qos

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QOS (QUALITY OF SERVICE) Q

QoS (Quality of Service)QoS is the idea that transmission rates, error rate, and other characteristics can be measured, improved upon, and to some extent guaranteed. It refers to a defined level of performance in a data communications system and to the mechanisms in the network software that make the actual determination of which packets have priority .

• Today, everything is built around IP, and there are a variety of methods competing to provide QoS in IP networks.

QoS can be measured and guaranteed in terms of the average delay at the gateway, variation in delay, and the transmission error rate.

QoS typically uses methods that employ reservation of the bandwidth (ex. RSVP).

For example, to ensure that Real Time voice and video are delivered without annoying blips, a guarantee of bandwidth is required. The plain old telephone system (POTS) has delivered the highest quality of service for years, because there is a dedicated channel between parties. However, when data is broken into packets that travel through dozens of routers and can even take different routes, QoS mechanisms must be built in to give one data stream priority over another.

You can think of QoS as ‘micro-level’ traffic control and CoS (Class of Service) as ‘macro-level’ traffic control.

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Q QOS (QUALITY OF SERVICE)

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ROUTERS R

RoutersA Router is a device that forwards data packets from one local area network (LAN) or wide area network (WAN) to another. A router works much like a bridge except a router decides whether to forward a packet by looking at the TCP/IP addresses rather than the MAC Address. A router puts the network-addressed packet into a new packet labeled with the MAC Address, either of the next device or of the final target computer.

Many routers can also function as bridges by connecting to at least two networks and deciding which way to send each information packet based on routing tables and routing protocols. Based on this information, routers read the network address in each transmitted frame and make a decision on how to send it, based on the most expedient route (traffic load, line costs, speed, bad lines, etc.).

Example:

Routers provide the ability to connect two networks together. Rather than a single network becoming congested by the number of users in various locations, a router can be utilised to split a network and keep users who exchange information regularly in one network.

• Routers work at Layer 3 in the protocol stack, whereas bridges and switches work at Layer 2.

Other related Information:

• A Layer-3 switch is a switch that can perform routing functions.

• An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network.

• A brouter is a network bridge combined with a router.

• Routers offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

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R ROUTERS

A router uses the information in the Network Layer header (Layer 3) of a packet to determine how and where to route it.

• The most important piece of routing information is the network address of the packet. This address is different from the MAC Address found in the Data Link Layer header of a packet.

• While the MAC Address is a unique physical address assigned to each NIC, the network address is a user-defined software address.

• The router determines the best path to the destination based on routing update protocols.

A network of routers is often used to link multiple LANs in an internetwork.

Routers maintain tables of adjacent LANs and other routers in the internetwork.

• When a router receives a packet, it checks its table to see if it can send the packet directly to its destination. If not, it determines the location of a router that can forward the packet to its destination.

• Routers are also used to send information between VLANs.

Host sends packet to router’s MAC address with the IP (Network Layer) address of the Destination Host

Router examines packet’s destination address & determines if it knows how to forward the packet to the next hop

If no, packet drops

If yes, changes physical address to that of next hop and forwards the packet

321

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SUBNET S

SubnetLarge networks can be divided into smaller segments which are called subnets. Dividing or subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, security, and the most common reason; to control network traffic.

In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, therefore, subnets are used to create a more efficient network.

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S SUBNET

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SUBNET ADDRESS S

Subnet Address A Subnet address is an extension of the IP addressing scheme. This subdivision of the network is not visible to external networks, however, the internal network becomes much more structured.

Subnetting

Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that broadcast domain. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to separate IP networks to minimize the amount of traffic each segment must receive.

An IP Address consists only of a network address portion and a host address portion, therefore, an additional element must be added to distinguish the subnet mask. The network portion and host portion of the IP Address are determined by using the subnet mask.

For example, an IP Address of 204.171.64.2 would be a Class C Network and the subnet mask would start at 255.255.255.0.

Network Class First OctetDefault Subnet Mask # of Networks # of Hosts per network

Class A 1 - 126 255.0.0.0 126 16, 777, 214

Class B 128 - 191 255.255.0.0 16, 384 65, 534

Class C 192 - 223 255.255.255.0 2, 097, 152 254

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S SUBNET ADDRESS

ANDing Process

If information sent on a network is for the same subnet, the information is sent directly to the host. If the destination address is a different network, the information is sent to a router which will direct the information to the correct network. This process of identifying a host’s subnet or network address is defined by using the Boolean ANDing operation. To understand the Anding function, convert your IP Address into binary format. The logical AND operation com-pares 2 bits and if they are both "1", then the result is "1"; otherwise, the result is "0".

Example for IP:204.171.64.2

Subnet mask: 255.255.255.0

In this Class C address, the Network ID is 204.171.64.0.

In this Class C address, the Network ID is 204.171.64.160.

Bit values for an 8-bit binary number

Add the numeric values of each 1 bit in the binary number to convert to the decimal equivalent.

Example 1 10000000 binary is 128 decimal

Example 2 11000000 binary is 128 + 64 = 192 decimal

Example 3 11000010 binary is 128 + 64 + 2=194 decimal

IP Address 204 171 64 2

Binary Equivalent 1 1 0 0 1 1 0 0 1 0 1 0 1 0 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0

Subnet mask 255 255 255 0


Network ID 204 171 64 0


IP Address 204 171 64 160


Subnet mask 255 255 255 224


Network ID 204 171 64 160


Bit Number Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1

27 26 25 24 23 22 21 20

Numeric Value 128 64 32 16 8 4 2 1

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SUBNET ADDRESS S

Netmasks Expanded (/24 through /32)

1 Subnet

2 Subnets

4 Subnets

8 Subnets

Netmask 255.255.255.0 /24 bit (11111111.11111111.11111111.00000000)

LOW IP HI IP

x.x.x.0 x.x.x.255

Netmask 255.255.255.128 /25 (11111111.11111111.11111111.10000000)

LOW IP HI IP

x.x.x.0 network address x.x.x.127 Broadcast address

x.x.x.128 network address x.x.x.255 Broadcast address

Netmask 255.255.255.192 /26 (11111111.11111111.11111111.11000000)

