Asynchronous Transfer Mode and its Virtual Connections

Asynchronous Transfer Mode (ATM) is derived from standards developed by the ITU-T that were based on BISDN (Broadband ISDN) technology.

ATM is a connection-oriented service in which transmitted data is organized into fixed-length cells. Upper-layer protocols and user data such as an IP packet are segmented into 48-byte protocol data units (PDUs). These PDUs are prepended with a 5-byte ATM header, and the resulting 53-byte cells are input into an ATM switch and multiplexed together. These cells then contend for vacant slots in the outgoing ATM cellstream.

Each ATM cell header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI), which together define the ATM virtual circuit the cell needs to follow on its path toward its destination. The arrival rate, or delay, of one particular cell stream is not periodic. Therefore, the cell transfer is referred to as Asynchronous Transfer Mode, in contrast to synchronous transfer, such as TDM transport, which uses fixed time periods for frame transmission and reception.

Each ATM cell contains information that identifies the virtual connection to which it belongs. That identification has two parts : a virtual channel identifier and a virtual path identifier. Both the VPI and VCI are used at the ATM layer. The virtual channels and the virtual paths are contained within the physical transmission path, as shown in the picture below.

The virtual channel is a unidirectional communication capability for the transport of ATM cells. To originate or terminate a virtual channel link, a VCI is either assigned or removed. Virtual channel links are concatenated to form a virtual channel connection (VCC), which is an end-to-end path at the ATM layer.

A virtual path is a group of virtual channel links, all of which have the same endpoint. To originate or terminate a virtual path link, the VPI is either assigned or removed. Virtual path links are concatenated to form a virtual path connection (VPC).

Reference :
“Advanced MPLS Design and Implementation”, Vivek Alwayn, Cisco Press, 2002.

The Principle and Configuration of a Virtual Leased Line (VLL) network

A Virtual Leased Line (VLL) service is a point-to-point data communication using ADSL network (over ATM) for communicating any data between two sites. The data speed of VLL depends on customer requirements such as 64, 128, 256, 512 Kbps.

In actual network, multiple users are connected to DSLAM ; furthermore, the service provider assigns the VLL ratio for bandwidth sharing, when each customer sends data simultaneously. This is a disadvantage of VLL service when compared with Leased line (LL) service. For the latter service, it can use a full bandwidth which is not shared with other customers. The benefit of a VLL service is that it is cheaper than a LL service.

The basic VLL configuration is shown in the picture below.

From this picture, the VLL configuration is divided into two sites : the provider site and the customer site. In the service provider, there are two DSLAMs (Digital Subscriber Line Access Multiplexer) and two ATM switches for transmitting and receiving any data traffic between two customer sites. DSLAM is used for combining data signal from multiple customers and then sends data to ATM switch. After receiving data from DSLAM, ATM switch searches the best route automatically to the other site by only configuring VPI / VCI parameters at the DSLAM connection and ATM switch connection. At DSLAM’s destination side (closing to ATM-SW), the VPI / VCI of DSLAM-A must be the same as ATM SW-A’s, and the VPI/VCI of DSLAM-B must be the same as ATM SW-B’s.

In each customer site, there are an ADSL Router and work stations (a server, a PC, or other LAN equipment). Customers must configure their ADSL routers in VLL mode by setting the connection type to “RFC1483 Route”. At DSLAM’s source side (closing to ADSL Router), the VPI / VCI of DSLAM-A must be the same as ADSL Router-A’s, and the VPI/VCI of DSLAM-B must be the same as ADSL Router-B’s. These parameter values depend on each telecommunication provider’s assignment.

The Principle and Configuration of a Leased Line (LL) network

A Leased Line (LL) service is a point-to-point data communication for communicating any data between two sites. The channel bandwidth reserved for each customer isn’t shared with other customers, and the data speed of LL depends on customer requirements.

A Leased line network composes of a Data Terminal Equipment (DTE), a Data Communication Equipment (DCE), and work stations (a PC, a server, or other LAN equipment) at each customer site. A DCE generates the clock signal for synchronizing between a DCE and a DTE.

In the provider site, there are LL nodes used for transmitting the data signal from the source to the destination of customer sites.

Mainstreet is an example of LL networks ; it is one of Alcatel’s network equipment. Mainstreet DCE is called Data Terminal Unit (DTU), and there are various types of mainstreet nodes in a provider network. A DTE is customer’s router connecting to a DTU with a serial port. The mainstreet configuration is displayed below :

LAN equipment or PCs (LAN port) <--> (LAN port) Router (Serial port) <--> (V.35 port) DTU (RJ-11 port) <--> (Line port) Mainstreet node (E1 port) <--> To the other site.

A customer’s router can directly connect to a mainstreet node without a DTU, if a mainstreet node use a V.35 card.