ATM adaptation layer. A series of protocols enabling various types of traffic, including voice, data, image, and video, to run over an ATM network.

AAL5 mode

ATM adaption layer 5. One of four AALs recommended by the ITU-T. AAL5 is used predominantly for the transfer of classical IP over ATM. AAL5 is the least complex of the current AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability. It is a Layer 2 circuit transport mode that allows you to send ATM cells between ATM2 IQ interfaces across a Layer 2 circuit-enabled network. You use Layer 2 circuit AAL5 transport mode to tunnel a stream of AAL5-encoded ATM segmentation and reassembly protocol data units (SAR-PDUs) over an MPLS or IP backbone. See also cell-relay mode, Layer 2 circuits, standard AAL5 mode, and trunk mode.


Area border router. Router that belongs to more than one area. Used in OSPF. See also OSPF.

access concentrator

Router that acts as a server in a Point-to-Point Protocol over Ethernet (PPPoE) session, for example, an E-series router.

access point name

See APN.

accounting services

A method of collecting network data relating to resource usage.


Address and Control Field Compression. Enables routers to transmit packets without the two 1-byte address and control fields (0xff and 0x03) normal for PPP-encapsulated packets, thus transmitting less data and conserving bandwidth. ACFC is defined in RFC 1661, The Point-to-Point Protocol (PPP). See also PFC.

active route

Route chosen from all routes in the routing table to reach a destination. Active routes are installed into the forwarding table.

adaptive services

A set of services or applications that you can configure on an Adaptive Service PIC (AS PIC). The services and applications include stateful firewall, Network Address Translation (NAT), intrusion detection services (IDS), Internet Protocol Security (IPSec), Layer 2 Tunneling Protocol (L2TP), and voice services. See also tunneling protocol.

add/drop multiplexer

See ADM.

Address and Control Field Compression


address match conditions

The use of an IP address as a match criterion in a routing policy or a firewall filter.

Address Resolution Protocol

See ARP.


Portion of the local routing information that pertains to the reachability of a single neighbor over a single circuit or interface.


Logical software table that contains BGP routes received from a specific neighbor.


Logical software table that contains BGP routes to be sent to a specific neighbor.


Add/drop multiplexer. SONET functionality that allows lower-level signals to be dropped from a high-speed optical connection.


Asymmetrical digital subscriber line. A technology that allows more data to be sent over existing copper telephone lines public switched telephone network (PSTN). ADSL supports data rates from 1.5 to 9 Mbps when receiving data (downstream rate) and from 16 to 640 Kbps when sending data (upstream rate).

ADSL2 interface

ADSL interface that supports ITU-T Standard G.992.3 and ITU-T Standard G.992.4. ADSL2 allocates downstream (provider-to-customer) data rates of up to 12 Mbps and upstream (customer-to-provider) rates of up to 1 Mbps.

ADSL2+ interface

ADSL interface that supports ITU-T Standard G.992.5. ADSL2+ allocates downstream (provider-to-customer) data rates of up to 25 Mbps and upstream (customer-to-provider) rates of up to 1 Mbps.


ITU-T Standard G.992.1 that defines how ADSL works over existing copper telephone (POTS) lines. See also ADSL and PIM.


ITU-T Standard G.992.1 that defines how ADSL works over Integrated Services Digital Network (ISDN) lines. See also ADSL and PIM.

ADSL interface

Asymmetrical digital subscriber line interface. Physical WAN interface that connects a router to a digital subscriber line access multiplexer (DSLAM). An ADSL interface allocates line bandwidth asymmetrically with downstream (provider-to-customer) data rates of up to 8 Mbps for ADSL, 12 Mbps for ADSL2, and 25 Mbps for ADSL2+ and upstream (customer-to-provider) rates of up to 800 Kbps for ADSL and 1 Mbps for ADSL2 and ADSL2+, depending on the implementation.

Advanced Encryption Standard

See AES.


Advanced Encryption Standard. Defined in FIPS PUB 197. The AES algorithm uses keys of 128, 192, or 256 bits to encrypt and decrypt data in blocks of 128 bits.

aggregate route

Combination of groups of routes that have common addresses into a single entry in a routing table.