LOW IP HI IP

x.x.x.0 x.x.x.63

x.x.x.64 x.x.x.191

x.x.x.192 x.x.x.255

Netmask 255.255.255.224 /27 (11111111.11111111.11111111.11100000)

LOW IP HI IP

x.x.x.0 x.x.x.31

x.x.x.32 x.x.x.63

x.x.x.64 x.x.x.95

x.x.x.96 x.x.x.127

x.x.x.128 x.x.x.159

x.x.x.160 x.x.x.191

x.x.x.192 x.x.x.223

x.x.x.224 x.x.x.255

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S SUBNET ADDRESS

16 Subnets

32 Subnets

Netmask 255.255.255.240 /28 (11111111.11111111.11111111.11110000)

LOW IP HI IP LOW IP HI IP

x.x.x.0 x.x.x.15 x.x.x.128 x.x.x.143

x.x.x.16 x.x.x.31 x.x.x.144 x.x.x.159

x.x.x.32 x.x.x.47 x.x.x.160 x.x.x.175

x.x.x.48 x.x.x.63 x.x.x.176 x.x.x.191

x.x.x.64 x.x.x.79 x.x.x.192 x.x.x.207

x.x.x.80 x.x.x.95 x.x.x.208 x.x.x.223

x.x.x.96 x.x.x.111 x.x.x.224 x.x.x.239

x.x.x.112 x.x.x.127 x.x.x.240 x.x.x.255

Netmask 255.255.255.248 /29 (11111111.11111111.11111111.11111000)


x.x.x.0 x.x.x.7 x.x.x.128 x.x.x.135

x.x.x.8 x.x.x.15 x.x.x.136 x.x.x.143

x.x.x.16 x.x.x.23 x.x.x.144 x.x.x.151

x.x.x.24 x.x.x.31 x.x.x.152 x.x.x.159

x.x.x.32 x.x.x.39 x.x.x.160 x.x.x.167

x.x.x.40 x.x.x.47 x.x.x.168 x.x.x.175

x.x.x.48 x.x.x.55 x.x.x.176 x.x.x.183

x.x.x.56 x.x.x.63 x.x.x.184 x.x.x.191

x.x.x.64 x.x.x.71 x.x.x.192 x.x.x.199

x.x.x.72 x.x.x.79 x.x.x.200 x.x.x.207

x.x.x.80 x.x.x.87 x.x.x.208 x.x.x.215

x.x.x.88 x.x.x.95 x.x.x.216 x.x.x.223

x.x.x.96 x.x.x.103 x.x.x.224 x.x.x.223

x.x.x.104 x.x.x.111 x.x.x.232 x.x.x.239

x.x.x.112 x.x.x.119 x.x.x.240 x.x.x.247

x.x.x.120 x.x.x.127 x.x.x.248 x.x.x.255

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SUBNET ADDRESS S

64 Subnets

Other related Information:

See also IP Addressing.

Netmask 255.255.255.252 /30 (11111111.11111111.11111111.11111100)


x.x.x.0 x.x.x.3 x.x.x.128 x.x.x.131

x.x.x.4 x.x.x.7 x.x.x.132 x.x.x.135

x.x.x.8 x.x.x.11 x.x.x.136 x.x.x.139

x.x.x.12 x.x.x.15 x.x.x.140 x.x.x.143

x.x.x.16 x.x.x.19 x.x.x.144 x.x.x.147

x.x.x.20 x.x.x.23 x.x.x.148 x.x.x.151

x.x.x.24 x.x.x.27 x.x.x.152 x.x.x.155

x.x.x.28 x.x.x.31 x.x.x.156 x.x.x.159

x.x.x.32 x.x.x.35 x.x.x.160 x.x.x.163

x.x.x.36 x.x.x.39 x.x.x.164 x.x.x.167

x.x.x.40 x.x.x.43 x.x.x.168 x.x.x.171

x.x.x.44 x.x.x.47 x.x.x.172 x.x.x.175

x.x.x.48 x.x.x.51 x.x.x.176 x.x.x.179

x.x.x.52 x.x.x.55 x.x.x.180 x.x.x.183

x.x.x.56 x.x.x.59 x.x.x.184 x.x.x.187

x.x.x.60 x.x.x.63 x.x.x.188 x.x.x.191

x.x.x.64 x.x.x.67 x.x.x.192 x.x.x.195

x.x.x.68 x.x.x.71 x.x.x.196 x.x.x.199

x.x.x.72 x.x.x.75 x.x.x.200 x.x.x.203

x.x.x.76 x.x.x.79 x.x.x.204 x.x.x.207

x.x.x.80 x.x.x.83 x.x.x.208 x.x.x.211

x.x.x.84 x.x.x.87 x.x.x.212 x.x.x.215

x.x.x.88 x.x.x.91 x.x.x.216 x.x.x.219

x.x.x.92 x.x.x.95 x.x.x.220 x.x.x.223

x.x.x.96 x.x.x.99 x.x.x.224 x.x.x.227

x.x.x.100 x.x.x.103 x.x.x.228 x.x.x.231

x.x.x.104 x.x.x.107 x.x.x.232 x.x.x.235

x.x.x.108 x.x.x.111 x.x.x.236 x.x.x.239

x.x.x.112 x.x.x.115 x.x.x.240 x.x.x.243

x.x.x.116 x.x.x.119 x.x.x.244 x.x.x.247

x.x.x.120 x.x.x.123 x.x.x.248 x.x.x.251

x.x.x.124 x.x.x.127 x.x.x.252 x.x.x.255

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S SUBNET ADDRESS

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SWITCH S

SwitchSwitches are used primarily to physically segment congested Ethernet networks. They are very fast and also improve network performance by making intelligent transfer decisions based on the MAC (Media Acess Control) Layer information of a data packet.

LAN “switching” was pioneered by a company called Kalpana (now Cisco) in the early ’90s. Switches are available for both Ethernet and Token Ring networks. Ethernet switches are actually fast multiport bridges (using ASIC chips) with eight to 24 ports or hardware-based bridges.

• Each port supports 10 Mbps or 100 Mbps Ethernet. Like a bridge, a switch’s function is to switch packets coming in on one port over to the port connected to the Ethernet segment where the destination station resides.

• Each port of a switch makes a forward/don’t forward decision.

