Network Topologies

(Adapted from Lecture notes for FDU students by Professor Nikolai Bezroukov)

Contents

• Introduction

• Four Network Topologies

• LAN Media and IEEE identifiers

• Network devices

• Network Categories

  • Peer-to-Peer Communication in TCP/IP Model

  • Encapsulation and Decapsulation

Introduction

A local area network (LAN) is generally defined as a communication network used by a single organization over a limited distance, which permits users to share information and resources. This general definition, while stating the basic nature and purpose of a local area network, is incomplete because LANs can span very large areas through gateways and routers, and because advanced applications are increasingly available that not only permit but encourage interdepartmental usage of the same

The PC LAN (local area network) emerged in the mid-1980’s as a requirement to meet the need of sharing local resources and information on personal computers, which had begun to complement the centralized computing facilities of companies. A network consists of multiple computers connected using some type of interface, each having one or more interface devices such as a Network Interface Card (NIC) and/or a serial device for PPP networking. Each computer is supported by network software that provides the server or client functionality. The hardware used to transmit data across the network is called the media. It may include copper cable, fiber optic, or wireless transmission. The most popular cabling is 100Base-TX category 5 Ethernet cable. This is twisted copper cabling which looks  similar to TV coaxial cable. Like 10BASE-T, the proper pairs are the orange and green pairs (canonical second and third pairs).  It is terminated on each end by a connector that looks like enlarged phone connector. Its maximum segment length is 100 meters.  With all 100BASE-T Ethernet (including 100BASE-TX), the raw bits of a packet to be transmitted -- a series of 0 and 1 bits at 100 Mbit/s -- are typically transferred 4 bits at a time clocked at 25 MHz to the Ethernet hardware. In its typical configuration, 100BASE-TX uses one pair of twisted wires in each direction, providing 100 Mbit/s of throughput in each direction (full-duplex). See IEEE 802.3 for more details.

The configuration of 100BASE-TX networks is very similar to 10BASE-T. When used to build a local area network, the devices on the network (computers, printers etc.) are typically connected to a hub or switch, creating a star network. Alternatively it is possible to connect two devices directly using a crossover cable.

Four Network Topologies

The network topology describes the method used to do the physical wiring of the network. The main ones are:

1. Bus - Both ends of the network must be terminated with a terminator. A barrel connector can be used to extend it.
2. Star - All devices revolve around a central hub, which is what controls the network communications, and can communicate with other hubs. Range limits are about 100 meters from the hub.
3. Ring - Devices are connected from one to another, as in a ring. A data token is used to grant permission for each computer to communicate.
4. VLAN - Virtual area networks (VLAN) recently become that standard way of structuring networks. In this case all systems are connected to the same physical device and this device (typically a switch) is capable of organizing multiple logical networks out of all connected notes. Advantages:
   • Using VLAN reduces size of broadcast domain.
   • You can move devices between VLANs without any hardware reconfiguration.

The dominant LAN topology is star topology since that is what Ethernet networks use.

LAN Media and IEEE identifiers

Many different types of physical connectors between networking nodes are used. Most popular options include twisted pairs and fiber. Initially coaxial capable were used and now coaxial cables with dual wires might stage a return as a media for ten gigabit networking. IEEE identifies for media consist of three parts:

• The fist represents media speed
• The second represents the type of signaling. For example BASE means baseband -- typew of signaling when the entire bandwidth of the cable used foe one signal. In this case two systems cannot transmit signals at the same time over such a cable.
• The third represent the segment type  (T for twisted pair, F for fiber) and approximate segment length in hundred meters (for example 5 in 10 BASE-5 means 500 meters)

IEEE 802.3 is a collection of IEEE standards defining the physical layer and the media access control (MAC) sublayer of the data link layer of wired Ethernet. This is generally a LAN technology with some WAN applications. Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable.

802.3 is a technology that can support the IEEE 802.1 network architecture.

The maximum packet size is 1518 bytes, although to allow the Q-tag for Virtual LAN and priority data in 802.3ac it is extended to 1522 bytes. If the upper layer protocol submits a PDU (Protocol data unit) less than 64 bytes, 802.3 will pad the data field to achieve the minimum 64 bytes.

