The NIC fits inside the station and provides the station with a 6-byte physical address. The Ethernet address is a 6 byte, normally written in Hexadecimal notation using a hyphen to separate bytes from each other as shown below:

Example: 06-A3-56-2C-4B-01

Unicast, Multicast and Broadcast Addresses
A source address is always a unicast address the frame comes from only one station, the destination address, however, can be unicast, multicast or broadcast. The below example shows that how to distinguish a unicast address from the multicast address.


 * Source: always0
 * Destination: Unicast 0, Multicast 1

Byte 1      Byte 2 ……………………………………………..Byte 6

A Unicast address defines only one recipient, the relationship between the sender and receiver is one to one. A Multicast addresses defines a group of addresses, the relationship is one to many. The Broadcast address is a special case of the multicast address;the recipients are all the stations on the networks. A destination broadcast address is 48 1s.

Types Of Ethernet
There are 3 types of Ethernet available in the market right now.


 * 1) Traditional Ethernet – 10 Mbps
 * 2) Fast Ethernet – 100 Mbps
 * 3) Gigabit Ethernet – 1000Mbps

1 Physical Layer
This layer encodes and decodes data. Traditional Ethernet uses Manchester encoding with rate of 10 Mbps.

2 Transceiver
It is a transmitter and receiver. It transmits signals over the medium;it receive signals over the medium, and also detects collision. It can be internal or external. If it is external then we need to connect attachment unit interface between the station.

Fast Ethernet-100Mbps
The purpose of the evaluation of  Ethernet from 10Mbps to 100Mbps is to keep the MAC Sublayer untouched. The access method is the same for the 100 Mbps. But there are 3 techniques by which we can change 10Mbps to 100Mbps.


 * 1) Auto negotiation:- It allows incompatible device to connect to one another, as a example a device designed for 10Mbps can communicate with a device desined for 100Mbps. It also allows a station to check a hub’s capabilities.
 * 2) Reconciliation Sublayer:- In fast Ethernet, this layer replaces the Physical layer of the 10Mbps, because encoding in fast ethernet is medium dependent. And encoding decoding moves to the transceiver layer.
 * 3) Medium Independent interface:- It provides a compatible path for both 10mbps and 100 mbps, it also provides the link between Physical Layer and Reconciliation  Layer.

Gigabyte Ethernet-1000Mbps


When we move from 100 mbps to 1000mbps, our idea was to leave Mac layer untouched which is not satisfied eventually.

Access Method
Gigabyte Ethernet has two approaches, one being half-duplex using CSMA/CD or full duplex with no need for CSMA/CD. The former one being interesting but complicated and not practical. while in full duplex we don’t need CSMA/CD. Generally full duplex approach is preferred over half-duplex.

Gigabyte medium independent interface
It is the specification defining reconciliation is to be connected to PHY transceiver. In this there is a chip which can work on 10mbps and 100mbps.

Management function are included and there is no cable or connector.

Bridged Ethernet
It has two effects mostly, raising the bandwidth and separating the collision domains.

1 Raise the Bandwidth
Stations shares the total bandwidth they are provided with. when we have more than one station they share the provided bandwidth hence we can put the bridge we can spilt the stations so that they can share the same bandwidth but the number of stations are less as it’s divided between stations. For example, We have 10 stations sharing 10 mbps now if we bridge them over 5-5 stations, we will have 5 stations sharing the same 10mbps bandwidth, hence bridged connection can increase the bandwidth by above mentioned method.

2 Separating collision domain
Separation of the collision domain is another advantage of bridged network. By bridging the collision domain becomes much smaller and te probability of collision is reduced by doing so.

Full duplex Ethernet
In full duplex each station has separate channel for transmitting the signal and also has receiving channel, hence collision is reduced.

Interconnecting LAN Segment
LAN segmentation simply means breaking one LAN into parts, with each part called a segment. With a single hub, or multiple hubs you have a single segment. With the use of bridge, switch, or router we can split one large LAN into small LAN segments.

First, many university and corporate departments have their own LANs. There is a need for interaction for different kind of LANs, so bridges are needed.

Second, the organization may be geographically spread over several buildings separated by considerable distances. It is cheaper to have separate LANs in each building and connect them with bridges or switches.