Once it is determined that a packet should be forwarded, it is sent directly to and only to the port that connects to the destination segment, where it is transmitted onto that network. A switch is designed to accommodate high volumes of traffic between its ports, so that transit latency between any two ports is very low, which makes switch throughput very fast.

More recently, switches have been utilized in multiple applications and are operating at several layers of the OSI Model. If required, each switch at a higher layer can also perform the tasks at lower layer as well.

A Layer 1 switch is often used to replace aging or slower hubs.

A Layer 2 switch works at the MAC layer transfering data from one MAC address to another

A Layer 3 switch, if supporting Network protocols such as IP and IPX, makes routing decisions based on Layer 3 information.

A Layer 4 switch is able to make forwarding decisions based on specific TCP and UDP ports an application is connected to.

See the specific section that details the differences in the Switches on the following pages.

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S SWITCH

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SWITCH, LAYER 1 S

Switch, Layer 1 Switches operating at Layer 1 are functioning as hubs. In a fully switched network, hubs are replaced with switches, providing a dedicated segment for each device. Since, the only elements on that segment are the switch and the device, the switch receives all data transmissions before it reaches other devices (instead of broadcast transmissions that each device on a hub receives).

Many organizations have replaced their hubs with switches throughout their networks. Hubs typically run about $50.00 per port while switches can be purchased for as little as $75.00 per port; so for a 50% increase in cost, an organization can increase available bandwidth.

The benefits are significant:

• Switches allow devices to act as if they were the only device on the switch.

• This means less waiting to send packets and network efficiency is greatly improved.

• Switches also allow for queues and priority levels for packets and provide support for CoS and QoS.

Most IT people are looking for a good reason to go to this architecture and VoIP is a perfect business case.

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S SWITCH, LAYER 1

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SWITCH, LAYER 2 S

Switch, Layer 2

Layer 2 Switches

Layer 2 Switches offer a simple, cost-effective means of breaking a network up into smaller segments, each with less traffic, while still maintaining full connectivity.When well-designed, each smaller segment has much less traffic and, therefore, the users have greater available bandwidth and higher throughput for their data transfers.

Multiple switches can be connected together to form what is sometimes called a switched backbone network. At the physical layer, each device receives its own uncontested 10 Mbps or 100 Mbps Ethernet segment. This is done by attaching a single station directly to a switch port. Once this is done any packets sent to that device will have a full 10 or 100 Mbps bandwidth, without any congestion resulting from collisions, as there are no other stations on that LAN segment. This is ideal for VoIP solutions - assign each IP phone its own port.

The Switch captures the MAC Addresses of all devices connected to the Switch within a table. When the Switch receives packets, it checks the table to see if the device is on its network segment and, if so, will only transmit the data to that port. Some switches have buffering ability built into the ports and many will support either 10 / 100 Mbps depending on the device connected. Switches can support Full Duplex - simultaneous incoming and outgoing transmis-sion of data, effectively doubling the bandwidth.

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S SWITCH, LAYER 2

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SWITCH, LAYER 3 S

Switch, Layer 3

Layer 3 Switches

A Layer 3 Switch is created by having routing capability within a Layer 2 Switch. This is achieved by installing a Router card within an Layer 2 Switch, thus permitting the switch to operate using Network protocols such as IP and IPX making routing decisions based on this information layer. Operating at this level allows the Switch to prioritize packets and queue data effectively to avoid a reduction in quality which is most apparent in Voice over IP applications.

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S SWITCH, LAYER 3

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TCP/IP MODEL OR STACK T

TCP/IP Model or StackThe TCP/IP protocol does not conform to the OSI model 7-layer standard. The TCP/IP Model has a similar intention and since conception, the two models have emerged with a certain compatibility. The TCP/IP model consists of four layers, some of which include multiple layers from the OSI Model.

The following diagram illustrates how the two standards define their layers.

Application Layer

Corresponds to Layers 7, 6 (sometimes 5) of the OSI model. An interface is provided for applications such as file and print services, name resolution services, etc. This layer effectively interfaces downward to the UDP or TCP ports in the Transport Layer from the end or upper applications.

Typical examples of applications would include:

• Telnet – a remote terminal protocol

• FTP – File Transfer Protocol

• SMTP – Simple Mail Transfer Protocol

• SNMP - Simple Network Management Protocol

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T TCP/IP MODEL OR STACK

Transport Layer

Corresponds to Layer 4 (sometimes 5) of the OSI model. It provides two transparent protocols for the application information. TCP provides session management and error correction. A TCP session is made before data is transferred. UDP is a connectionless protocol that sends datagrams as required. No session is established and no error correction is used.

Two protocols used:

• TCP – Transmission Control Protocol

• UDP – User Datagram Protocol

Internet Layer

Corresponding to the Network layer in the OSI model, this layer creates the TCP/IP portion. IP addressing of the host determines a local or remote destination via routing tables.

Typical Examples of protocols used:

• IP – Internet Protocol

• ICMP – Internet Control Message Protocol

• IGMP – Internet Group Management Protocol

• ARP – Address Resolution Protocol

• RARP – Reverse Address Resolution Protocol

Network Access Layer

Corresponds to the physical and data link layers of the OSI/RM. This layer accepts and handles higher layer datagrams and transmits them over the attached network by handling all hardware details of the network media.

This typically contains:

• The Operating Systems Device Driver

• The Interface Card NIC

• The Physical Connection

As a voice packet is prepared for broadcast to the network, each layer may add a Header and a Trailer to its Data (which consists of the next higher layer's Header, Trailer and Data as it moves through the layers). The Headers contain information that specifically addresses lay-er-to-layer communication. For example, the Transport Header (TH) contains information that only the Transport layer sees. All other layers below the Transport layer pass the Transport Header as part of their Data. A more detailed look at the content of the packet can be found in the following diagram.