Authentication header. A component of the IPSec protocol used to verify that the contents of a packet have not changed, and to validate the identity of the sender. See also ESP.


ATM line interface. Interface between ATM and 3G systems. See also ATM.

alternate priority queuing

See APQ.


American National Standards Institute. The United States' representative to the ISO.


Access point name. When mobile stations connect to IP networks over a wireless network, the GGSN uses the APN to distinguish among the connected IP networks (known as APN networks). In addition to identifying these connected networks, an APN is also a configured entity that hosts the wireless sessions, which are called Packet Data Protocol (PDP) contexts.

application-specific integrated circuit



Alternate priority queuing. Dequeuing method that has a special queue, similar to strict-priority queuing (SPQ), which is visited only 50 percent of the time. The packets in the special queue still have a predictable latency, although the upper limit of the delay is higher than that with SPQ. Since the other configured queues share the remaining 50 percent of the service time, queue starvation is usually avoided. See also SPQ.


Automatic Protection Switching. Technology used by SONET ADMs to protect against circuit faults between the ADM and a router and to protect against failing routers.


  1. Routing subdomain that maintains detailed routing information about its own internal composition as well as routing information that allows it to reach other routing subdomains. In IS-IS, an area corresponds to a Level 1 subdomain.


  1. In IS-IS and OSPF, a set of contiguous networks and hosts within an autonomous system that have been administratively grouped together.


area border router

See ABR.


Address Resolution Protocol. Protocol used for mapping IP addresses to method authenticity check (MAC) addresses.


Autonomous system. Set of routers under a single technical administration. Each AS normally uses a single interior gateway protocol (IGP) and metrics to propagate routing information within the set of routers. Also called a routing domain.


Autonomous system boundary router. In OSPF, a router that exchanges routing information with routers in other ASs.

ASBR Summary LSA

OSPF link-state advertisement (LSA) sent by an area border router (ABR) to advertise the router ID of an autonomous system boundary router (ASBR) across an area boundary. See also ASBR.

AS external link advertisement

OSPF link-state advertisement sent by AS boundary routers to describe external routes that they know. These link-state advertisements are flooded throughout the AS (except for stub areas).


Application-specific integrated circuit. Specialized processors that perform specific functions on the router.


Adaptive Services Module. On a Juniper Networks M7i router, provides the same functionality as the AS PIC.

AS path

In BGP, the route to a destination. The path consists of the AS numbers of all routers that a packet must go through to reach a destination.


Adaptive Services PIC. See adaptive services.

asymmetrical digital subscriber line


Asynchronous Transfer Mode

See ATM.


Asynchronous Transfer Mode. A high-speed multiplexing and switching method utilizing fixed-length cells of 53 octets to support multiple types of traffic.

ATM adaption layer

See AAL.

ATM-over-ADSL interface

Asynchronous Transfer Mode (ATM) interface used to send network traffic through a point-to-point connection to a DSL access multiplexer (DSLAM). ATM-for-ADSL interfaces are intended for asymmetrical digital subscriber line (ADSL) connections only, not for direct ATM connections.

ATM line interface

See ALI.


Smallest possible operation. An atomic operation is performed either entirely or not at all. For example, if machine failure prevents a transaction from completing, the system is rolled back to the start of the transaction, with no changes taking place.


Authentication center. Part of the Home Location Register (HLR) in third-generation (3G) systems; performs computations to verify and authenticate a mobile phone user.

authentication center

See AUC.

authentication header

See AH.

automatic policing

A policer that allows you to provide strict service guarantees for network traffic. Such guarantees are especially useful in the context of differentiated services for traffic engineered LSPs, providing better emulation for ATM wires over an MPLS network.

Automatic Protection Switching

See APS.

autonomous system

See AS.

autonomous system boundary router


autonomous system external link advertisement

OSPF link-state advertisement sent by autonomous system boundary routers to describe external routes that they know. These link-state advertisements are flooded throughout the autonomous system (except for stub areas).

autonomous system path

In BGP, the route to a destination. The path consists of the autonomous system numbers of all the routers a packet must pass through to reach a destination.


One of three methods of electing and announcing the rendezvous-point-to-group address mapping in a multicast network. This is a vendor-proprietary specification supported by the JUNOS software. See RP.