Although it is not technically correct, the terms "packet" and "frame" are used interchangeably. The ISO/IEC 8802-3 ANSI/IEEE 802.3 Standards refer to MAC sub-layer frames consisting of the Destination Address, Source Address, Length/Type, data, and FCS fields. The Preamble and SFD are (usually) considered a header to the MAC Frame. This header plus the MAC Frame constitute a "Packet".

The original Ethernet is called "Experimental Ethernet" today. It was developed by Robert Metcalfe in 1972 (patented in 1978) and was based in part on the wireless ALOHAnet protocol. It is not in use anywhere, but is thought to be the only Ethernet by some purists. The first "Ethernet" that was generally used outside Xerox was the DIX Ethernet. However, as DIX Ethernet was derived from Experimental Ethernet, and as many standards have been developed that are based on DIX Ethernet, the technical community has accepted the term Ethernet for all of them. Therefore, the term "Ethernet" can be used to name networks using any of the following standardized media and functions:

Ethernet Standard	Date	Description
Experimental Ethernet	1972	2.94 Mbit/s (367 kB/s) over coaxial cable (coax) cable bus
Ethernet II (DIX v2.0)	1982	10 Mbit/s (1.25 MB/s) over thin coax (thinnet) - Frames have a Type field. This frame format is used on all forms of Ethernet by protocols in the Internet protocol suite.
IEEE 802.3	1983	10BASE5 10 Mbit/s (1.25MB/s) over thick coax - same as DIX except Type field is replaced by Length, and an 802.2 LLC header follows the 802.3 header
802.3a	1985	10BASE2 10 Mbit/s (1.25 MB/s) over thin Coax (thinnet or cheapernet)
802.3b	1985	10BROAD36
802.3c	1985	10 Mbit/s (1.25 MB/s) repeater specs
802.3d	1987	FOIRL (Fiber-Optic Inter-Repeater Link)
802.3e	1987	1BASE5 or StarLAN
802.3i	1990	10BASE-T 10 Mbit/s (1.25 MB/s) over twisted pair
802.3j	1993	10BASE-F 10 Mbit/s (1.25 MB/s) over Fiber-Optic
802.3u	1995	100BASE-TX, 100BASE-T4, 100BASE-FX Fast Ethernet at 100 Mbit/s (12.5 MB/s) w/autonegotiation
802.3x	1997	Full Duplex and flow control; also incorporates DIX framing, so there's no longer a DIX/802.3 split
802.3y	1998	100BASE-T2 100 Mbit/s (12.5 MB/s) over low quality twisted pair
802.3z	1998	1000BASE-X Gbit/s Ethernet over Fiber-Optic at 1 Gbit/s (125 MB/s)
802.3-1998	1998	A revision of base standard incorporating the above amendments and errata
802.3ab	1999	1000BASE-T Gbit/s Ethernet over twisted pair at 1 Gbit/s (125 MB/s)
802.3ac	1998	Max frame size extended to 1522 bytes (to allow "Q-tag") The Q-tag includes 802.1Q VLAN information and 802.1p priority information.
802.3ad	2000	Link aggregation for parallel links
802.3-2002	2002	A revision of base standard incorporating the three prior amendments and errata
802.3ae	2003	10 Gbit/s (1,250 MB/s) Ethernet over fiber; 10GBASE-SR, 10GBASE-LR, 10GBASE-ER, 10GBASE-SW, 10GBASE-LW, 10GBASE-EW
802.3af	2003	Power over Ethernet
802.3ah	2004	Ethernet in the First Mile
802.3ak	2004	10GBASE-CX4 10 Gbit/s (1,250 MB/s) Ethernet over twin-axial cable
802.3-2005	2005	A revision of base standard incorporating the four prior amendments and errata.
802.3an	2006	10GBASE-T 10 Gbit/s (1,250 MB/s) Ethernet over unshielded twisted pair(UTP)
802.3ap	exp. 2007	Backplane Ethernet (1 and 10 Gbit/s (125 and 1,250 MB/s) over printed circuit boards)
802.3aq	2006	10GBASE-LRM 10 Gbit/s (1,250 MB/s) Ethernet over multimode fiber
802.3ar	exp. 2007	Congestion management
802.3as	exp. 2006	Frame expansion
802.3at	in work group	Power over Ethernet enhancements
802.3au	2006	Isolation requirements for Power Over Ethernet (802.3-2005/Cor 1)
802.3av	in study group	10 Gbit/s EPON
802.3 HSSG	2009	Higher Speed Study Group. 100 Gb/s up to 100 m or 10 km using MMF or SMF optical fiber respectively