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TCP/IP MODEL OR STACK T

Voice Data

VoIP Ethernet II Packet Format Bits Bytes Standard

V=2 2 bitsP 1 bitsX 1 bitsCC 4 bitsM 1 bitsPT 7 bitssequence number 16 bitstimestamp 32 bitssynchronization source (SSRC) identifier 32 bits

Voice Payload 160

RTP Container 12 RFC1889

UDP Container 8 RFC768Source Port 16 bitsDestination Port 16 bitsLength 16 bitsChecksum 16 bits

IP Container 20 RFC791Version 4 bitsIHL 4 bitsType of Service 8 bitsTotal Length 16 bitsIdentification 16 bitsFlags 3 bitsFragment Offset 13 bitsTime to Live 8 bitsProtocol 8 bitsHeader Checksum 16 bitsSource Address 32 bitsDestination Address 32 bits

MAC Container MAC Preamble 7 IEEE 802.3MAC Start of Frame De-limiter 1Destination MAC 6Source MAC 6Qtag Prefix 4Frame Type 2

Frame CRC 4Inter-Packet Gap 12

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T TCP/IP MODEL OR STACK

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TAGGED AND UNTAGGED PACKETS T

Tagged and Untagged PacketsIn a voice and data network, traffic needs to be prioritized. In most networks, we can provide priority by physically separating the devices; voice on one subnet and data devices on a different subnet.

This requires two physical connections to each desktop for voice and data. This configuration works and is effective, however, the duplication of equipment to support a group of users adds cost. The promise of one infastructure requires a “single drop to the desk".

Typical networks provide a high priority to data packets, however, voice transmission must be synchronized as delays caused by heavy traffic will cause the voice quality to drop. To manage the prioritization of packets, the 802.1p/Q standard was developed. This is more commonly called Virtual Local Area Network (VLAN) tagging. The tagging identifies a voice packet for prioritization on the VLAN.

The switch can handle both Untagged and Tagged translation, as well as straight VLAN traffic. On a VLAN-aware switch, the PC 0800 frames are tagged at ingress and the tag is removed at egress, but the phone traffic 8100 is not. The VLAN that does the conversion is known as the ‘Native_VLAN’ in Cisco devices and the VLAN deemed as ‘Untagged’ in HP devices (typically default_VLAN(1)). The settings are different, but the functionality is the same.

Devices on the network that contain a Layer 2 switch (Ethernet switch, IP phone) understand tagged packets. Those that do not contain a switch (some Servers, routers or PC and printers) do understand tagged packets. The IP phone contains a Layer 2 switch, so it can look at the packets and route them to the PC or to the phone itself.

Definition Frame Type VLAN ID Priority

"Untagged" 0800 None None

"Untagged" 8100 0 0 to 7

"Tagged" 8100 1 to 4095 0 to 7

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T TAGGED AND UNTAGGED PACKETS

.

Ports that connect to devices that do not understand tags must take incoming untagged packets from the devices and put them on a VLAN. These ports also send packets on the VLAN out to these devices as untagged.

Note: MiNet packets and keep alive messages are sent untagged using R-UDP (reliable user datagram protocol).

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TOPOLOGY (NETWORK) T

Topology (Network)Network topology is the specific physical, (Real, or Virtual) arrangement of the elements of a network. Every LAN has a topology, or the way that the devices on a network are arranged and how they communicate with each other.

Most LANs are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via a dedicated digital network, leased bandwidth or public carrier network. A system of LANs connected in this way is called a Wide Area Network (WAN).

Topologies are either physical or logical.

Physical topology

The physical layout of devices on a network, the way the workstations are connected to the network through the actual cables that transmit data -- the physical structure of the network -- is called the physical topology.

There are many different types of LANs, with Ethernet being the most common for PCs.

Logical Topology (also called signal topology)

The logical topology, in contrast to the physical topology, is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices.

Logical topologies are bound to the network protocols that direct how the data moves across a network. The Ethernet protocol is a common logical bus topology protocol. Localtalk is a common logical bus or star topology protocol. IBM's token ring is a common logical ring topology protocol.

A network's logical topology is not necessarily the same as its physical topology. For example, twisted pair Ethernet is a logical bus topology in a physical star topology layout. While IBM's token ring is a logical ring topology, it is physically set up in a star topology.

Design Considerations

The topology of a network is most closely tied to the enterprise's geography (who's on what floor, which server sits where, and so on). Other considerations also come into play.

Network design decisions are most often constrained by the amount of money to be spent, and such logistical issues as how long the enterprise plans to stay in a building.

Nowadays, no matter the budget or platforms, segmenting (star topology) is not only the option, but is the preferred design approach.

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T TOPOLOGY (NETWORK)

LAN Topology Types

• Bus

• Star

• Ring

• Tree

• Wireless

Bus Topology

Two basic types of Bus LANs

• Thick Ethernet

• Thin Ethernet

In networking, a bus is a central cable that connects all devices on a Local Area Network (LAN). It is also called the backbone. There are now three types of bus topologies. Thin Ethernet, thick Ethernet and gigabit Ethernet (which supports data rates of 1 gigabit (1,000 megabits) per second. The trouble with bus topologies is that if something fails along the trunk, the whole network could go down.

Star Topology

Star topology is the mostly widely used Local Area Network today.

A star physical topology means that the nodes/devices are all connected to a centralized hub or switch and is commonly used for 10BASE5, 10BASE-T or 100BASE-TX.

Cabling is inexpensive, easy to use, more reliable and easier to manage because hubs allow you to route around defective cable segments. Locating and repairing bad cables is easier because of the concentrators and network growth is easier.

All nodes receive the same signal, therefore, dividing bandwidth. Maximum computers on a LAN is 1,024. Maximum UTP cable length (unshielded twisted pair) is 100 meters (approxi-mately 330 feet). Minimum distance between computers is 2.5 meters.

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TOPOLOGY (NETWORK) T

The main advantages of a star network are that one malfunctioning node doesn't affect the rest of the network, and it's easy to add and remove nodes. The main disadvantage of star networks is that they require more cabling than other topologies, such as bus or ring networks.

However, if the central connection point (Switch or Hub) fails, the entire network becomes unusable.

Ring Topology

In a Ring topology, all of the nodes are connected in a closed loop. Messages travel around the ring, with each node reading those messages addressed to it. One of the advantages of ring networks is that they can span larger distances than other types of networks, such as bus networks, because each node regenerates messages as they pass through. The disadvantage of this topology is that if one node fails, the whole network goes down.

Tree Topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable.

Wireless Topology

Wireless Topology refers to a geographic area, divided into cells, combining a wireless structure with point-to-point and multipoint design for device attachment. The devices in each cell directly communicate with a central station called a hub.

This is becoming more common in business locations on a location-by-location basis.

