DSL and Networks Information

Because of the abstraction provided by encapsulation, IP can be used over a heterogeneous network (i.e., a network connecting two computers can be any mix of Ethernet, ATM, FDDI, Wi-Fi, token ring, etc.) and it makes no difference to the upper layer protocols. Each data link layer can (and does) have its own method of addressing (or possibly the complete lack of it), with a corresponding need to resolve IP addresses to data link addresses. This address resolution is handled by the Address Resolution Protocol (ARP).

Data from an upper layer protocol is encapsulated inside one or more packets/datagrams (the terms are basically synonymous in IP). No circuit setup is needed before a host tries to send packets to a host it has previously not communicated with (this is the point of a packet-switched network), thus IP (Internet protocol) is a connectionless protocol. This is quite unlike Public Switched Telephone Networks that require the setup of a circuit before a phone call may go through (a connection-oriented protocol).

A virtual network provides the virtual links between nodes in a physical computer network to form a virtual network. The implementation of these virtual links may or may not correspond to physical connections between nodes.

The terms VLAN, VPN, and VPLS are all used to describe different types of virtual network.

A VLAN is a partitioning of a network into multiple subnets using a VLAN ID. The partitioned network can be on a single router, can be on multiple routers that would otherwise form a single network, or can be on a VPN.

A VPN is multiple remote routers (or networks) joined by some sort of tunnel over another network, usually a third party network. Two such routers constitute a ‘Point to Point Virtual Private Network’ (or a PTP VPN). Connecting more than two routers by putting in place a mesh of tunnels creates a ‘Multipoint VPN’.

A VPLS is a specific type of Multipoint VPN. VPLS are divided into Transparent LAN Services (TLS) and Ethernet Virtual Connection Services. A TLS sends what it receives, so it provides geographic separation, but not VLAN subnetting. An EVCS adds a VLAN ID, so it provides geographic separation and VLAN subnetting.

A Network Interface Controller (NIC) is a hardware interface that handles and allows a network capable device access to a computer network such as the internet. The NIC has a ROM chip that has a unique Media Access Control (MAC) Address burned into it. The MAC address identifies the vendor and the serial number of the NIC which is unique to the card. Every NIC has a unique MAC address which identifies it on the LAN. The NIC exists on both the ‘Physical Layer’ (Layer 1) and the ‘Data Link Layer’ (Layer 2) of the OSI model.

Sometimes the word ‘controller’ and ‘card’ is used interchangeably when talking about networking because the most common NIC is the Network Interface Card. Although ‘card’ is more commonly used, it is in less encompassing. The ‘controller’ may take the form of a network card that is installed inside a computer, or it may refer to an embedded component as part of a computer motherboard, a router, expansion card, printer interface, or a USB device.

A MAC Address is a unique 48 bit network hardware identifier that is burned into a ROM chip on the NIC to identify that device on the network. The first 24 bits is called the Organizationally Unique Identifier (OUI) and is largely manufacturer dependent. Each OUI allows for 16,777,216 Unique NIC Addresses.

Smaller manufacturers that do not have a need for over 4096 unique NIC addresses may opt to purchase an Individual Address Block (IAB) instead. An IAB consists of the 24 bit OUI, plus a 12 bit extension (taken from the ‘potential’ NIC portion of the MAC address)

OUI and IAB blocks are available for purchase from the IEEE Registration Authority.

A local area network is a network that spans a relatively small space and provides services to a small number of people. Depending on the number of people that use a Local Area Network, a peer-to-peer or client-server method of networking may be used. A peer-to-peer network is where each client shares their resources with other workstations in the network. Examples of peer-to-peer networks are: Small office networks where resource use is minimal and a home network. A client-server network is where every client is connected to the server and each other. Client-server networks use servers in different capacities. These can be classified into two types: Single-service servers, where the server performs one task such as file server, print server, etc.; while other servers can not only perform in the capacity of file servers and print servers, but they also conduct calculations and use these to provide information to clients (Web/Intranet Server). Computers are linked via Ethernet Cable, can be joined either directly (one computer to another), or via a network hub that allows multiple connections.Historically, LANs have featured much higher speeds than WANs. This is not necessarily the case when the WAN technology appears as Metro Ethernet, implemented over optical transmission systems.

Currently, most ADSL communication is full duplex. Full duplex ADSL communication is usually achieved on a wire pair by either frequency division duplex (FDD), echo canceling duplex (ECD), or time division duplexing (TDD). FDM uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user. With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk issues that affect other lines in the same bundle.

There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used.

A common error is to attribute the A in ADSL to the word asynchronous. ADSL technologies use a synchronous framed protocol for data transmission on the wire.

Some variants of DSL connections, like ADSL and very high speed DSL (VDSL), typically work by dividing the frequencies used in a single phone line into two primary ‘bands’. The ISP data is carried over the high frequency band (25 kHz and above) whereas the voice is carried over the lower frequency band (4 kHz and below). (See the ADSL article on how the high frequency band is sub-divided). The user typically installs a DSL filter on each phone. This filters out the high frequencies from the phone, so that the phone only sends or receives the lower frequencies (the human voice). The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other.