What is defined in earlier IEEE 802.3 standards is often confused for what is used in practice: most network frames you will find on an Ethernet will be DIX frames, since the Internet protocol suite will use this format, with the type field set to the corresponding IETF protocol type. IEEE 802.3x-1997 allows the 16-bit field after the MAC addresses to be used as a type field or a length field, so that DIX frames are also valid 802.3 frames in 802.3x-1997 and later versions of the IEEE 802.3 Ethernet standard.

Network devices

Hub, switch look similar: There is a row of RJ-45 jacks, sometimes called "ports." (What's a "jack"? It looks like the hole in the wall where you'd plug in a telephone, only bigger.) There may be some lights to tell you it's working. There will be some way to provide electric power to it.

• Repeaters. Repeaters are devices that amplify and regenerate the data signal, bit by bit, to extend the distance of the transmission. Repeater does not read or interpret the data.
• Hubs are the central devices of a star topology network. It simply connects all the devices on its "ports" together. An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fiber optic Ethernet devices together, making them act as a single segment. Hubs are typically used on small segments were performance is not critical. Collisions commonly occurs on networks implemented using hubs as all systems are connected to a hum and listen to all the traffic on he network.
• Bridges. A bridge is a network-layer device that reads and interprets packet addresses for filtering or forwarding packets. Bridges connect two or more network segments. Collisions commonly occur on a bridged network because the collision domain consists of more than one system.  Bridges provide some traffic isolation.
• Switches are multiport devices that control the logical dynamic connection and disconnection between any two cable segments. Switches are high-bandwidth devices because multiple data paths can be established and used simultaneously. Switches reduce the number of collisions on a network by replacing a single shared data path with multiple dedicated data paths.

  A network switch (or just switch) is a networking device that performs transparent bridging (connection of multiple network segments with forwarding based on MAC addresses) at up to the speed of the hardware. Common hardware includes switches, which can connect at 10,100, or 1000 megabits per second, at half or full duplex. Half duplex means that the device can only send or receive any given time, whereas full duplex can send and receive, at the same time. The use of specially designed expansion also makes it possible to have large numbers of connections utilizing different mediums of networking, including Ethernet, Fibre Channel, ATM, 802.11, to name frequently used technologies.

  A switch can connect Ethernet, token ring, Fibre Channel or other types of packet switched network segments together to form a Internetwork.

  If a network has only switches and no hubs then the collision domains are either reduced to a single link or, if both ends support full duplex, eliminated altogether. The principle of a fast hardware forwarding device with many ports can be extended to higher layers giving the multilayer switch.

Network Categories and Types of Communication

There are two main types of network categories which are:

• Server based
• Peer-to-peer

In a server based network, there are computers set up to be primary providers of services such as file service or mail service. The computers providing the service are are called servers and the computers that request and use the service are called client computers.

In a peer-to-peer network, various computers on the network can act both as clients and servers. For instance, many versions of Microsoft Windows allow file and print sharing. These computers can act both as a client and a server and are also referred to as peers. Many networks are combination peer-to-peer and server based networks. The network operating system uses a network data protocol to communicate on the network to other computers. The network operating system supports the applications on that computer. A Network Operating System (NOS) includes Windows NT, Novell Netware, Linux, Unix and others.

Peer-to-Peer Communication in TCP/IP Model

Peer-to-peer communication is also the way adjacent layers of TCP/IP model communicate with each other. It occurs when one layer on a system communicates with the corresponding layer of another system. For example application layer of one system communicates with application layer on the other system.

Encapsulation and Decapsulation

Each TCP/IP layer on the sender encapsulates the data and adds header information about the corresponding protocol layer. The header information helps the receiving host decapsulate the data and direct the message to the appropriate application.

Data encapsulation occurs when:

• Data travels down through layers at the source system’s end
• Headers and trailers are added before the data is passed down to the next successive layer

Data decapsulation occurs when:

• Data travels up through layers at the destination system’s end. Headers and trailers are removed before the data is passed up to the next layer TCP/IP Protocols

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Last modified: March 12, 2019