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T TOPOLOGY (NETWORK)

Network Layers

Networks usually consist of different layers. The two main layers are the ‘core’ and ‘access’ network layers.

The ‘Core’ Network Layer

The ‘core’ network layer could potentially have data devices on dedicated links connecting to Layer 2 and Layer 3 switches. These units will almost certainly have a redundant configuration through cross-connections and UPS backup systems; so that if one device were to fail, it would not result in a total network failure.

The ‘Access’ Network Layer

The ‘access’ network layer connects to the core by either single or multiple connections.

This layer provides individual user connection (usually 10/100 Base T). If a device fails here, then only the locally failed device is affected.

Note: Other definitions have the network divided up into three divisions; core, distribution, and access. With our definition, the distribution layer has been divided between the core and the access layer.

Example of a Typical Network

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UTILITIES FOR TCP/IP CONNECTIVITY U

Utilities for TCP/IP Connectivity A number of utilities are available to troubleshoot and configure TCP/IP.

Network dysfunction typically results from one of a handful of common problems. The TCP/IP utilities help you uncover a problem and trace them to their source.

The four top network connectivity problems are usually a version of one of the following:

• The protocol software doesn’t work or isn’t configured properly.

• A hardware problem. A cable isn’t plugged in or isn’t working or is the wrong type of cable. A hub, switch, or router isn’t working.

• Faulty name resolution

• Excessive trafficIP Address

Useful Utilities

Ping

Verifies IP-level connectivity to another TCP/IP computer by sending Internet Control Message Protocol (ICMP) Echo Request messages. The receipt of corresponding Echo Reply messages are displayed, along with round-trip times. Ping is the primary TCP/IP command used to troubleshoot connectivity, reachability, and name resolution. Used without parameters, pingdisplays help.

Syntax

ping [-t] [-a] [-n Count] [-l Size] [-f] [-i TTL] [-v TOS] [-r Count] [-s Count] [{-j HostList| -k HostList}] [-w Timeout] [TargetName]

Parameters

-t

Specifies that ping continue sending Echo Request messages to the destination until interrupt-ed. To interrupt and display statistics, press CTRL-BREAK. To interrupt and quit ping, press CTRL-C.

-a

Specifies that reverse name resolution is performed on the destination IP Address. If this is successful, ping displays the corresponding host name.

-n Count

Specifies the number of Echo Request messages sent. The default is 4.

-l Size

Specifies the length, in bytes, of the Data field in the Echo Request messages sent. The default is 32. The maximum size is 65,527.

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U UTILITIES FOR TCP/IP CONNECTIVITY

-f

Specifies that Echo Request messages are sent with the Don't Fragment flag in the IP header set to 1. The Echo Request message cannot be fragmented by routers in the path to the destination. This parameter is useful for troubleshooting Path Maximum Transmission Unit (PMTU) problems.

-i TTL

Specifies the value of the TTL field in the IP header for Echo Request messages sent. The default is the default TTL value for the host. For Windows XP hosts, this is typically 128. The maximum TTL is 255.

-v TOS

Specifies the value of the Type of Service (TOS) field in the IP header for Echo Request messages sent. The default is 0. TOS is specified as a decimal value from zero to 255.

-r Count

Specifies that the Record Route option in the IP header is used to record the path taken by the Echo Request message and corresponding Echo Reply message. Each hop in the path uses an entry in the Record Route option. If possible, specify a Count that is equal to or greater than the number of hops between the source and destination. The Count must be a minimum of 1 and a maximum of 9.

-s Count

Specifies that the Internet Timestamp option in the IP header is used to record the time of arrival for the Echo Request message and corresponding Echo Reply message for each hop. The Count must be a minimum of 1 and a maximum of 4.

-j HostList

Specifies that the Echo Request messages use the Loose Source Route option in the IP header with the set of intermediate destinations specified in HostList. With loose source routing, successive intermediate destinations can be separated by one or multiple routers. The maxi-mum number of addresses or names in the host list is nine. The host list is a series of IP Addresses (in dotted decimal notation) separated by spaces.

-k HostList

Specifies that the Echo Request messages use the Strict Source Route option in the IP header with the set of intermediate destinations specified in HostList. With strict source routing, the next intermediate destination must be directly reachable (it must be a neighbor on an interface of the router). The maximum number of addresses or names in the host list is nine. The host list is a series of IP Addresses (in dotted decimal notation) separated by spaces.

-w Timeout

Specifies the amount of time, in milliseconds, to wait for the Echo Reply message that corre-sponds to a given Echo Request message to be received. If the Echo Reply message is not received within the time out, the "Request timed out" error message is displayed. The default time out is 4000 (4 seconds).

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TargetName

Specifies the destination, which is identified either by IP Address or host name.

/?

Displays help at the command prompt.

Tracert

Determines the path taken to a destination by sending Internet Control Message Protocol (ICMP) Echo Request messages to the destination with incrementally increasing Time to Live (TTL) field values. The path displayed is the list of near-side router interfaces of the routers in the path between a source host and a destination. The near-side interface is the interface of the router that is closest to the sending host in the path. Used without parameters, tracertdisplays help.

Syntax

tracert [-d] [-h MaximumHops] [-j HostList] [-w Timeout] [TargetName]

Parameters

-d

Prevents tracert from attempting to resolve the IP Addresses of intermediate routers to their names. This can speed up the display of tracert results.

-h MaximumHops

Specifies the maximum number of hops in the path to search for the target (destination). The default is 30 hops.

-j HostList

Specifies that Echo Request messages use the Loose Source Route option in the IP header with the set of intermediate destinations specified in HostList. With loose source routing, successive intermediate destinations can be separated by one or multiple routers. The maxi-mum number of addresses or names in the host list is nine. The HostList is a series of IP Addresses (in dotted decimal notation) separated by spaces.

-w Timeout

Specifies the amount of time in milliseconds to wait for the ICMP Time Exceeded or Echo Reply message corresponding to a given Echo Request message to be received. If not received within the time out, an asterisk (*) is displayed. The default time out is 4000 (4 seconds).

TargetName

Specifies the destination, identified either by IP Address or host name.

-?