Third, it may be necessary to split what is logically a single LAN into separate LANs to accommodate the load. Like many universities uses different servers for file server and web sever. Multiple LANs connected by bridges are used. Each LAN contains a cluster of workstation with its own file server so that most traffic is restricted to a single LAN and does not add load to the backbone.

Fourth, in some situations, a single LAN would be adequate in terms of the load, but the physical distance between the most distant machines is too great. The only solution is to partition the LAN and install bridges between the segments. Using this technique the total physical distance covered can be increased.

Fifth, a bridge can be programmed to exercise some discretion about what is forwards and what it does not forward. This can enhance reliability by splitting the network.

Sixth, by inserting bridges at various places and being careful not to forward sensitive traffic, a system administrator can isolate parts of the network so that its traffic cannot escape and fall into the wrong hands.

Bridge Issues

 * 1) For connecting different 802 architecture communication bridge change the frame and reformat it that takes CPU time, requires a new checksum calculation, and introduces the possibility of undetected errors due to bad bits in the bridge’s memory.
 * 2) Interconnected LANs do not necessarily run at the same data rate.
 * 3) Different 802 LANs have different maximum frame lengths. When a long frame must be forwarded onto a LAN that cannot accept it. Splitting the frame into pieces is out of the question in this layer. Basically, there is no solution for frames that are too large. They must be discarded.
 * 4) Both 802.11 and 802.16 support encryption in the data link layer. Ethernet does not. So some encryption which used by wireless is lost when traffic passes over an Ethernet.
 * 5) Both 802.11 and 802.16 provide QoS provide it in various forms, the former using PCF mode and the letter using constant bit rate connection. Ethernet has no concept of quality of service, so traffic from either of the others will lose its quality of service when passing over an Ethernet.

Format Conversation And Reformatting
Sender A resides on wireless network and receiver B resides on Ethernet. The packet descends into the LLC sub layer and acquires an LLC header (shown in black in the figure). Then it passes into the MAC sub layer and an 802.11 header is prepended to it. This unit goes out over the air and picked up by the base station. Which sees that it needs to go to the fixed Ethernet. When it hits the bridge connecting the 802.11 network to the 802.3 network; it starts in the physical layer and works its way upwards. In the MAC sublayer in the bridge, the 802.11 header is stripped off. The bare packet is then handed off to the LLC sublayer in the bridge. In this example, the packet is destined for an 802.3 LAN, so it works its way down the 802.3 side of the bridge and off it goes on the Ethernet.

Note: A bridge connecting k different LANs will have k different MAC sublayers and k different physical layers, one for each type.

Traffic Isolation
Bridges come in two main forms. One type of bridge is what is known as a transparent or learning bridge. This type of bridge is transparent to the device sending the packet. At the same time this bridge will learn over time what devices exist on each side of it. This is done by the bridge’s ability to read the Data-Link information on each packet going across the network. By analyzing these packets, and seeing the source MAC address of each device, the bridge is able to build a table of which exist on what side of it. There usually is a mechanism for a person to go in and also program the bridge with address information as well; learning bridge references an internal table of address. This table is either learned by the bridge, from previous packet deliveries on the network, or manually programmed into the bridge.

Another type of bridge is a source routing bridge. This type of bridge is employed on a token-ring network. A source routing bridge is a bridge that reads information in the packet will state the route to the destination segment on the network. A source routing bridge will analyze this information to determine whether or not this stream of data should or should not be passed along.

Bridges, however, cannot join LANs that are utilizing different network addresses, this is because bridges operate at the layer 2 of the OSI model and depends on the physical address of devices and not at the Network Layer which relies on logical network addresses.

Forwarding Table & Backward Learning
Bridges build the bridge table by listening to incoming frames and examining the source MAC address in the frame. If a frame enters the bridge and the source MAC address is not in the bridge table, the bridge creates an entry in the table. The MAC address is placed into the table, along with the interface in which the frame arrived. This is known as self address learning method.

For filtering the packets between LAN Segments Bridge uses a bridge table. When a frame is receive and destination address is not in the bridge table it broadcast or multicast, forward on all ports except the port in which the frame was received. If the destination address is in the bridge table, and if the associated interface is not the interface in which the frame arrived, forward the frame out the one correct port. Else filter the frame not forward the frame.

STP’s working
The spanning tree algorithm places each bridge or switch port into either a forwarding state or a blocking state. All the ports in the forwarding state are considered to be in the current spanning tree.

