LAN and WAN Frames - Layer 2 Protocols

In a TCP/IP network, every OSI Layer 2 protocols work with IP at OSI Layer 3. but, the Layer 2 protocol used depends on the logical topology and the physical media.

every protocol performs media access control for specific Layer 2 logical topologies. This means that a number of different network devices can work as nodes that operate at the data link layer while implementing these protocols. These devices contain the NICs on computers as well as the interfaces between routers and Layer 2 switches.

The Layer 2 protocols used for a particular network topology is determined by the equipment used to apply that topology. The technology is, in turn, determined by the size of the network - in terms of the number of hosts and the geographic scope - and the services to be provided over the network.

A LAN usually uses a high-bandwidth technology that is able of supporting huge figures of hosts. A LAN's moderately small geographic area (a single building or a multi-building campus) and its high density of users, make this technology cost-effective.

on the other hand, WANs cover a large geographical area, using a high bandwidth technology is usually not cost-effective for WANs. The cost of the long distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity. Traditionally, WANs have been implemented using one of two technologies: circuit switching and packet switching. Recently, frame relay and ATM networks have assumed major roles. The differentiation in bandwidth usually results in the use of different protocols for LANs and WANs.

Data link layer protocols

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  Ethernet

  
  


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  802.11 Wireless

  
  


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  Frame Relay

  
  


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  Point-to-Point Protocol (PPP)

  
  


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  HDLC

  
  

Click to view LAN and WAN Frame Demo

 

 

Pakistan Super League 2017 Schedule

The second edition of the PSL will start on from February 9 in Dubai. The matches will be played in Sharjah and Abu Dhabi apart from Dubai with the PSL 2017 final to be played in Lahore Insha Allah Ho.There is the schedule of Pakistan Super Leag matches

Date	Local Time	Time (GMT)	PSL Match	Venue
9-Feb	20:00	16:00	Peshawar vs Islamabad	Dubai
10-Feb	15:30	11:30	Lahore vs Quetta	Dubai
10-Feb	20:00	16:00	Karachi vs Peshawar	Dubai
11-Feb	15:30	11:30	Islamabad vs Lahore	Dubai
11-Feb	20:00	16:00	Quetta vs Karachi	Dubai
12-Feb	20:00	16:00	Peshawar vs Lahore	Dubai
15-Feb	20:00	16:00	Islamabad vs Quetta	Sharjah
16-Feb	20:00	16:00	Lahore vs Karachi	Sharjah
17-Feb	15:30	11:30	Peshawar vs Quetta	Sharjah
17-Feb	20:00	16:00	Karachi vs Islamabad	Sharjah
18-Feb	15:30	11:30	Quetta vs Lahore	Sharjah
18-Feb	20:00	16:00	Islamabad vs Peshawar	Sharjah
19-Feb	20:00	16:00	Peshawar vs Karachi	Sharjah
20-Feb	20:00	16:00	Lahore vs Islamabad	Sharjah
23-Feb	20:00	16:00	Karachi vs Quetta	Dubai
24-Feb	15:30	11:30	Peshawar vs Lahore	Dubai
24-Feb	20:00	20:00	Quetta vs Islamabad	Dubai
25-Feb	15:30	11:30	Lahore vs Karachi	Dubai
25-Feb	20:00	16:00	Peshawar vs Quetta	Dubai
26-Feb	20:00	16:00	Karachi vs Islamabad	Dubai
28-Feb	20:00	16:00	Playoff 1: Team 1 vs Team 2	Sharjah
1-Mar	20:00	16:00	Playoff 2: Team 3 vs Team 4	Sharjah
3-Mar	20:00	16:00	Winner Playoff 2 vs Runner Up Playoff 1	Dubai
7-Mar			PSL 2017 Final (Subject to security so time will be announced later)	Lahore

Sports and cricket are not the subjects of this blog I upload these detail only for the interest of my viewer in Pakistan super league. so this will be continued till PSL ending. 

Data Link Layer Frame

In networking, a frame is layer 2 (data link layer of the OSI model) data unit that is transmitted between network points. This frame contains complete addressing, necessary protocol, and control information. The data link layer received layer 3 (Network layer of the OSI model) PDU from layer 3 within the data field and then prepare this PDU as layer 2 frame for carrying across network media. Before transmission, data link layer protocol encapsulates this with trailer and header. There are a lot of different data link layer protocols that describe data link layer frames. At the receiving end again data link layer protocols explain and de-encapsulate this frame. Figure 3.24 Illustrate frame which has three basic parts:-

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  Header

  
  


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  Data

  
  


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  Trailer

  
  

Figure 3.24 Illustrate the layer 2 frame general structure.

The frame fields

The general frame as shown in above mention image but, the structure of the frame contains fields in the header and trailer varies according to the protocol. There is no frame structure that meets the requirements of all data transportation across all type of media. Depending on the background, the size of control information needed in the frame varies to match the access control requirements of the media and logical topology. So frame has no standard size its varies according to the media and environment.

 

Typical frame fields are following:-

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  Start and stop indicator fields – These fields  explain the start and end restrictions of the frame

  
  


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  Addressing fields- indicate the source nodes and destination nodes.

  
  


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  Type  - this field explains layer3 protocols in the data field.

  
  


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  Control - identify special flow control services like QoS (Quality of Service).

  
  


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  Data -The frame payload (Network layer packet)

  
  


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  Error Detection- These frame fields are used for error detection and are included after the data to form the trailer.

  
  

Figure 3.25 Illustrate the frame fields 

Carrier Sense Multiple Access (CSMA)

Carrier Sense Multiple Access is a networking protocol that listens to network signals on the carrier/medium before transmitting any data. CSMA is implemented in Ethernet networks with more than one computer or network device attached to it. CSMA is part of the Media Access Control (MAC) protocol.

Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is the most widely used transmission method used in half-duplex Ethernet networks.  On Ethernet, any device can attempt to send a frame at any time. Each device senses whether the line is idle and therefore available to be used. If it is, the device begins to transmit its first frame. If another device has tried to send at the same time, a collision is said to occur and the frames are discarded. Each device then waits a random amount of time and retries until successful in getting its transmission sent.

Carrier Sense Multiple Access with Collision Avoidance

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a method for carrier transmission in 802.11  networks. CMSA/CA uses a method similar to CSMA/CD to detect if the media is clear. CMSA/CA also uses additional techniques. This method does not detect collisions but attempts prevent collisions before happen. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.

In CSMA/CA, when a node receives a traffic that is to be sent, it checks to be sure that the channel is clear (no other node is transmitting at the time). If the channel is clear, then the packet is sent. If the channel is not clear, the node waits for a randomly chosen period of time and then checks again to see if the channel is clear. This period of time is called the backoff factor and is counted down by a backoff counter. If the channel is clear when the backoff counter reaches zero, the node transmits the packet. If the channel is not clear when the backoff counter reaches zero, the backoff factor is set again, and the process is repeated. Carrier sense multiple access with collision avoidance (CSMA/CA) is,  the least popular of the access methods. This access method is now used with WLANs.

Half and Full Duplex Communication

Duplex communication refers to the direction of data transmission between two devices. Half-duplex communications limit the exchange of data to one direction at a time while full-duplex allows the sending and receiving of data at the same time.

Half-duplex communication

In this type of communication where one side can talk at a time. When one side has complete transmitting its data, the other side can reply. Only one node can talk at a time. If both try to talk at the same time, a collision will take place on the network. So both device can transmit and receive on the media but cannot do so at the same time. The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-Duplex is used with contention-based access methods. this method of communication is not very efficient and requires more time to send/receive larger amounts of data. Older networks work in half-duplex mode, due to the constraints of the network medium (coax cable) and hardware equipment (hubs). Figure 3.22 illustrate the half-duplex communication.

Full-duplex communication 

Full-duplex is the type of communication where Both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both nodes at any time. Of this type, there is no danger of a collision and therefore the transfer of data is completed much faster. Ethernet switches operate in full-duplex mode by default but can operate in half-duplex if connecting to a device such as an Ethernet hub. Figure 3.23 shows full-duplex communication.

Today, all networks make use of switches instead of hubs and UTP Ethernet cabling instead of co-axial cabling, which allow full-duplex communication between all connected hosts. It is important that both interconnected interfaces operate using the

Physical LAN Topologies

Physical LAN topologies define how the end systems are physically interconnected. In shared media LANs, end devices can be interconnected using the following physical topologies:

Star topology

In Star topology, all end devices are connected to the central device.Early star topologies interconnected end devices using Ethernet hubs. However, star topologies now use Ethernet switches. The star topology is easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot.

All the data on the star topology passes through the central device before reaching the intended destination. Hub/Switch acts as a junction to connect different nodes present in Star Network, and at the same time it manages and controls the whole of the network. Depending on which central device is used, “hub” and “Switch”  can act as repeater or signal booster. Central device can also communicate with other hubs and switch of different network.

Extended Star Topology

In an extended star topology, additional Ethernet switches interconnect other star topologies.

Bus Topology

Bus Topology is the simplest of network topologies. All end systems (computers as well as servers)  are connected to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Bus topologies using coax cables were used in legacy Ethernet networks because it was inexpensive and easy to set up.

Ring Topology

In the ring topology, end systems are connected to a single circle of cable. Unlike the bus topology, the ring does not need to be terminated. The signals travel around the loop in one direction and pass through each computer, which acts as a repeater to boost the signal and send it to the next computer. Ring topologies were used in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.

Figure 3.21 illustrate physical LAN topologies

 

Network Topology

The network topology is the arrangement or relationship of the network devices, including its nodes and connecting lines. The network really has two shapes or two types of topology; one is physical and the other is logical.

The physical topology

The physical topology of a network is the actual physical and geometric layout of the network that we can see such as devices like routers, switches, and wireless access points, nodes and cables. There are several common physical topologies like point-to-point, ring, bus, star and mesh topologies. 

The logical topology

A logical topology is how devices appear connected to the user. This topology defines the way a network transfers frames from one node to the next. This topology consists of virtual links between the nodes of a network. These logical paths are defined by data link layer protocols.  The data link layer deals the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and media access control used.

Common Physical WAN Topologies

WAN's are generally interconnected using the following physical topologies:

Point-to-Point 

Only two devices are involved in a point-to-point connection, with one wire (or air, in the case of wireless) sitting between them.This is the simplest topology in networking. For this reason, this is a very popular WAN topology. Figure 3.18 illustrated the physical point-to-point topology.

Hub and Spoke

A hub and spoke network is a traditional and widely used topology for all types of networks. It's also called the star topology. In this topology, a central site interconnects branch sites using point-to-point links. The Central site is called a hub and branch site is called spokes. Communication between two spokes always travels through the hub. Figure 3.19 illustrated the physical hub and spoke topology.

Mesh Topology

A mesh network is a network topology in which each node relays data for the network so this topology provides high availability but requires that every end system is interconnected to every other system. Therefore the administrative and physical costs can be very high. Each link is essentially a point-to-point link to the other node. Variations of this topology include a partial mesh where some but not all of the end devices are interconnected. 3.20 illustrated the physical mesh topology.

Next - Common Physical LAN Topologies