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Arp

Displays and modifies entries in the Address Resolution Protocol (ARP) cache, which contains one or more tables that are used to store IP Addresses and their resolved Ethernet or Token Ring physical addresses. There is a separate table for each Ethernet or Token Ring network adapter installed on your computer. Used without parameters, arp displays help.

Syntax

arp [-a [InetAddr] [-N IfaceAddr]] [-g [InetAddr] [-N IfaceAddr]] [-d InetAddr [IfaceAddr]] [-s InetAddr EtherAddr [IfaceAddr]]

Parameters

-a [InetAddr] [-N IfaceAddr]

Displays current ARP cache tables for all interfaces. To display the ARP cache entry for a specific IP Address, use arp -a with the InetAddr parameter, where InetAddr is an IP Address. To display the ARP cache table for a specific interface, use the -N IfaceAddr parameter where IfaceAddr is the IP Address assigned to the interface. The -N parameter is case sensitive.

-g [InetAddr] [-N IfaceAddr]

Identical to -a.

-d InetAddr [IfaceAddr]

Deletes an entry with a specific IP Address, where InetAddr is the IP Address. To delete an entry in a table for a specific interface, use the IfaceAddr parameter where IfaceAddr is the IP Address assigned to the interface. To delete all entries, use the asterisk (*) wildcard character in place of InetAddr.

-s InetAddr EtherAddr [IfaceAddr]

Adds a static entry to the ARP cache that resolves the IP Address InetAddr to the physical address EtherAddr. To add a static ARP cache entry to the table for a specific interface, use the IfaceAddr parameter where IfaceAddr is an IP Address assigned to the interface.

/?


Ipconfig

Displays all current TCP/IP network configuration values and refreshes Dynamic Host Config-uration Protocol (DHCP) and Domain Name System (DNS) settings. Used without parameters, ipconfig displays the IP Address, subnet mask, and default gateway for all adapters.

Syntax

ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns] [/displaydns] [/registerdns] [/showclassid Adapter] [/setclassid Adapter [ClassID]]

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Parameters

/all

Displays the full TCP/IP configuration for all adapters. Without this parameter, ipconfig displays only the IP Address, subnet mask, and default gateway values for each adapter. Adapters can represent physical interfaces, such as installed network adapters, or logical interfaces, such as dial up connections.

/renew [Adapter]

Renews DHCP configuration for all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter is available only on computers with adapters that are configured to obtain an IP Address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

/release [Adapter]

Sends a DHCPRELEASE message to the DHCP server to release the current DHCP configuration and discard the IP Address configuration for either all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter disables TCP/IP for adapters configured to obtain an IP Address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

/flushdns

Flushes and resets the contents of the DNS client resolver cache. During DNS troubleshooting, you can use this procedure to discard negative cache entries from the cache, as well as any other entries that have been added dynamically.

/displaydns

Displays the contents of the DNS client resolver cache, which includes both entries preloaded from the local Hosts file and any recently obtained resource records for name queries resolved by the computer. The DNS Client service uses this information to resolve frequently queried names quickly, before querying its configured DNS servers.

/registerdns

Initiates manual dynamic registration for the DNS names and IP Addresses that are configured at a computer. You can use this parameter to troubleshoot a failed DNS name registration or resolve a dynamic update problem between a client and the DNS server without rebooting the client computer. The DNS settings in the advanced properties of the TCP/IP protocol determine which names are registered in DNS.

/showclassid Adapter

Displays the DHCP class ID for a specified adapter. To see the DHCP class ID for all adapters, use the asterisk (*) wildcard character in place of Adapter. This parameter is available only on computers with adapters that are configured to obtain an IP Address automatically.

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/setclassid Adapter [ClassID]

Configures the DHCP class ID for a specified adapter. To set the DHCP class ID for all adapters, use the asterisk (*) wildcard character in place of Adapter. This parameter is available only on computers with adapters that are configured to obtain an IP Address automatically. If a DHCP class ID is not specified, the current class ID is removed.

/?


Netstat

Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, IPv4 statistics (for the IP, ICMP, TCP, and UDP protocols), and IPv6 statistics (for the IPv6, ICMPv6, TCP over IPv6, and UDP over IPv6 protocols). Used without parameters, netstat displays active TCP connections.

Syntax

netstat [-a] [-e] [-n] [-o] [-p Protocol] [-r] [-s] [Interval]

Parameters

-a

Displays all active TCP connections and the TCP and UDP ports on which the computer is listening.

-e

Displays Ethernet statistics, such as the number of bytes and packets sent and received. This parameter can be combined with -s.

-n

Displays active TCP connections, however, addresses and port numbers are expressed numer-ically and no attempt is made to determine names.

-o

Displays active TCP connections and includes the process ID (PID) for each connection. You can find the application based on the PID on the Processes tab in Windows Task Manager. This parameter can be combined with -a, -n, and -p.

-p Protocol

Shows connections for the protocol specified by Protocol. In this case, the Protocol can be tcp,udp, tcpv6, or udpv6. If this parameter is used with -s to display statistics by protocol, Protocol can be tcp, udp, icmp, ip, tcpv6, udpv6, icmpv6, or ipv6.

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-s

Displays statistics by protocol. By default, statistics are shown for the TCP, UDP, ICMP, and IP protocols. If the IPv6 protocol is installed, statistics are shown for the TCP over IPv6, UDP over IPv6, ICMPv6, and IPv6 protocols. The -p parameter can be used to specify a set of protocols.

-r

Displays the contents of the IP routing table. This is equivalent to the route print command.

Interval

Redisplays the selected information every Interval seconds. Press CTRL+C to stop the redisplay. If this parameter is omitted, netstat prints the selected information only once.

/?


Ftp

Transfers files to and from a computer running a File Transfer Protocol (FTP) server service such as Internet Information Services. Ftp can be used interactively or in batch mode by processing ASCII text files.

Syntax

ftp [-v] [-d] [-i] [-n] [-g] [-s:FileName] [-a] [-w:WindowSize] [-A] [Host]

Parameters

-v

Suppresses the display of FTP server responses.

-d

Enables debugging, displaying all commands passed between the FTP client and FTP server.

-i

Disables interactive prompting during multiple file transfers.

-n

Suppresses the ability to log on automatically when the initial connection is made.

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-g

Disables file name globbing. Glob permits the use of the asterisk (*) and question mark (?) as wildcard characters in local file and path names.