First Root Bridge is selected. It is selected by lowest serial number. All ports of root bridge are designated port. Each non-root bridge receives the hello packet from root bridge. After that each bridge compares path cost to the root bridge with each port. The port which has lowest path cost is declared as a root port for the non-root bridge. That is known as root port. The root port of each bridge is placed into a forwarding state.

Finally each Lan segment has an STP designated bridge on that segment. Many bridges can attach to the same Ethernet segment. The bridge with the lowest cost from itself to the root bridge port, as compared to the other bridges attached to the same segment, is the designated bridge for that segment. The interface that the bridge uses to connect to that segment is called the designated port for that segment, the port is placed into a forwarding state. STP places all other ports into a blocking state.

In the intelligent bridges and switches STP runs automatically and no need for manual configuration. The STP algorithm continues to run during normal operation.

VLAN
A group of device on one or more LANs that are configured(using management software) so that they can communicate as if they were attached to the same wire, when in fact, they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

Each switch has two VLANs. On the first switch, the send of VLAN A and VLAN B occurs through a single port, which is trunked. These VLANs go to both the router and, through another port, to the second switch. VLAN C and VLAN D are trunked from the second switch to the first switch and, through that switch, to the router. This trunk can carry traffic from all four VLANs. The trunk link from the first switch to the router can also carry all four VLANs. In fact, this one connection to the router actually allows the router to appear on all four VLANs. The appearance is that the router has four different physical ports with connection to the switch.

The VLANs can communicate with each other via the trunking connection between the two switches. This communication occurs with use of the router. For example, data from a computer on VLAN A that need to get to a computer on VLAN B must travel from the switch to the router and back again to the switch. Because of the transparent bridging algorithm and trunking, both PCs and the router think that they are on the same physical segment. LAN switches can make a big difference in the speed and quality of your network. VLAN 1 is the default VLAN; it can never be deleted. All untagged traffic falls into this VLAN by default.

There are the following types of Virtual LANs:


 * 1) Port-Based VLAN: each physical switch port is configured with an access list specifying membership in a set of VLANs.
 * 2) MAC-based VLAN: a switch is configured with an access list mapping individual MAC addresses to VLAN membership.
 * 3) Protocol-based VLAN: a switch is configured with a list of mapping layer 3 protocol types to VLAN membership - thereby filtering IP traffic from nearby end-stations using a particular protocol such as IPX.
 * 4) ATM VLAN - using LAN Emulation (LANE) protocol to map Ethernet packets into ATM cells and deliver them to their destination by converting an Ethernet MAC address into an ATM address.

Advantages of VLAN

 * Reduces the broadcast domain, which in turn reduces network traffic and increases network security (both of which are hampered in case of single large broadcast domain)
 * Reduces management effort to create sub networks
 * Reduces hardware requirement, as networks can be logically instead of physically separated
 * Increases control over multiple traffic types

802.1Q
The IEEE’s 802.1Q standard was developed to address the problem of how to break large networks into smaller parts so broadcast and multicast traffic wouldn’t grab more bandwidth than necessary. The standard also helps provide a higher level of security between segments of internal networks.

Frame Format
The 802.1q frame format is same as 802.3. the only change is the addition of 4 bytes fields. The first two bytes are the VLAN protocol ID. It always has the value of 0X8100. The second 2-bytes field contains three subfields.


 * 1) VLAN identifier
 * 2) CFI
 * 3) PRI


 * VID- VLAN ID is the identification of the VLAN, which is basically used by the standard 802.1Q. It has 12 bits and allow     the identification of 4096 (2^12) VLANs. Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum possible VLAN configurations are 4,094.
 * User Priority- Defines user priority, giving eight (2^3) priority levels. IEEE 802.1P defines the operation for these 3 user priority bits.
 * CFI- Canonical Format Indicator is always set to zero for Ethernet switches. CFI is used for compatibility reason between Ethernet type network and Token Ring type network. If a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to an untagged port • User Priority- Defines user priority, giving eight (2^3) priority levels. IEEE 802.1P defines the operation for these 3 user priority bits.

Because inserting this header changes the frame, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer.

Problems

 * 1) Sketch the Manchester Encoding for the bit stream: 0001110101
 * 2) Sketch the differential Manchester Encoding for the bit stream of the previous problem. Assume the line is initially in the low state.

0001110101 = LHLHLHHLLHHLHLLHLHHL ( Differential Manchester Encoding Pattern)