-s:FileName

Specifies a text file that contains ftp commands. These commands run automatically after ftpstarts. This parameter allows no spaces. Use this parameter instead of redirection (<).

-a

Specifies that any local interface can be used when binding the FTP data connection.

-w:WindowSize

Specifies the size of the transfer buffer. The default window size is 4096 bytes.

-A

Logs onto the FTP server as anonymous.

Host

Specifies the computer name, IP Address, or IPv6 address of the FTP server to which to connect. The host name or address, if specified, must be the last parameter on the line.

/?


Tftp

Transfers files to and from a remote computer, typically a computer running UNIX, that is running the Trivial File Transfer Protocol (TFTP) service or daemon. Used without parameters, tftp displays help.

Syntax

tftp [-i] [Host] [{get | put}] [Source] [Destination]

Parameters

-i

Specifies binary image transfer mode (also called octet mode). In binary image mode, the file is transferred in one-byte units. Use this mode when transferring binary files. If -i is omitted, the file is transferred in ASCII mode. This is the default transfer mode. This mode converts the end-of-line (EOL) characters to an appropriate format for the specified computer. Use this mode when transferring text files. If a file transfer is successful, the data transfer rate is displayed.

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Host

Specifies the local or remote computer.

put

Transfers the file Destination on the local computer to the file Source on the remote computer. Because the TFTP protocol does not support user authentication, the user must be logged onto the remote computer, and the files must be writable on the remote computer.

get

Transfers the file Destination on the remote computer to the file Source on the local computer.

Source

Specifies the file to transfer.

Destination

Specifies where to transfer the file. If Destination is omitted, it is assumed to have the same name as Source.

/?


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VLAN (VIRTUAL LOCAL AREA NETWORK) V

VLAN (Virtual Local Area Network)A VLAN is a logical subgroup within a Local Area Network created with software instead of hardware.

The benefit of using a VLAN is the controller can change or add workstations, manage load balancing and bandwidth allocation more easily than with a physical picture of the LAN. Network management software maps the virtual local area network with the actual physical network.

Common reasons to implement VLANS

• Performance / Bandwidth - Reduces the number of router hops and increases the apparent bandwidth for users

• Broadcast / Traffic Flow - Does not pass broadcast traffic to other nodes that are not part of the VLAN

• Security / Access Lists - Provides a method of control for network administrators

• Separating Devices - Provides packet priority by having tagged packets obtain priority over untagged packets

• Flexibility - Does not require rewiring each time a device moves physical locations

A "VLAN," combines user stations and network devices into a single unit, regardless of the physical LAN segment they are attached to and allows traffic to flow more efficiently within populations of mutual interest.

VLANs are implemented in port-switching hubs and LAN switches and generally offer proprietary solutions.

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V VLAN (VIRTUAL LOCAL AREA NETWORK)

VLANs function at Layer 2. Since their purpose is to isolate traffic within the VLAN, in order to bridge from one VLAN to another, a router is required. The router works at the higher Layer 3 network protocol, which requires that network layer segments are identified and coordinated with the VLANs.

Methods of Separating Voice and Data Traffic

If the switch is a managed switch, VLANs can be created by simply logging into the switch and entering the VLAN parameters (name, domain, and port assignments).

Typically, all ports on a switched network are in the same broadcast domain. A broadcast issued by a particular station within a VLAN, however, will only propagate to stations on the same VLAN.

On a completely "flat" switched network, any broadcast issued by any station on the campus LAN would have to be processed by every station on that LAN. The interruption of each device's CPU is probably a more serious issue than the bandwidth consumption associated with broad-casts in a LAN environment.

VLANs provide a mechanism for creating multiple broadcast domains in a switched network. As noted earlier, you can have more than one VLAN on a switch, but communication between the VLANs requires a router.

Note: A VLAN is essentially synonymous with an IP subnet. In a switched environment, if two stations are on the same VLAN then they must also be on the same IP subnet.

Separate Connections in Network

• Phones and PCs are segregated through separate IP Addresses and physical connections (Layer 2 switches).

• Phones and PCs cannot share the same infrastructure (careful planning is needed to implement).

• No Priority (It’s OK if voice does not share same connections as other devices, such as PCs.)

Port-based VLANS

• IP Phones do not generate VLAN information.

• A separate ‘drop’ is required for a PC and an IP Phone at the desk. (You cannot use a second port on a phone with this method.)

• There are no VLAN settings in the DHCP server (simpler implementation).

• Moves are restricted between configured phone ports (set to the same type).

Default VLAN

• Phones are capable of generating/receiving VLAN information - phones get assigned to VLAN ‘x’ with high priority for packets.

• Data devices don’t usually use NICs with 802.1p/Q, therefore, data devices get put on the default VLAN ‘y’ at the switch port and VLAN ‘y’ has low priority for packets.

• A router is needed between VLANs so that information can be sent between VLANS.

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VLAN (VIRTUAL LOCAL AREA NETWORK) V

• A DHCP server needs to support access to native/default VLANs and voice VLANs.

VLANs impose a level of security similar to that associated with routed subnets. If this port is assigned to a particular VLAN, the device on that port will only detect broadcasts associated with that VLAN rather than for the entire LAN.

Note: Security policies can also be configured on the router that controls the inter-VLAN communication, just as for conventional LAN segments.

The IP Address plan may also, in part, dictate the VLAN strategy. For example, if you're using a 26-bit mask for LAN subnets, the maximum number of hosts per subnet is 60. This means that the entire LAN cannot simply remain "flat" if it contains more than 60 hosts. If your switched LAN has a large number of hosts, you must create VLANs with a maximum of 60 hosts per VLAN.

VLANs go some way towards combining the intelligence of a routed network with the flexibility of a switched LAN. For example, a user on a particular VLAN can remain on that VLAN after moving to a different physical location within the campus. You simply need to change the relevant switch configurations; there is no need for a hardware change or re-patching of cables.

Note: You can extend VLANs across multiple switches using a VLAN trunking protocol. The VLAN Trunking Protocol (VTP) is the protocol used by switches to allow VLANs to communicate with each other.

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V VLAN (VIRTUAL LOCAL AREA NETWORK)

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VPN (VIRTUAL PRIVATE NETWORK) V

VPN (Virtual Private Network)A network service offered by public carriers in which the customer is provided a network connection that, in many ways, appears as if it is a private network, but which, in fact, is provided over the carrier's public network facilities. This takes advantage of the economies of scale and management facilities of large networks.

VPNs are widely used by enterprises to create wide area networks (WANs) that span large geographic areas, to provide site-to-site connections to branch offices and to allow mobile users to dial up their company LANs.

• A virtual private network (VPN) is a private data network that makes use of the public telecommunication infrastructure, maintaining privacy through the use of tunneling protocol and security procedures.

• A virtual private network can be contrasted with a system of owned or leased lines that can only be used by one company. The idea of the VPN is to give the company the same capabilities at a much lower cost, by using the shared public infrastructure, rather than a private one.

• Phone companies have provided secure, shared resources for voice messages. A virtual private network makes it possible to have the same secure sharing of public resources for data.

• Companies today are looking at using a private virtual network for both extranets and wide area intranets.

For years, common carriers have built VPNs that appear as a private national or international network to each customer, but, in fact, share the same physical backbone trunks with many customers. VPNs have been built over X.25, Switched 56, frame relay and ATM technologies, as well as IP networks. For added security, encryption is often used.

• Encrypted connections over the Internet are popular for linking remote offices and mobile users; however, for top secret communications, the maximum security is still only afforded by networks of totally private lines.

Internet VPNs

The PPTP and L2TP protocols are used to provide a point-to-point link over the public Internet using SSL and IPsec encryption.

Frame Relay VPNs from Carriers

Carriers offer point-to-point and multipoint VPNs using frame relay. Customer equipment converts packets to frame relay packets. Frame relay VPNs support any network protocol (IP, IPX, SNA, etc.), but adding a location in a multipoint VPN means provisioning virtual circuits from that site to all other sites, which can be costly.

Virtual IP VPNs from Carriers

Carriers offer multipoint networks that accept only IP packets from the customer and run over an IP core. These virtual routed private networks (VPRNs) connect the customer's IP router to the provider's IP router and require some coordination.

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V VPN (VIRTUAL PRIVATE NETWORK)

Ethernet VPNs from Carriers

Carriers offer services that encapsulate Ethernet frames and deliver them across their network to an Ethernet connection on the other end. A "LAN interconnect" service connects one Ethernet to another. A "transparent LAN" service (TLS) offers multipoint connectivity, typically using frame relay, but is problematic, due to the differences in Ethernet and frame relay architecture. A virtual private LAN service (VPLS) is the latest implementation of a transparent LAN for Ethernet, using an IP and MPLS (Multi Protocol Label Switching) core to route traffic.

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VOICE QUALITY V

Voice QualityVoice quality can be explained in these terms: delay, jitter, echo and priority. Voice Quality is also dependent on bandwidth and packet loss.

Delay

This refers to the amount of time taken to receive data at its destination. Also known as propogation delay, this time is somewhat transparent to data users, but greater than 50ms may become unacceptable to IP telephony users.

Once a packet takes longer than 80 - 200 milliseconds to travel to its location, voice quality starts to transform from acceptable (toll quality) to unacceptable.

• Each element in a network (bridges, switches, routers, firewalls, CODECs and distance) adds to the overall delay of the packet being sent.

• Research has shown that most people greatly dislike long transmission delays in telephone calls.

• Too much latency makes it difficult to carry on an interactive dialog, which is typically the case in voice communications.

There are several factors that can cause delay of voice traffic in an IP or other packet network. These include message serialization delay, the time needed to process voice samples and compress/decompress them in a CODEC, network delays including queuing, and the need to accommodate delay variation at the receiver in a jitter buffer.

Jitter

Jitter is a distortion or fluctuation caused by the variation of a signal from its references, which can cause data transmission errors, particularly at high speeds.

• Jitter is often caused by congestion in the switch and router (i.e. a slow WAN link and large packets).

For example, in a network transmission, jitter would be a bit arriving either ahead or behind a standard clock cycle.

Echo

Echo is a repetition of a signal in a communications line. Received data is transmitted back to the sending station, allowing the user to inspect visually what was received. A local echo displays what you type on your screen.

• Echo results when IP calls leave the LAN through poorly administered TDM (analog) trunks to the PSTN.

• Differences in electrical characteristics at opposite ends can cause the echo.

• Echo is caused by poor termination or acoustic feedback.

Most gateway devices include echo cancellation up to 64 ms The IP Phone includes echo-sup-pression to remove acoustic echo.

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V VOICE QUALITY

Priority

Priority refers to a particular order, or sequence, in which things take place (items processed, users served, etc.). A priority is based on a predetermined assignment of value, or importance, to different types of events and people.

Priority Queuing (PQ)

Each packet is assigned a priority and placed into a hierarchy of queues based on priority. When there are no more packets in the highest queue, the next-lower queue is serviced. The problem with this method is that lower-priority packets may get little attention.

Techniques to Improve Network Quality

Managing Voice Quality

• Delay can be controlled through prioritization of voice traffic:

• Implementing VLANs with 802.1p/Q on switches

• Implementing DiffServ/ToS on routers

• Increasing the bandwidth (increasing the size of the pipe)

• Changing the network topology (segmenting traffic with more subnets)

• Compression (of voice traffic)

Tolerance Mechanisms

Care has been taken in the design of the IP phones and ICP controllers to cater for delay through the inclusion of echo cancellation devices.

Jitter buffers

Jitter and a certain degree of packet loss are also taken care of by the inclusion of jitter buffers. The current jitter buffers are static in size, but they are adaptive to the network conditions. In other words, they adaptively correct for clock slip and lost packets, but they do not dynam-ically change their size.

Also, IP phones have additional tolerance techniques built into them to help reduce delay, jitter, and packet loss through the rejection of packets that it deems to be too long to be a voice packet.

Voice Sample Rates

Mitel Networks samples the voice transmission every 20ms (other sample durations include 40ms or 60ms). A 8khz sample every 20ms was chosen in order to assist with potential packet loss. The sample rate allows for some packets to be lost, yet not affect the quality of the voice.

Example: Sampling the voice every 20ms will compensate for packet loss because you do not have to wait another 40 milliseconds in a 60ms duration to resend new packets.

qos

Documents

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